Optimization of hammers in Braille printers

2006:215 CIV

M A S T E R ' S T H E S I S

Optimization of Hammers in Braille Printers

Peter Engström

Luleå University of Technology MSc Programmes in Engineering

Electrical Engineering

Department of Computer Science and Electrical Engineering Division of EISLAB

2006:215 CIV - ISSN: 1402-1617 - ISRN: LTU-EX--06/215--SE

Optimization of Hammers in Braille Printers

Peter Engstr¨ om

Lule˚ a University of Technology

Dept. of Computer Science and Electrical Engineering EISLAB

March 19, 2006

A BSTRACT

Tactile print has been used since the beginning of 1800[1], at first not for visually impaired but as an invention (called Sonography) for the artillery to read and write in darkness and for making several copies of a document at once, though it was never used as intended because of it’s complexity with characters representing sound and not letters. Sonography characters were also to big to be covered by a single finger tip wich made reading slow.

Later on Sonography was presented at a school for blind students. It was first rejected by the teachers but the students pushed on. One of the first students to encounter the Sonography was Louis Braille. He reworked the 12-dot Sonography to a six dot literal system. Over the years the Braille system has been improved and is now used for science, mathematics and also music purpose. With the possibility to connect Braille printers to personal computers even graphics can be made, making abstract tables of figures visualized by diagrams and charts.

This master thesis is about optimizing hammers in Braille printers. There has been some problems with dot quality in Braille printers. The first problem is that some dots are not embossed to the same height. The problem is somewhat coupled to increased temperature in the printing head depending on low efficiency. An other problem is how to measure striking force. Dynamic laser measurement turned out to be the best method to measure the striking force. By knowing speed and mass of movement, energy can be calculated with ease. Optimization of the hammers striking force was made by static measurement and modelling using Femlab

and Matlab

. The Femlab static modell was verified by measurement with a force sensor at different displacement. Modifications modelled in Femlab and verified in static force measurement were then dynamically tested and measured with laser to make sure that they worked in reality. Finally hammers were tested in a printing head on different paper qualities. It turned out that the dot quality was highly dependent on the rubber shock absorbers in the hammers rather than solenoid efficency.

1

Femlab from Comsol (www.comsol.com) is a program for physics modeling with finite element method.

2

Matlab is a programming language from Mathworks (www.mathworks.com) wich can be used in conjunction with Femlab.

iii

P REFACE

The work on this master thesis was done in the spring of 2005 at Index Braille AB.

During a job like this one always encounter some persons that makes the work a lot easier than it first appers to be. I would like to thank my examiner Dr Kalevi Hyypp¨ a, for introducing this thesis for me. I must admit that I was not interested at first but after a few weeks thinking I said: -”Ok”. After a few days of work I was convinced that the decision was right although the work did not include electronic design. Special thanks to Per Gren at the Sirius Laboratory for helping me out with the laser measurements and

˚ Ake Wisten at EISLAB for great discussions on electromagnetics. Most of all I would thank my supervisor at Index Braille AB, Per Burman for great ideas and guidancy.

Peter Engstr¨ om

v

C ONTENTS

Chapter 1: Introduction 1

1.1 Index Braille AB . . . . 1

1.2 The Braille Printers . . . . 1

1.3 Objectives . . . . 4

1.4 Delimitation . . . . 7

1.5 Method . . . . 7

1.6 Thesis Outline . . . . 8

Chapter 2: Theory 9 2.1 Maxwells equations . . . . 9

2.2 Magnetic Circuit Analysis . . . . 10

2.3 Theory of Hammers . . . . 12

2.4 Dynamics . . . . 13

Chapter 3: Measure Hammer Performance 15 3.1 The Measuring System from Microbit 2.0 AB . . . . 15

3.2 Visualizing Rapid Movements . . . . 16

3.3 Measure Impact Velocity with Laser . . . . 19

3.4 Static Force Measurements . . . . 21

3.5 Dot Quality on Print-out . . . . 21

Chapter 4: Hammer Opimization 23 4.1 Research . . . . 23

4.2 Description of the Parts in the Hammer . . . . 23

4.3 Braille Printer Head . . . . 25

4.4 Hammer Optimization . . . . 25

4.5 Braille Printer Electronics . . . . 27

Chapter 5: Results 31 5.1 Measuring System . . . . 31

5.2 Hammer Optimization . . . . 32

5.3 The Printer Head . . . . 36

5.4 The Printer Electronics . . . . 38

Chapter 6: Summary and Conclutions 43

6.1 Conclusions . . . . 43 6.2 Further work . . . . 44

Appendix A:Datasheets for displacement sensor 45

viii

C HAPTER 1 Introduction

1.1 Index Braille AB

Index Braille AB in Gammelstad, Lule˚ a, Sweden is a Braille printer company established in 1982. With eleven employees and producing about 1400 printers for shipment all over the world they can not be ignored on the market. Most parts for the printers are made by subsuppliers, but final assembly, programming, tests and shipping is made in Gammelstad, Lule˚ a.

1.2 The Braille Printers

Index Braille AB has several models of printers in their line of production. The models Basic-S, Basic-D (the foremost printer in figure 1.1) and 4Waves PRO (figure 1.2) uses continious paper with tractor-feeding. The models 4x4 PRO and Everest (figure 1.3 shows paper loading) uses cut sheet paper. Every model except Basic-S can print double sided and every model can print graphics. One can print braille on ordinary paperquallities but the result is best with paper weight of 120g/m

to 200g/m

. With lighter paper the dots can easily be destroyed.

The printers can print ordinary text files just by sending the file trough a serial or paralell cable without any special formatting. The printers automaticly formats the document double sided. This mean they are virtually unbounded by any operating system. If one include graphics such as maps or diagrams in a document a program called WinBraille

has to be used and at the time of writing it only works with Windows

. The

1

WinBraille can convert and send a document with embedded text and graphics to any Index Braille printer.

2

Windows is operating system for personal computers and is also a registered trademark owned by

1

2 Introduction

Figure 1.1: Two printer models from Index Braille AB. In front the Basic printer for use with tractor feed paper. In the background the Everest printer for use with cut sheet paper. Both printers can print double sided.

connection to a computer can be done by serial, paralell, USB 1.1 or Network cable. To make the printers user friendly for visually impaired persons, every button on the panel is marked with Braille and the printer speaks out all settings done by the user.

1.2.1 A More Technical View

When it comes to printing the paper is feed through the printer head by a stepper motor.

The printing head moves from side to side over the paper and is also run by a stepper motor. While the printer head is moving over the paper the hammers strikes to form the

Microsoft corp.

