pcb drilling machine 2

5/24/2018 PCB Drilling Machine 2

1/7

GENERALINTEREST

60 Elektor Electronics 4/2001

Part 2: T he controller

Design by T. Mller (Radix GmbH) www.radixgmbh.de

Our PCB drilling machine provides a link between a Windows PC and themotors and solenoids of the machine itself: a microcontroller integrated

with a high current drive circuit.

PCB

Drilling Machine (2)


2/7

operating system, has other things to do

w hile generating our output w aveform.

Updat ing the mouse position, checking for

incoming e-mail, reading audio data from a

CD, communicat ing w ith peripherals : a l l

these ta ke processor time.Window s is event-driven, w hich means

that the various parts of the machine can

caus e the operat ing syst em to carry out oper-

at ions. This can b e seen w hen printing, for

example: Window s launches a process t o

communicate w ith the printer, and s upplies

i t w i th new da ta as and w hen it i s ready to

accept it. When that ha ppens and how much

dat a is transferred at a t ime is tota lly unde-

fined, but nevertheless consumes processor

time. This ca n interfere w ith the timing of our

sq uare wa ve and ca use var ia t ions in i ts

period.There are tw o w ay s to overcome this prob-

lem. We could shut d ow n all other process es

and disable multitasking w hile w e generate

our sq uare w ave. Then w e might as w ell use

DOS, w hich is w hat most manufacturers of

this type of control softw are for small

ma chines do. Under DOS, an app lica tion pro-

gram can use the full pow er of the processor

for itself. Of course, w e sa crifice the ea se of

use of Window s, a nd, in spite of i ts pow er,

the computer cannot be used for anything

else.

There a re various operating s ystem ad d-ons a vailable for Window s w hich ameliorate

this situation and provide quasi-real-time

fac ilities . Thes e a re most ly VXD or TSR pro-

In the previous instalment (March

2001) w e des cribed in det ail the

ba sic principles of t he CNC d rilling

machine. The controller is just as

unconventional: the machine is not

driven directly from a PC, but indi-

rectly via a specia l controller circuit.For trouble-free communication

betw een the Window s PC a nd the

controller over the Centronics inter-

face w e need to make use of several

tr icks tha t ta ke us beyond the s tan-

dard Centronics protocol.

Lets start b y ta king a look at t he

rather straightforw ard hardw are of

the controller boa rd. The b as ic circuit

consists on one side of a microcon-

troller w ith a PC interface and on the

other the output d rive circuits, con-

nect ed to I/O-port pins. Of coursethere is rather more to the design

than tha t .

Real-time with Windows?

How ca n w e control a st epper motor

from a PC? I t goes w ithout sa ying

tha t w e need an output d rive circuit

capable of supplying the current

required by the motor, since none of

the PCs interfaces ca n drive a motor

directly. And then, in order to make

the motor step smoothly, w e needvery accurate timing.

At first blush it w ould be ideal if

w e could somehow c onnect the out-

put s tages d irect ly to the PC and

han dle practica lly a ll the control sig-

nals d irectly a s b its from the PC. The

circuit w ould t hen be very simple,

consist ing of just a couple of latch es

and an address decoder.

Unfortunately, Window s ha s a

rather unpleasa nt cha racteristic: its

operations a re tot ally as ynchronous.

I t is impossible to say at exactlyw hat t ime an event w ill occur.

Approximate timing is possible, but

as t he specified time betw een

events g ets s maller and more pre-

cise, something eventually w ill go

w rong. Windows on its ow n w as not

designed for control applications,

and indeed is not suitable for them.

I t may b e easy to use, but from the

hardw are point of view it is no more

suitable than older operating sys-

tems such a s DOS.

