Development of microprocessor controlof a model-making machine

D. Ha worth, P. Purssell and M.F. Hessey

Indexing terms: Controllers, Computerised control, manufacturing computer control

Abstract: A model-making machine has been developed as part of a project on computer-aided design andmanufacture. In order to accurately machine the complex three-dimensional surfaces which are designed bythese methods the machine has been provided with a microprocessor-based control system. This accepts datain a compact form on paper tape and controls the operation of three stepping motors which drive themachine axes. The microprocessor provides linear interpolation and other control functions. Few problemswere experienced during development: the most recalcitrant being electrical noise. The microprocessor hasproved a cheap, reliable and easily developed control system for the machine.

1 Introduction

At the Lucas Group Research Centre a project is beingcarried out on computer-aided design and manufacture.One of the main subjects being investigated is the design ofcomplex three-dimensional surfaces, such as those ofvehicle lamps and aircraft components (References 1—4).In this project extensive use is being made, at the designstage, of visualisation aids such as computer graphics andgraph plotting. While these are ideal at the early stages ofdesign it is often desirable, when dealing with complex sur-faces, to be able to see and handle the surface at the laterstages of design — that is, to have a model. With this in

mind it was decided to carry out a design study on a systemfor the production of such a model. It was realised that asystem of this kind would have a very practical use andcould be of real benefit when applied in industry, as it isoften necessary to produce models as a step in the pro-duction process. The machine (Fig. 1) is essentially alight-weight numerically controlled three-axis milling machineintended for machining soft materials such as polyurethanefoam. A typical model produced using the machine isshown in Fig. 2.

The three axes of the machine are driven by steppingmotors, the control of which is performed by means of amicroprocessor. This was required to accept data punchedin a compact format on paper tape by the computer-aideddesign system and generate from this the necessary incre-mental movements of the three stepping motors.

Fig. 2 Model produced by the machine

\

Fig. 1 Model-making machine

Paper T205 C first received 5th September 1977 and in revised form18th April 1978The authors are with the Logic Systems Division, Joseph Lucas Ltd.,PO Box 34, Highlands Road, Shirley, Solihull, West Midlands, _. _ . , . , .England F|9- 3 Microprocessor control unit

COMPUTERS AND DIGITAL TECHNIQUES, A UGUST1978, Vol. 1, No. 3 IS

0140-1335/78/205C-0075 $1-50/0

The attraction of a microprocessor control lay in itssimplicity, ease of development and low cost. The device isalso well suited to carrying out linear interpolation andcould also be used for more sophisticated interpolationfunctions, although this has not yet been found necessary.The microprocessor control unit is shown in Fig. 3.

2 The model-making machine

The model-making machine (Fig. 1), has an overall size ofl-6mx l-2mx 2m, and is capable of machining a size0-6 m x 0-45 m x 0-3 m with a resolution of 12-5 jum. It iscompletely enclosed by sheet metal and Perspex coversin order that dust produced when machining soft materialssuch as wood and polyurethane foam does not escape intothe general workshop area.

A simple extraction system is fitted to the base of themachine. While this is not sufficiently powerful to removeall dust during machining, it does prevent dust leakage andserves to encourage the dust to settle at the base of themachine thus reducing the possibility of dust being releasedwhen the Perspex machine cover is raised.

The wooden table to which the workpiece is attached isfitted to the Y axis motion. Each axis slides on twin parallelbars, and is moved under the action of a lead screw. Limitswitches prevent excessive travel on each axis. All threeaxes are driven by stepper motors attached to the leadscrews. The machine operates in an open-loop control

external, reset

STSTB 822Ua

•hold req.

sync 02 01 resetRDY DB1N b int.

WR 00-07/ HLDA A0-A15

118228 2x8212

TT.

data highway

mode, that is the controller outputs to the stepper motorsand no check is made that the requested motions have infact been performed. As a check against errors a counterhas been fitted to each axis which gives a visible indicationof position. The cutter motor and cutter may be rotatedmanually about two pivot points during setting-up in orderto provide clearance for the cutter motor and workpieceduring machining.

3 Development of controller hardware

When initially built the control system for the model-making machine was based around a minicomputer. It wasrealised that for a task of this sort a minicomputer wasvastly underutilised, so development work in the field ofmicroprocessors being carried out within the ResearchCentre led to the idea of replacing the minicomputer with amicroprocessor. This idea was pursued with the result thatthe control system is now based around an 8080 micro-processor board.

This board was not designed specifically to drive themodel-making machine, but instead was developed as ageneral purpose board which would be capable of ex-pansion to suit the possible future needs of other systems.In the case of the model making machine, the basic require-ments of the microprocessor board were:

(a) The ability to read in data from a high-speed paper-tape reader and check that data for errors.

(b) To perform linear interpolation on the data.(c) To output the appropriate series of pulses to the

stepper-motor drive units.

emergency"stop

2x8702A

control highway | |

nmaddress highway

8212

P.t.r.

Linnr

P.t.r.command

Fig. 4 Block diagram of controller hardware

a Clock generator and driverb 8080 Ac.p.u.c System controllerd Address buffers/decoder

e E.P.R.O.M.s/ R.A.M.sg Input/output peripheral interface

76 COMPUTERS AND DIGITAL TECHNIQUES, AUGUST 1978, Vol. l,No. 3

To achieve the requirements, a fairly minimal systemconfiguration was found to be sufficient consisting of only256 words of random access memory (r.a.m.) containingthe stack and intermediate data storage, 512 words oferasable programmable read-only memory (e.p.r.o.m.),containing the control program, one eight-bit input port,one eight-bit output port, a single input bit for a deviceflag, and an interrupt facility. A block diagram of thissystem is shown in Fig. 4.