1.2. The Braille Printers 3

Figure 1.2: 4Waves PRO Braille printer for large volume printing. In 4Waves PRO four Braille printers are coupled in series in order to increase the printing speed.

dots on both side of paper simultaniously. Double sided printing is done by offsetting the backside dots one dot horizontally and one dot vertically. As the printers are fully software driven a dot can be formed anyware on the paper.

As seen in figure 1.4 the printer head is a quite a big piece. The figure 1.5 and 1.6

shows how the hammers are placed in the printer head. As one can see, the hammers are

mounted on the same side in the printer head and the double sided printing is obtained

by using concave and convex hammers. While printing large documents (that is 30 pages

or more) the Braille dots starts to get slightly faded. The larger the document the more

faded dots becomes, some almost disapear. The company thought this had something

to do with the hammers. The hammers are mounted about 2mm above the paper slit in

the printer head. The hammer is energized the first 1.8mm of travel so when it hits the

paper after 2mm it is unforced and will bounce back to it’s initial position forced by the

return spring. A cross-section view of a hammer can be seen in figure 1.7.

4 Introduction

Figure 1.3: Paper loading on an Everest printer.

1.3 Objectives

To form the dots on paper, Index Braille AB have since the start used hammers

from Impact Devices Inc.(USA), but they have become too expensive over the years, so pro- ducing own hammers were a topic. While making a new Braille embosser for high volume production of Braille (see figure 1.2) some problems arise. This thesis was founded with these objectives:

• Develope a measuring system for the striking force.

• Determin the maximal printing frequency.

• Reduce the variation of dot quality while printing large documents.

• Reduce the temperature in the printing head.

• Reduce sensitivity of abrade on the hammer and temperature variations.

3

A hammer is a solenoide with a striking pin to form the dots in the paper.

1.3. Objectives 5

Figure 1.4: Removal of Printer Head on an Everest printer.

Figure 1.5: Hammer bore layout in printer head.

A few weeks before I started my work the problem with high temperature inside the printer head was solved. During a meeting with the manufacturer they found that the striking pin was not polished and thereby increased the temperature by friction. The same week I begun my work, the new printer head with corrected hammers arrived. A few tests were done and they noticed quit an increase in dot quality and reduced temperature.

To measure the striking force Microbit 2.0 AB

developed a system based on a force sensor. The hammer hits a force sensor behind a sheet of rubber. A data acqisition system measures the voltage over the force sensor, but since the this system is not accurate enough it is not used.

4

Microbit 2.0 AB is a company located in Kalix, Sweden

6 Introduction

Figure 1.6: Side view of printer head: 1-Paper feed through, 2-Anvil rubber, 3-Hammer.

Figure 1.7: Hammer cross-section view with material description in parentesis: 1-Piston(Remco C steel), 2-Striking pin(stainless steel), 3-Return spring(steel), 4-Air gap,

5-Bearing(conductive PTFE plastic), 6-Solenoid(220 turns of 0.30mm copper wire on a plastic

former), 7-Housing(Remco C steel), 8-Shock absorber(rubber), 9-Washer(stainless steel), 10-

End cap(aluminium).

1.4. Delimitation 7

1.4 Delimitation

As much as possible of the existing measuring system is to be reused. Outer dimensions of the hammers are kept as on the original to avoid a reconstruction of the printer head.

The magnetic material for the hammers is not altered because it’s too hard to find reliable suppliers for low volume consumers.

1.5 Method

1.5.1 Measuring System

The company wanted a new system for measuring hammer performance based on the existing one, so tests on this was done to find out what was wrong with the used method.

1.5.2 Hammer Optimization

To get more knowledge about hammers articles on solenoids were studied, usually they were about actuators from the vehicle industry, but some parts were interesting. Several hammers of earlier batches from Index Braille AB and Impact Devices Inc. were dis- assembled, to get a feeling of the development until today. Hammers were also studied with a stroboscope to see the movement of the striking pin.

The optimization of the hammers is based on practical tests together with analysis in Femlab. The tests are divided in tree parts: static measurements, dynamic measurement and real world tests in Braille printers where dot quality is evaluated. Static measure- ments where conducted in a test bench where force versus displacement was evaluated.

This was done to find optimum use of the force distribution. Dynamic analysis was made using laser triangulation where time versus displacement was captured in order to measure the velocity at the time when the hammer hits the paper. By knowing impact velocity the impact energy can be calculated. The last analysis method was to mount solenoides in a real Braille printer and check the dot quality. To determine maximum printing speed a stroboscope was used to determine when the hammer enters the steady state after excitation. The stroboscope was also used to test different shock absorbers.

The FEM analysis where made to simulate different changes in the solenoid before trying it out in the real world. The coupling of FEM to real world measurements was quite good. For FEM-analysis Femlab 3.1 from Comsol

was used.

Most of the work is done on a Everest printer but all printers uses same technic and only the dimension of the printer head is different.

5

www.comsol.com

8 Introduction

1.6 Thesis Outline

Chapter 2: A short introduction to electromagnetics and FEM-analysis with Femlab.

Chapter 3: Here some methods to analyse performance of the hammers are discussed.

Chapter 4: The work of hammer opimization.

Chapter 5: Result from tests and simulations.

Chapter 6: Conclutions on final designs.

Appendix A: Data sheet of laser sensor M7.

C HAPTER 2 Theory

In this chapter I will try to explain the theory behind magnetic circuit analysis and the hammers.

2.1 Maxwells equations

All electromagnetics problems are based on the well known Maxwell’s Equations:

(∇ × E) = − ∂B

∂t (2.1)

(∇ × H) = J + ∂D

∂t (2.2)

(∇ · D) = ρ (2.3)

(∇ · B) = 0 (2.4)

Constitutive relations

D = 

E (2.5)

B = µ

H (2.6)

Where 2.1 is called Faradays law and 2.2 is called Amperes law. The equation 2.3 express the relation between the electric flux density and charge desity. The equation 2.4 states that there can be no magnetic free charges. The equation 2.5 and 2.6 are constituive laws that describes how electric and magnetic flux desities are related to field intesites in free space. In the presens of matter they are described as:

D = 



E (2.7)

B = µ

µ

H (2.8)

9

10 Chapter 2: Theory

2.2 Magnetic Circuit Analysis

Magnetic circuit analysis is quite comparable with the electrical circuit analysis by the fact that the magnetomotive force can be compare to the voltage source in electrical circuit analysis. The magnetic flux Φ

compares to the current I and the reluctance R

compares to resistance R. The magnetomotive force (mmf) is formed by Ampere’s law:

I

H · d` = N I = mmf (2.9)

Where N =number of turns in the coil and I =current through the coil. This can be formed to ”Ohm’s law” of magnetic circuit:

N I = R

Φ

(2.10)

Compare with:

U = RI (2.11)

A simple example with this so called lumped parameters can be seen in figure 2.1.

a) b)

Figure 2.1: The transformation from the a) magnetical circuit to the b) lumped parameter circuit.