First of all, modern programminglanguages offer hardly any com-

ma nds for acc ess t o the I/O-ports ;

and second, multita sking prevents

direct hardw are access by programs

that a ttempt to bypass t he operating

sys tem. Special drivers a re req uired,

such as por t . dl l found in the Elek-

t or Elect ronicsbook PC Ports under

Window s (to be published soon).Let us suppose that you have

ava i lab le a programming language

that lets you control the port signa ls

at w ill . We w ish to generate a con-

tinuous sq uare w ave on one of the

da ta pins of the Centronics port at a

given freq uency. In softw are this is a

trivial matter: w e simply need to tog -

gle the bit in question regularly. In

order to obtain the desired fre-

quency, the appropriate delay must

be used. I f the delay is reasona bly

large, say 0.5 s, w e obtain a perfectoutput s ignal w ith a freq uency of

1 Hz.

If, how ever, w e reduce the delay

to sa y 0.5 ms and hence raise the fre-

quency to 1 kHz, we hi t a problem.

The right freq uency app ears a t the

output, but only w hen averaged over

a long period. The individua l pulse

lengths are a little longer or shorter

in a random pattern, and some are

very different from the specified

period. Why is this?

Of course, mod ern processors a refast enough to toggle a bit 2,000

times per second . No, the reason is

tha t Window s, as a multi tas king

GENERALIN TEREST

614/2001 Elektor Electronics

A un iver sal con t r o l l e r boar d

The contro ller board, w ith its pow er output stages and integrated microcontroller, has

been designed not just for use with the PCB drilling machine, but also with enough flexibil-

ity to be used for many other purposes. The driver stages make it a very handy controller

board for other equipment or machines that employ stepper motors and other high cur-rent devices. For this reason there are a few places in the layout w here we have made

additions that are not strictly necessary for the drilling machine application.

To t ake one example, the output stages are controlled by a GAL. The GAL is simply pro-

grammed to combine the PWM signal w ith a tw o-bit address to activate the solenoid drive,

and (with an enable input) the drill motors. This function could easily have been carried out

using a simple address decoder such as a 74138. However, by using a GAL we can com-

pletely reconfigure the eight output stages. To this end, you will see that we have routed

extra signals from the microcontroller to the GAL, in particular signals that can be assigned

special functions in the microcontroller.

Series resistors and capacitors can be fit ted a t t he inputs to the driver stages. The pow er

MOSFETs used in the drilling machine contro ller do not require these; but if you w ish to

use bipolar transistors in the drive stages, the spaces we have left on the board will come in

handy.


3/7

grams that are loaded w hen Window s sta rts

up and then run in the b ackground. These

programs run briefly, but at an exact moment

in time; and they run at the highest priority

to guarant ee having the necessary processor

pow er ava ilab le to them.

These progra ms a re rat her critica l and con-siderably increase the cha nces of Window s

crashing. They a lso permanently consume

memory and processor time, because t hey are

alw ays present and must alw ays be ready to

spring into a ction w hen req uired.

These programs a re also often w ritten

using undocument ed features or ba ck doors

in the operat ing syst em, and so a re usually

only fully compa tible w ith one part icular ver-

sion of the sys tem.

Offloading the synchronisation

problemWe have therefore chosen an alternative

approach to d riving a s tepper motor under

Window s. Synchronisat ion is not left up to

Window s softw are, but rather trans ferred to

a microcont roller. The microcont roller receives

a control command (for example for a mot or

pulse), a nd rat her than executing it immedi-

at ely, w aits for further timing information to

indicate w hen the data are to be processed.

In this w ay, as long a s the overall system is

fast enough, w e can obta in perfectly syn-

chronised results using Window s.The information pa sse d to t he microcon-

troller to generat e a sq uare w ave runs as fol-

low s: bit high in 500 s, bit low in 500 s,

Repeat. This information is sufficient for the

microcontroller to produce the signal inde-

pendent ly, requiring no further intervention

from the PC . The PC mus t deliver the da ta

fast enough, each command arriving w ithin

500 s. This s hould not present an y d ifficulty

for a reasona bly fas t PC.