The eight-bit input port is used by the high-speed paper-tape reader to take in the data to be processed. Output tothe model-making machine is by way of the first six bitsof the output port. One of the two remaining bits of thisport is used as a command for the paper-tape reader.

A hand-held stop button is connected to the interruptrequires line on the 8080 central-processing unit. Thisallows the machine to be stopped without the loss of data.A system reset pushbutton is also provided to initialisethe control program.

The pulses output from the microprocessor to drive thestepper motors on the model machine are 5 V t.t.l. com-patible. These are converted by external circuitry to becompatible with the drive units which were supplied withthe stepping motors.

The system is capable of expansion on its single boardby the addition of extra memory or input/output whichwould allow for the implementation of further facilitiessuch as feed-rate control and the ability to perform circularinterpolation.

The main problems encountered during development ofthe system were found to result from electrical noise inthe power supplies. These were overcome by attachingcapacitors across the supply rails adjacent to each inte-grated circuit.

Table 1: Tape format

Character

1234567

8

Control characterX — Low orderX — High orderY — Low orderY — High orderZ — Low orderZ — High order

Checksum

All characters need not be present in each recordCon trol characterBit

01234567

Definition

X low order byte presentX high order byte presentY low order byte presentY high order byte presentZ low order byte presentZ high order byte presentEnd of tapeNot used

4 Development of the microprocessor program

The program was originally written in Fortran to run on aminicomputer. This provided a useful background to thewriting of the program for the microprocessor, since theIntel 8080 assembler was unfamiliar to those concernedwith the controller development at the time.

As the program was translated from Fortran to anassembler language it ran faster than the original despite be-ing in a slower processor. However, certain facilities thathad been available in the Fortran program for testing thesystem had to be sacrificed. In order for the program tobe able to drive the machine at a high speed it had to bewritten so that it did not include any multiplication ordivision. The program uses 16-bit addition for all its cal-culations. These calculations are used to produce a linearinterpolation from the current position to the given offset,which is specified on the data tape. The program calculatesthe order in which to output pulses to the three axes toobtain the closest possible approximation to a straight line,using discrete steps. The input is from 8-hole paper tape inthe format given in Table 1. The format of this tape wasbrought about by the way in which the computer-aideddesign programs output their data although there is nopractical reason why a standard tape format could not beused. The X, Y, Z values are 2 s-complement offsets relativeto the current position of the machine.

Initially, the program was written and tested using anIntel m.d.s. (microprocessor development system), themonitor of which included the routines required to drivethe reader. Use was also made of the m.d.s. monitor todebug the program and modify various parameters so thatthe system would perform satisfactorily.

It would be quite possible to connect the microprocessorcontroller directly to the computer which generates thepaper tape. The controller would appear to the computer asa punch or teletype, and hence both tape reader and punchwould be eliminated without requiring changes to either thecomputer or microprocessor software. This has not yetbeen attempted since in practice it has been preferred tokeep the operation of this machine separate from thecomputer.

5 Reasons for the use of a microprocessor and its choice

At the time that the model-making machine was beingdesigned with stepper motor drive it was considered that acomputer would offer a convenient and straightforwardmeans of controlling the axis movement. However, theintention was that the machine should be reproduced foruse both elsewhere in the company and for sale to externalcustomers. A microprocessor was therefore an extremelyattractive proposition since it offered an order-of-magnitudereduction in the cost of the controller, resulting in a moreattractive final price for the product.

When the microprocessor was originally being selectedthere was far less choice than there is now. The decidingfactors then were the ready availability of the Intel 8080and most important, the strong support availability forthis device.

An interesting point is that the new Intel 8748 hassufficient capacity to replace the board completely for thisapplication. Unfortunately, it became available two yearstoo late for this project.

6 Conclusions

The microprocessor has proved to offer a cheap, reliableand easily developed control system for the model-makingmachine. Three-axis linear interpolation was easily imple-mented, and although parabolic, circular and other forms ofinterpolation have not at present been implemented, theyshould present little difficulty if required. No serious prob-lems were encountered in developing the microprocessor

COMPUTERS AND DIGITAL TECHNIQUES, AUGUST 1978, Vol. l,No. 3 77

controller, although some time was spent in eliminatingelectrical noise.

6 References

1 HESSEY, M.F., and DEANE, J.N.S.: "The computer aided designand manufacture of vehicle lamps', Proceedings of the 17thmachine tool design and research conference, 1976, pp. 11-17

2 DEANE, J.N.S., and HESSEY, M.F.: 'Interactive design and

three dimensional modelling applied to vehicle lamp manu-facture', Society of Manufacturing Engineers, Technical PapersMS 76-737,1976SNEAD, J.C., HESSEY, M.F., and DEANE, J.N.S.: 'The Lucassystem for complex three dimensional surfaces', Paper presentedat the interactive designs systems conference, Stratford-on-Avon,13-15 April, 1977ASHDOWN, J., and COX, M.D.: 'Application of the LUCAD/LUCAM', Paper presented at the interactive designs systems con-ference, Stratford-on-Avon, 13-15 April, 1977

Errata

VOGEL, A.: 'Multiple bridging faults in monotone net-works', IEE J. Comput & Digital Techniques, 1978,1, (2),pp. 49-52

The following errors occurred in the printed text:

The third line of Definition 1 should read (i) %DC

The first line of Definition 5 should read as follows:

LetSeDll and#(S)c)R forKE{6, U}

The fifth line of the proof of theorem 5 should start

S" = d(u,v)S'

The fourth line of theorem 7 should read

\U\

n + l

kwhere k —

n + l

78 COMPUTERS AND DIGITAL TECHNIQUES, AUGUST 1978, Vol. 1, No. 3