The reluctance R

can be calculated as:

R

= l

µ

µ

A (2.12)

Where l is the magnetic path length and A is the cross sectional area of the magnetic iron. The reluctance for the airgap is calculated as follows:

R

= l

µ

A (2.13)

These calculations are however not completely accurate as the flux of current never leaves

the conducting material but the magnetic flux does. Air gaps in a magnetic circuit acts

like a resistor for current. The smaller the air gap the less reluctance. This means that

2.2. Magnetic Circuit Analysis 11

Figure 2.2: Magnetic flux: 1-Field lines, 2-Air gap, 3-Magnetic material.

large error in the calculations of a magnetic circuit are induced as the dimensions of the magnetic material becomes smaller relative to the air gaps, this is called fringing (see figure 2.2). In order to handle this several papers states their methods[2][3].

However modern computers are fast enough to allow for calculations of large matrixes of equations. In order to increase the accuracy of the calculations and be able to handle fringing one can make use of thoose fast computers and use the Finite Element Method.

There are several programs on the market but in this master thesis Femlab is used. In Femlab a drawing of the magnetic construction is made with the built in drawing tools or a CAD

-drawing can be imported. When the drawing is finished the magnetic properties (such as relative permeability and conductivity) of every detail must be defined. After that a mesh for over wich the computation should be taken is generated as in figure 2.3

Figure 2.3: A 2D rotational symetry drawing of a hammer with applied mesh. As can be seen the mesh is addaptive and becomes finer in critical areas.

1

Acronyme for Computer Aided Design

12 Chapter 2: Theory

Figure 2.4: The original piston gives a force of 3.9N (the spring force is neglected)

Figure 2.5: The modified piston with spring seat removed gives a force of 6.2N at the same position as in figure

When the electromagnetical properties and the mesh is defined the solver is started.

When the solver is finished it can show the result in many ways such a surface plot over the magnetic flux density in the piston (see figure 2.6). It is also possible to do a integration over the piston to calculate force. In Femlab one can make small changes to an existing modell to see the differens in force. This is shown in figure 2.4 and 2.5.

If the force calculations are done for different positions of the piston a curve that represents force vs. displacement is obtained. To automate these calculations Matlab can be used togheter with Femlab. The position of the piston and striking pin is easily changed by a for-loop in Matlab. The downside of Femlab is that the drawing interface is not the easiest to work with and the import tool for CAD-drawings leaves many wishes. It is not so easy to realise a complex drawing and therefor my drawings is a litle simplified with sharper edges than in reality. The downside is that sharp edges makes some matematical noise is the solutions.

2.3 Theory of Hammers

Hammers (see figure 1.7) are built around a solenoid

. The solenoid produces a magnetic field which forces the piston to attempt to close the airgap for maximum inductance.

2

Thightly wound insulated wire around a cylindrical former.

2.4. Dynamics 13

Figure 2.6: A surface plot over the flux density for the piston. To the right the drawing with the piston selected.

The force developed on the piston depends on the cross sectional area of the piston and the flux density. Mathematically stated as:

F = 1 2

Z

BHds (2.14)

In order to increase the flux density the housing around the solenoid is made of magnetic steel and the magnetic field tries to flow along the housing as seen in the Femlab simu- lation figure 2.7 because the housing has a lot lower reluctance than air. To identify the parts in the 2D rotational symetry Femlab simulation see figure 1.7.

2.4 Dynamics

Before the movement of the piston takes place the magnetic forces must be higher than the spring preload. The build-up of this is not instant due to delays formed by the inductance of the coil. The movement of the piston follows the ordinary differential equation:

m¨ x = F

− F

− kx (2.15)

Where

F

Magnetic force

14 Chapter 2: Theory

Figure 2.7: Field lines calculated with Femlab (simulated in 2D with rotational symetry around horizontal axis). The outer field lines are slightly compressed because of the mesh size used.

Finite element method finds solutions within this mesh, thus with a smaller mesh the faster solution but this also introduces some errors as seen on the field lines.

F

Spring preload k Spring constant and

F

= 1

2µ

µ

B

A (2.16)

The magnetic circuit of the hammer is very complex and a fully mathematical description

(closed-form) of the dynamics is not possible. The use of a finite element method or

partial differens method is required.

C HAPTER 3 Measure Hammer

Performance

Performance measurement of the hammers, is fundamental when trying to optimize them.

The performance can be measured by static, dynamic and quality analysis by looking at the formed dots on a print-out. The static analysis is done by applying a known current to the hammers and measure the obtained force at a different distances ranging from 0.2mm to 3.0mm

. In the dynamic measurement the full cycle is analysed. In this special case where dots are to be formed on a sheet of papper, impact energy is the most important quantity to be measured.

3.1 The Measuring System from Microbit 2.0 AB

The existing measuring system from Microbit 2.0 AB is based on a real printer circuit board to activate the hammers. The hammer is placed in a test bench and strikes against a sheet of rubber to simulate paper. The force sensor

is placed on the opposite side of the rubber (see figure 3.1).

The printer circuit board is controlled by a Labview

program. The Labview test program uses a DAQPad-6020E

from National Instruments for capturing data. The program prompts for ”hammer ok” or ”hammer not ok” based on how much energy it can deliver into the force sensor and the travelling time for the striking pin to enter the

1

Actual distances are 0.2mm, 0.3mm, 0.5mm, 0.7mm, 0.9mm, 1.0mm, 1.35mm, 1.8mm, 2.0mm and 3.0mm

2

Honeywell FSG-15N1A (www.honeywell.com)

3

Labview from National Instruments(www.ni.com) is a graphical programming tool for making con- trol, measure and test programs.

4

DAQPad-6020E is a analog to digital converter with 16 inputs and a resolution of 12-bit. Maximum sampling rate is 100kS/s.

15

16 Chapter 3: Measure Hammer Performance rubber sheet. The idea of using rubber in front of the force sensor was to simulate paper and to reduce the shock into the force sensor as it was not intended to be used in shock measuring.The program also showed nice figures of the force vs time (figure 3.2) but as temperature and wear alters the properties of rubber one could see that these curves were altered over quite a short time (a few seconds). This is not a reliable measuring system and therfore not used. However everything in this system could be reused to make a better one.

3.2 Visualizing Rapid Movements

The hammers has a working cycle of about 5.6 to 7.8ms and hits the paper after a movement of 2mm. The cycle time depends on the return spring used and excitation current. To visualize such a fast movement I used a stroboscope. As the time between two strikes of the hammer is not constant, a free running stroboscope could not be used.

The workaround for this is to use a stroboscope with a variable delay that triggers on the excitation pulse for the hammer. This will make a snapshot of the hammer at a fixed time.

3.2.1 The Delay Box

The delay box is designed by myself and built around an integrated circuit called 74HC221 which contains two independent monostable multivibrators. The first monostable makes the actual delay and the second forms the output to a short pulse. Two potentiometers are used to allow coarse and fine tune of the delay time (see schematic in figure 3.3).

The delay time is measured with an oscilloscope where channel one on the oscilloscope is connected to one of the inputs of the delay box and the other channel connected to one of the outputs. The difference of the two channels gives the time delay of the delay box. The last output is connected to the stroboscope. All connectors are BNC-type. (see figure 3.4).

3.2.2 Stroboscope Modification

For the first tests I used an old high quality stroboscope which I borrowed from Lule˚ a

University of Technology, but later on I modified Index Brailles own stroboscope. The

original stroboscope was a free running stroboscope with adjustable frequency and no

connection for external trigg. The modification was the implementation of an external

connection (see figure 3.4). This was done by cutting the signal wire from the internal

oscillator to the lightning unit and attach a 3.5mm phono-connector. Inside the phono-

connector there is a switch. This switch has been wired to switch over to internal oscillator

when the phono-plugg is not inserted.

3.2. Visualizing Rapid Movements 17

Figure 3.1: Sketch of Hammertest from Microbit 2.0 AB

Figure 3.2: A curve from Hammertest.

18 Chapter 3: Measure Hammer Performance

Figure 3.3: Schematic of the delay box.

Figure 3.4: Picture of the delay box with attached oscilloscope.

3.3. Measure Impact Velocity with Laser 19

3.3 Measure Impact Velocity with Laser

The main advantage of using a laser for measurements is that no friction or mass in- terference is added to the system. The laser used measures displacement of the piston.

The measurements are sampled with a time stamp. Thus the velocity of the striking pin can be calculated. By knowing impact velocity and mass of the moving parts the impact energy W

can be calculated by this well known formula:

W

= mv

2 (3.1)

3.3.1 The Equipment

The system is based on the existing hammertest by Microbit 2.0 AB. (figure 3.5). A BNC-connector has been mounted on the front panel to allow connection for the laser.

A switch has also been mounted to switch between force sensor and laser. As the laser output voltage is ±10V and the force sensor output is 0-5V a voltage divider is mounted in the hammertest box. Slight adjustments to the Labview program has been done to allow bipolar measurements. The laser is a M7L-20 from MEL Mikroelektonik GmbH. The laser uses triangulation to determine displacement and output is an analog signal from

−10V to 10V which represents a displacement of ±10mm. The resolution is 0.009mm, for more data on the laser see Appendix A

3.3.2 The Measurements

The hammer is mounted in a test bench as shown in figure 3.6 with the laser looking at the top of the striking pin. To get accurate readings from the laser the top of the strikin pin has to be flat and not too shiny. This is achived with a small amount of Tack-It (or ”sticky blue” as a brittish researcher entering the laboratory called it) applied on the top. Without the Tack-It the measurement became too noisy because a small radial movement also includes an axial movement as the top is not flat. No stopper is mounted on the test bench. This makes the hammer strike to it’s full length. The real striking lenght is about 2mm. A stopper that covers the full diametre of the striking pin could not be mounted on the test bench as it has to be clear for the laser to see the striking pin. No material would be clear after some strikes. A half diameter stopper could not be mounted in the test bench because Tack-It or glue would fall off when the hammer starts striking and the laser has to be excanged to another one with a smaller measuring spot.

This makes one think about measuring from behind of the hammer through the air hole, but the air hole is too small and the piston is too far away from the opening to enable laser triangulation. It is possible to measure from behind through the air vent hole, but this requires a more expensive equipment (laser interferometry).

Every measurement has been done on two hammers of each type. All data were sampled

at a rate of 100k samples/s and every test covered 16 cycles. All measurements were done

20 Chapter 3: Measure Hammer Performance at Lule˚ a University of Technology by assistance of Per Gren at the Sirius Laboratory

.

Figure 3.5: Setup for laser measurement.

Figure 3.6: Close view of laser measurement.

5

Sirius Laboratory is a division at Lule˚ a University of Technology.

3.4. Static Force Measurements 21

3.4 Static Force Measurements

Static force measurement were done by applying a constant current source to the solenoid and changing the distance to the force sensor. With this method, optimization of the force distribution can be made. Estimation on dynamical performance such as impact velocity can be calculated by these formulas:

a(x) = 1

m F (x) (3.2)

and

v(t) = Z

0

a(τ )dτ + v

(3.3)

Though in this typical case it is not so accurate because of two major things. First, the static force will be slightly bigger than from dynamical tests because no eddy-currents are taken acount for. Second, the delay of the magnetic field due to inductance in the coil makes a slow build up of the magnetic field.

3.4.1 The Equipment

The hardware design from Microbit 2.0 AB is reused and a power supply with adjustable current is used. To measure the output from the force sensor the testprogram which comes with the analog-to-digital converter is used.

3.5 Dot Quality on Print-out

This test is done by Index Braille AB on every printer before shipment. All models of

printers have a built-in selftest, that prints 30 pages with a special apperance to test

all hammers in the printer head. The last page is inspected for dot quality. If the dot

quallity is not good enough the printer head is replaced. Checking for dot quality on

print-out while developing hammers is and will always be a more or less subjective view

of performance, but it’s still decisive in final design.

C HAPTER 4 Hammer Opimization

4.1 Research

A lot of articles where read to get a picture on how solenoids work and how to make them faster. Most of the articles were about speeding up numerical methods for modelling solenoid[4][5] behaviour and not the speed of the solenoid itself. The articles about speeding up solenoids ended up in a too complex manufacturing of the solenoid e.g.

reducing eddy-currents by making a slit into the hammer housing[6] or very complex energizing models with several transistors[7] for one solenoid. None of the methods is cheap.

4.2 Description of the Parts in the Hammer

4.2.1 The Piston

The piston (labeled 1 in figure 1.7 on page 6) is an object for abrade when it hits the stainless steel washer. After about 100.000 cycles the wear is obvious. Small particles of metal fall out of the airhole in the back of the hammer. This is caused by deformation of the soft iron piston when it hits the harder stainless steel washer. Due to the deformation of the piston pieces of the Nickel coating fall off. This abrade of the piston seems not to be a problem for the hammers performance, but on some models of printers the metal particles falls directly on the main circuit board and could possibly cause electrical shortcuts, though this has never been reported by the users or by service personal at Index Braille AB.

23

24 Chapter 4: Hammer Optimization

4.2.2 The Striking Pin

The striking pin (labeled 2 in figure 1.7) is made of stainless steel. The top of the striking pin is either convex or concave depending on which side of the paper the dots are to be formed.

4.2.3 The Return Spring

The return spring (labeled 3 in figure 1.7) forces the piston to it’s initial position after excitation. The stronger the spring the faster and more stable return. Stability in return is related to friction against the walls. A strong spring makes the friction neglectable but the drawback is lower impact velocity into the paper. To measure the spring constant I used a scale and a M5 screw. The screw depress the spring against the scale. One turn of the M5 bolt makes a displacement of 0.8mm. After each turn of the bolt I read the scale and wrote the measurement down. After a few measurements one can determine the spring constant.

4.2.4 The Bushings

The bushings (labeled 5 in figure 1.7) are made of PTFE.

The bushing are made con- ductive to avoid static electricity.

4.2.5 The Solenoide

The solenoide (labeled 6 in figure 1.7) consists of 220 turns of enameled copper wire wound on a plastic former. The diameter of the wire is 0.30mm.

4.2.6 The Housing

The housing (labeled 7 in figure 1.7) is made of a steel called Remco C or Din:17405 RFE100.

4.2.7 Shock Absorbers

The purpose of the shock absorbing rubber washer (labeled 8 in figure 1.7), is to reduce bumping when the piston returns to it’s initial position. This is essential to ensure that the piston is at rest when the next excitation occurs. Otherwise one can not know how hard the next strike will be. This was mainly observed in stroboscope light. Several rubber materials were tested to find the right stuff. The material used until this thesis was written is EPDM 70 shore.

1

Poly Tetra Fluoro Ethylene, the company DuPont has invented the brand name Teflon.

4.3. Braille Printer Head 25

4.2.8 Washer

The purpose of the washer (labeled 9 in figure 1.7) is to reduce abrade of the shock absorber. In the original design with the washer made of stainless steel it also reduces the impact velocity into the shock absorber. The reduction of velocity is about 10%

calculated by this formula:

m

v

= m

v

(4.1)

Piston weight m

=4.7 · 10

kg. Piston and Washer weight m

=5.3 · 10

kg. Return velocity v

=1.3m/s

v

= m

v

m

= 4.7 · 10

× 1.3

5.3 · 10

= 1.15m/s

The reason to use stainless steel is that it is not magnetic. The difference of using magnetic material can be studied in chapter 5

4.3 Braille Printer Head

The printer head consists of an aluminium profile with thirteen hammers in two lines.

The upper line is formed by seven concave striking pins and the lower line by six convex ones. On the opposite side of the printer head every hammer has a anvil with contrary shape. Inbetween these parts the paper is fed through the printer head. By the use of both convex and concave striking pins it is possible to write on both sides of the paper simultaneously.

4.3.1 Hammers

The thirteen hammers are placed in two lines as shown in figure 1.5. They are loosely fitted and held in place with a strong spring.

4.3.2 Anvil Rubber Damping

The anvils in the printer head are loosely fitted. Behind the line of anvils a rubber strip holds them in place as seen in figure 1.6. This makes the anvils selfadjusting and also reduces vibration and noise in the printer head.

4.4 Hammer Optimization

4.4.1 Increasing the Force

The increased force of the hammers can be used diffrently depending of what you wish to

obtain. If temperature is the issue, the increased force can be used to lower the current

26 Chapter 4: Hammer Optimization and thereby reduce resistive heating. Another advantage of reducing current is that a cheaper power supply can be used. More force can also be used to increase the printing speed and impact energy for higher print quality.

There are several ways to increase the force of the hammers; increasing the number of turns in the coil or increasing the current through the coil are the most obvious ways.

The disadvantage of this is higher temperature and by increasing the current a more expensive power supply is also needed. Increasing the cross sectional area of the piston is another thing to do, but then the outer size of the hammer has to be altered and the whole printer head has to be reconstructed. Some rewriting of the software is also needed. Altering the material in the magnetic part such as piston and housing could be done but as a low volume consumer it’s hard to find the right material. The material also has to be of a type that is easy to mill and turn which excludes many good materials.

The magnetic properties of the material used in the existing hammers are acceptable.

This leaves the alternative to make small changes in the existing hammer and to try to find the optimum position of the piston in order to make best use of the force. To find this position and to compare different hammers a static force measurement was done.

Simulations in Femlab was also a good help before trying piston position in reality.

Sharp edges on the front of the piston and in the housing are perfered to increase the force. On the housing it was not possible to sharpen the edges as it would make it impossible to insert the tool for moulding the bushings. The only thing that could be done on the piston is to reduce the springseat and make this as sharp as possible. This was done and the result can be seen in chapter 5.

By reduction of the thickness of the back bushing, force increases but this makes it harder to manufacture. Results of Femlab simulations are shown in chapter 5.

4.4.2 Speed Increase

To increase the speed of the hammer, not only the forward velocity must be high, but also the return velocity must be increased. The piston is forced back to it’s initial position by a spring. A stiffer spring makes a faster return. The original spring force was 85N/m and after some experiments the new spring force is set to 127N/m.

4.4.3 Reduction of Abrade in the Hammers

Abrade of the piston is a big issue as small metal dust falls onto the circuit board in some

models of the braille printers. The cause of abrade of the piston is that the hard stainless

steel washer meets the rather soft piston. This causes deformation at the bottom of the

piston and the nickel coating falls off as seen in figure 4.1. Metal that strikes against

another metal is always a problem, insertion of another material inbetween should solve

the problem. Therfore some tests where done. The first test was done with a bit of

plastic glued into a cavity of the stainless steel washer. The plastic used was polyamide

filled with fiber glass. The second test was done with a loose polyamide washer between

4.5. Braille Printer Electronics 27 a flat stainless steel washer and the piston. The third test was done with a polyamide washer pressed down in the cavity of the original stainless steel washer. The last test was done with a thick polyamide washer and no steel washer. The results are discussed in chapter 5

Figure 4.1: Abrade of piston after 100.000 cycles.

Another point on the piston that is exposed to abrade is the spring seat at the piston top (see figure 4.2). This is caused by too loose fit between the spring and the piston. A more snug fit solves this and with the use of a stiffer spring (see Chapter 5 section 5.2.1) the thicker spring wire will do the work.

4.5 Braille Printer Electronics

The Braille printers are built around an H8 16-bit processor from Renesas. This processor

handles the connection to a computer, reformats the file to a printable form, runs the

stepper motor to feed paper, moves the printer head and determines when it is time for

the hammers to strike. When a file comes into the printer the processor formats the data

28 Chapter 4: Hammer Optimization

Figure 4.2: Abrade of piston at spring seat after 1.5 million cycles.

to printable form and fills a FIFO

with control sequences to the stepper motors and hammers. The data in the FIFO is clocked out at a rate of 10000Hz but the frequency to the stepper motor that moves the printer head, is the frequency often referred to as it determines when the hammers are to strike. In most of the models the stepper motor which moves the printer head is run by a frequency of 1300Hz but in their latest model, 4-waves, the frequency is 1600Hz. A increased FIFO clock rate results in faster printing as the stepper motor moves faster. The FIFO output which controls the hammers are connected to a programmable current limited output stage which is controlled by the processor. The time that the hammer is on can be controlled with a resolution of 0.1ms.

4.5.1 Hammer Striking Sequence

One of the problems that occurs occasionally with the Braille printer is that some of the dots gets sligthly blurred. Sometimes it’s almost every second dot. The cause of this could be a too weak power supply. The power supply is constructed to handle two hammers striking at the same time. If the FIFO clock rate is increased for faster printouts

2

First in first out memory

4.5. Braille Printer Electronics 29 the number of hammers striking at the same time will increase. A program was written by Nils Huhta

to save data from the FIFO without exciting the hammers. To analyse the FIFO data I wrote a Matlab program to find the number of hammers active at the same time. Results are presented in chapter 5

4.5.2 Reduction of Temperature in Printer Head

Though the temperature is reduced by the polishing of the striking pin it can be further reduced by use of some electronics. The first alternative is to use zener diodes instead of schottky diodes as ”free wheeling” diodes but this require some research to find a fast diode. The other alternative is to insert series resistors with the schottky diodes.

The hammer is energized with a current from the MOSFET-transistor (see figure 4.3).

The current is limited by a PWM

-controller which uses the current sensing resistor for measuring the current. When the MOSFET-transistor turns off, the coil drives a current through the diode to deenergize itself. This current also flows through the coil with resistance R

. The current through this resistance induces heat. If there where no diode in the circuit the voltage at the drain V

of the MOSFET-transistor would become V

= V

+ v

where

v

= L di

dt (4.2)

and

I

= V

R

(4.3)

solving the first degree differential equation

i(t) = I

e

(4.4)

as R = ∞ (no resistor or diode connected) and i(t) = I

at t = 0

the immediate voltage becomes

v

= I

R + V

= ∞ (4.5)

The MOSFET-transistor will not survive this voltage. By attaching a diode over the inductor this voltage is reduced to V

+V

(V

=diode forward voltage) but a high current flows through the inductor and generates heat. To reduce the current in the deenergizing- state, a series resistor is connected to the diode as in the right view of figure 4.3. Thus the voltage at the drain becomes:

v

= V

+ V

+ (R + R

)I

e

(4.6) If the resistors R1 and R2 in figure 4.3 are chosen such that R1//R2 = R

then half of the produced heat in the deenergizing-state would end up in the resistors. This is of course

3

Programmer at Index Braille AB

4

PWM=pulse width modulation.

30 Chapter 4: Hammer Optimization only valid if the diode resistance r

= 0Ω and the diode forward voltage V

= 0V. Another influence on the values of R1 and R2 is wire resistance. Since the coil in the hammer has a rather low resistance of 2Ω the wires have to be included in the calculations. Because of the wire resistance the real values of R1 and R2 has to be lower than calculated to remove half of the heat in the deenergizing-state of the coil.

Figure 4.3: Schematic of drive-stage and modification of drive-stage.

C HAPTER 5 Results

5.1 Measuring System

5.1.1 Static Measuring System

The static measuring system based on Microbit 2.0 AB dynamic system was very useful when evaluating changes in the hammers. By analyzing the force curves an optimum piston position could be determined.

5.1.2 Dynamic Measuring System

The dynamic measuring system based on Microbit 2.0 AB;s dynamic system together with the laser sensor M7L-20 MEL Mikroelektronik GmbH gives an almost perfect view of how the hammers performs. The only drawback is that the hammer strikes at full length and not on a sheet of paper at the correct 2mm distance. The actual impact energy could be determined by a Matlab program.

5.1.3 Visualizing Rapid Movements

The delay box together with a stroboscope and a oscilloscope gives a good view of the hammers movement. If the stroboscope is of good quality, it is easy to measure the time when the striking pin hits the paper or how big the bounce is when the piston returns to it’s initial position. The stroboscope which I borrowed from Lule˚ a tekniska universitet was a good one

, but Index Brailles AB’s own rebuilt stroboscope

was not, because it was not fast enough. When triggered, it did not flash at once due to internal delays in

1

Strobotac 1531-AB

2

Lutron DT-2239A

31

32 Chapter 5: Results the construction. These delays where not constant so no exact measurement could be done with this. But still, one can see bounces and other odd behaviours.

5.2 Hammer Optimization

5.2.1 The Return Spring

The return spring forces the piston to its initial position. The stronger the spring the faster and more stable return. Stability in return is related to friction against the walls.

A strong spring makes the friction neglectable but the drawback is lower impact velocity into the paper. The spring constants K tested were 85N/m (which is the original), K=127N/m and K=170N/m. With an oscilloscope attached to the laser and the return spring constant set to 170 N/m the readout was almost frozen. With K=85N/m the curves on the oscilloscope was fluttering. With the spring constant set to 127N/m it was quite stable and the return time was about 6ms.

5.2.2 The Shock Absorbers

Since Index Braille AB started to make their own hammers EPDM 70 shore has been used as the shock absorber material. This gives a high bounce when the piston returns to it’s initial position as seen in figure 5.1 The high bounces come from hammers with EPDM rubber 60 and 70 shore

and the very low ones from a rubber from Impact Devices Inc (IDI). The drawback of the rubber from IDI is that it bounces even more than EPDM if the temperature increses above 30

C. After printing 60 sheets of text the temperature inside the printer head is about 45

C at an ambient temperature of 22

C. Other rubber materials tested were:

Natural rubber 40 shore.

Chloroprene rubber 60 shore.

Silicon 40 shore.

Fluor Silicon rubber 50 shore.

Fluor rubber 60 shore.

Viton rubber 70 shore.

Butyl rubber 60 shore.

Sylodamp HD100.

Sylodamp HD300.

Several plastic polymers has also been tested but they are only slightly better than EPDM at the best. Not all materials are tested by laser, but has been observed under stroboscope light. The best material tested for bounce reduction is Sylodamp HD300 and Fluor rubber when combined with a stainless steel washer. I could not find any

3

Shore is a unit for rubber hardness. The higher value the harder rubber.

5.2. Hammer Optimization 33 difference between them in stroboscope light. When a solid polyamide washer is used the butyl rubber and Sylodamp HD100 was about equal.

Figure 5.1: Displacements curves form laser measurements. RF70-Stainless steel washer with EPDM 70 shore, RF60-EPDM 60 shore, RFUSA-Rubber from IDI.

5.2.3 The Washer

To see what influence the mass of the stainless steel washer has on bounce reduction a aluminium washer was produced. The result can be seen in figure 5.2 where the darker plot shows the movement for the stainless steel washer and the lighter plot shows the aluminium washer. One can see that the bounces are slightly higher when the mass is reduced. Insertion of different polyamide washers between the stainless steel washer and the piston were made to reduce abrade of the piston.

The first test was done with a bit of plastic glued into a cavity of the stainless steel

washer as shown in figure 5.3. This combined with a shock absorber made of Sylodamp

HD300 was studied in stroboscope light and showed very good result. The plastic was

polyamide with small particles of glass. In order to avoid the glue, a polyamide washer

was pressed into the cavity of the stainless steel washer without glue, but the result was

34 Chapter 5: Results

Figure 5.2: Displacements curves form laser measurements. 0505vgUSA-Stainless steel washer with rubber from IDI, 0505sgUSA-Aluminium washer with rubber from IDI.

not good. The piston bounced very much, the cause of this is probably that the strain in the polyamide washer pushed the polyamide washer up from the bottom of the cavity. As a result of this a flat stainless steel washer combined with a loose polyamide washer was tested but the result was variable. About every third bounce was very high, probably depending on the position of the loosely fitted polyamide washer. Next try was to remove the stainless steel washer and replace it with a thick polyamide washer. The results was not as good as the combination of stainless steel washer and a polyamide piece glued into the cavity, but it was much better than the other tests.

5.2.4 Reducing the Back Bushing Thickness

To investigate how much influence the back bushing thickness has on the force, a Femlab

simulation was done. The influence is not very big as seen in figure 5.4 unless it becomes

very thin and thereby harder to manufacture.

5.2. Hammer Optimization 35

Figure 5.3: Washers: 1-Polyamide applied with glue, 2-Polyamide washer pressed into the cavity, 3-Loose fit polyamide washer, 4-Thick acetal washer

5.2.5 The Piston

The hammer strikes the paper at a distance of about 2mm. By looking at the static force-curve in figure 5.5 on hammer 0505 one can see that if the piston is moved forward about 1mm, the force curve would be used more efficiently. The 0505v hammer in the same figure 5.5 shows a 0505 hammer with the initial position moved forward 1mm. The more efficient use of the force-curve can also be observed as a velocity increase in figure 5.6.

5.2.6 Spring Seat Modification

When the spring seat in the piston was made shorter the static force increased as seen

in figure 5.5 where 0505 is the reference hammer and 0505vk is the hammer with shorter

spring seat. The shortening of the spring seat was made by turning the top of the piston

so the piston in 0505vk is 0.4mm shorter than in 0505v. The differences between old

and new spring seat can be seen in figure 5.7. In the static measurement (figure 5.8)

the increase of force is seen on hammers 30A and 30B. The hammers 30A and 30B are

manufactured on a manual turn so they are not completely alike. The simulated force

curve follows the measured curve quit well, the peak force is slightly off due to differences

in the drawing and the real hammer.

36 Chapter 5: Results

Figure 5.4: These curves shows the results from a Femlab simulation where different thickness of the back bushing on a 0505 hammer are modeled. The back bushin is 1.27mm on a original 0505 hammer. One can see that the back bushing has to be rather thin to have influence on the force-curve.

5.2.7 Abrade of Piston

By using a polyamide washer between the piston and the stainless steel washer abrade is reduced more than 40 times as seen in picture 5.9. No abrade is seen on the piston after 4 million cycles. The abrade of the piston at the spring seat is reduced as seen in figure 4.2 by a more snug fit of the spring in the seat.

5.3 The Printer Head

5.3.1 Anvil Rubber

While observing the hammers in stroboscope light I saw that the anvil was forced deep into the anvil rubber, but I took no notice of it. During a conversation with Per Burman

he said that he has been thinking of using a harder anvil rubber to see if it would result

4

My supervisor at Index Braille AB

5.3. The Printer Head 37

Figure 5.5: Static force curves: AM-Hammer from Impact Devices Inc., 0505-Index reference with polished striking pin, 0505R-Spring 170N/m, 0505v-Piston moved forward 1mm, 0505vr- Same as 0505v but with washer in magnetic material, 0505vk-Shorter piston with shorter spring seat and piston moved forward 1mm. The velocity calculations are based on the static forces and shows the impact speed at a distance of 2mm.

in better dots. Immediately I understod what I had seen in my tests. The original anvil rubber in the Everest model was made of natural rubber with a hardness of 40 shore, which is quite a soft rubber. A print out test with the anvil rubber changed to 70 shore was promising. A longer test was made with EPDM 80 shore and this time one could see that the lenght of the print out made almost no difference to the dot quality. Only slight difference could be seen with a trained eye between the first and the 60:th page.

The difference was also very slight between natural rubber shore 70 and EPDM 80 shore,

only on large printouts of 60 or more pages the trained eye preferred the EPDM 80 shore

over the natural rubber. The EPDM rubber was chosen because its properties does not

change so much over the temperature range of 20

C to 60

C. The difference between a

printout with natural rubber 40 shore behind the anvils and the 80 shore EPDM can be

seen in figure 5.10. In order to simulate the first 30 pages of printing I warmed the printer

head in a owen to about 60

C. After remounting the printer head the temperature had

dropped to about 50

C which is a little higher than if the printer had printed 30 sheets

of paper. This saved some paper while testing.

38 Chapter 5: Results

Figure 5.6: Velocity curves: 0505-Normal hammer, 0505v-Piston moved forward 1mm.

5.4 The Printer Electronics

5.4.1 Hammer Striking Sequence

The power supply of the Braille printer is designed to allow two hammers striking at the

same time. If more than two hammers are energized at the same time the striking force

will be less than expected. The analysis of the striking sequence can be seen in figure

5.11. The bottom curve shows that the mean number of active hammers are 1.9. As the

number of active hammers increases with increased FIFO frequency further increase of

printing speed is not possible.

5.4. The Printer Electronics 39

Figure 5.7: Foreground: Spring seat of a 0505 hammer. Background: Modified spring seat.

40 Chapter 5: Results

Figure 5.8: AM-Hammer from IDI, 0505-Hammer from Index Braille AB, 30A-Hammer with shorter spring seat and moved forward piston, 30B-Hammer like 30A, SIM-Simulated force curve.

a) b)

Figure 5.9: a) Abrade in piston after 100.000 cycles without modification. b) After 4.000.000

cycles with polyamide washer.

5.4. The Printer Electronics 41

a) b)

Figure 5.10: a) Dots after 30 sheets print out with 40 shore natural rubber behind the anvils.

b) with EPDM 80 shore.

Figure 5.11: Striking sequence of hammers at a printer head frequency of 1600Hz. The upper

curves shows the hammers striking in a test print out. The lower curves shows how many

hammer striking at the same time.The mean is 1.9.

C HAPTER 6 Summary and Conclutions

6.1 Conclusions

Most of the objectives stated in chapter 1 are achieved with good results and a lot of new ideas for making the printers even better are explored. The variations of the printing quality was mainly dependent on the anvil rubber in the printing head and not on the hammers.

The abrade of the piston, were small pieces of metall falls of, is completely absent with the use of a plastic washer insted of the one made in stainless steel. The drawback is slightly higher bounces when the piston returns. The abrade of the piston at the spring seat is reduced with the more tight fit of the stronger spring. The stronger spring makes the strikes of the hammers more repetable and an increase of the printing speed is possible. To reduce the bonces when the piston returns the EPDM rubber is changed to butyl rubber. A further reduction of the bounces is possible but that requires that the plastic washer is changed back to one in stainless steel with a fiber glas reinforced washer moulded onto it and a shock absorber made in Sylodamp HD300. This makes the manufacturing more expensive.

In order to increase the impact energy the pistons initial position is moved forward 1mm to make it optimized at a striking distance of 2mm instead of 3mm. The reconstructed spring seat also made an increase of impact energy.

Optimizing the hammers reduces the temperature in the printer head and this reduces variations in the properties of the anvil rubber and thereby making a more even quality of the printout.

The simulations in Femlab showed more force than real measurements but I did not include friction in the Femlab model and the actual manufactured hammer can differ slightly from the original drawings. I realised that the models in Femlab has to be quite

43

44 Chapter 6: Summary and Conclutions

a) b)

Figure 6.1: These result are not results from the same simulation but are representative to show the difference between sharp corners and round ones. a) In this simulation the corners in the model were sharp. b) Here the corners are rounded or phased off.

correct by means of corners. If one uses sharp corners in Femlab discontinuities are introduced in the results. In the manufactured hammer the corners are rounded. The difference of sharp and round corners can be seen in figure 6.1 allthough the simulations are not the same but one can see that the plot where corners are rounded is more calm.

I found that simulation of complex structures as a hammer are hard to make correct in Femlab as the graphical tools are too simple. The actual calculations and visualisation of the results are nice though. Importing graphical objects from Solid Works

was a quite hard work especially if one want to make small adjustments to the Femlab model.

6.2 Further work

To increase the impact energy a piston with a larger cross sectional area can be made with the existing coil former. This will of course need some rework on the housing.

The modification of the printer PCB

stated in section 4.5.2 (page 29) can not be done at this time because the MOSFET-transistors that are used are not rated to the increased voltage. A change to transistors with higher voltage range must be done. This modification should be done in the next revision of the printer electronics.

The objective of this work is not fully completed. The determination of the maximal printing frequency could not be done as the final design is not manufactured at the time of writing.

1

Solid Works is a CAD program which Index Braille AB uses to make the drawings of the printers.

2

Printed Circuit Board

A PPENDIX A Datasheets for displacement

sensor

45

46

47

M7L/ M7L/ M7L/ M7L/ M7L/ M7L/ M7L/ M7L/ M7L/ M7L/

Sensor

0,5 2 4 10 20 50 100 200 400 37/10

Range [mm] 0,5 2 4 10 20 50 100 200 400 10

Range begin [mm] 23,75 23 22 40 55 95 170 240 480 37

Linearity* ± [mm] 0,001 0,004 0,008 0,02 0,04 0,1 0,2 0,4 0,7 0,02

Resolution* [mm] 0,0002 0,0004 0,001 0,005 0,009 0,03 0,06 0,2 0,6 0,003

Light spot diameter [mm] 0,1 0,2 0,3 0,6 0,9 1,5 1,5 2 4 0,2

Laser protection class 2 2 2 2 2 2 2 2 3R 2

Light source Laser, wavelength 670 nm, red visible

Sampling frequency 54 kHz

Distance output ±10 V (optional 0 ... 10 V / 0 ... 5 V / ± 5 V) RS 232 / 4 ... 20 mA (optional 0 ... 20 mA)

Impedance approx. 0 Ohm (10 mA max.)

Angle error with 30° of inclination (A-axis): approx. 0,5% on white surface Reaction time T 0,1 ... 67 ms, see below "switch setting"

Bandwith F 0,015 ... 10 kHz (-3dB), see below "switch setting"

Temperature drift 0,02% / K of range

Analog outputs

Intensity output 0 ... 10 V

MIN +24 V / E2410 mA when lower than MIN, LED yellow

OK +24 V / 10 mA when higher than MIN and lower than MAX, LED green

MAX +24 V / 10 mA when higher than MAX, LED orange

Switching outputs

Error output +24 V / 10 mA, LED red

Switching hysteresis ca. 0,5% of range

Ambient light 20.000 LUX

Operation time 50.000 h for Laser-Diode

Isolation voltage 200 VDC, 0 V against case

max. Vibration 5 g bis 1 kHz (Sensor head, 20 g optional)

Operation temperature 0° ... +50°C

Storage temperature -20° ... +70°C

Humidity up to 90% RH

Protection Sensor: IP 64, Electronic module: IP 40

Supply +24 VDC / 200 mA (10 ... 30V)

* Measurement on object color white – bandwith 4 kHz

24.05.2005 - 186 - © 2004 MEL Mikroelektronik GmbH - http://www.melsensor.de