To prevent the PC ha ving to w ait until the

microcontroller is ready for a new comma nd,

the controller is eq uipped w ith a small FIFOmemory w ith spa ce for 40 entries. This a llow s

the cont roller to be w ell decoupled from the

PC. The PC can send off 40 precalculated

commands for sw itching the va rious output

signals to the controller and then has

40500 s to prepare new da ta or carry out

other tasks.

Shortly before the FIFO empties, this s ta te

is flag ged t o the PC, w hich can then refill the

FIFO w ith new da ta . This does not affect the

controller, w hich continues executing com -

mands one after another w ith perfect timing.

Using the Centronics interface

Communication w ith the controller could b e

carried out over any PC interface.

The Centronics and V24 (serial)

interfaces a re alw ay s ava ilab le, and

are particularly suita ble candidat es.

There are certa in problems a ss o-

ciated w ith the V24 interface: get-

ting the w iring right and a rrang inghand sha king can be full of pitfa l ls .

Matters a re also more complicat ed

for the microcontroller, w hich must

first convert the serial bit stream into

parallel bytes. And, if the microcon-

troller does not cont a in a UART, th e

controller must detect and interpret

the st art bits its elf. This consumes

processor time in the controller and

ultimately creates difficulties in

ensuring timely execution of com-

mands .

For these reas ons, w e have cho-sen the Centronics interface. This

interface is also superior from the

point of view of speed, because

eight bits a re transferred at a t ime:

this a lso makes the da ta easy to

process. The da ta are simply w ritten

to a specific I/O add ress a nd t he job

is done. It is also convenient for the

microcontroller to receive t he da ta in

a simple 8-bit format .

Eight bits may be a convenient

number, but it is too few to specify a

command w ith timing informat ion.Multiple bytes must be a ssembled

together to c onstruct a useful com-

mand so that the controller can

know w hat to do w i th the da ta . For

our application, tw o bytes a re

enough for each command, but that

is too much to t ransfer in one g o over

the Centronics interface. But w hen

the 16 bits a re divided into tw o inde-

pendent bytes , a synchronisation

problem can arise.

The strobe-edge trickHow can t he controller know w hich

of the byt es is the firs t a nd w hich

the second? Counting is one option,

but not a very reliable one: a sing le

transmission error and all successive

commands w ill be w rongly inter-

preted. The error must be d etect ed

and the controller reset. Alterna-

tively, the dat a bytes could include

further information to allow the con-

troller to identify w hich is w hich.

However, this also has i ts disad -vant ag es. The ma rker bits na turally

reduce the amount of real informa-

tion trans ferred, and so w e have

few er bits a vailable for commands .

For this reason w e distinguish the

bytes externally using a means

already provided w ithin the Cen-

tronics interface. Take a look at the

protocol as show n in Figure1. First

consider normal operation,described at the top of the f igure.

The interfac e, how ever, w ill let us

transfer two by tes at a t ime w ithout

modifica tion. How t hat is achieved is

described at t he bottom of the fig ure.

The hand sha king process is

unchanged : w e have s imply

chang ed the meaning of the signals.

Tw o bytes a re coalesced into a s in-

gle packet, w here the falling edg e of

the STROBE signal marks the first

byte, and the ris ing edge, the sec-

ond.If a n error occurs w hen the first

byte is a lready in the process of

being sent , STROBE w ill event ua lly

rise, if only because of the pull-up

resistor fi tted to the cont roller board.

The trans fer is then terminat ed, an d

the pa ir of bytes remains t ogether.

Synchronisat ion problems a re there-

fore impossible, even though the

contents of the tw o bytes w ill be in

error. I f on the other hand w e ha d

simply counted b ytes , then all sub-

seq uent bytes w ould be interpretedw rongly, as t hey w ould a ll be inter-

chang ed. I f w e w ere to continue in

this ma nner, the cont roller w ould

ceas e operating correctly hence the

syst em w ould ha ve to be re-ini-

tialised.

At st art-up the control softw are

sends a RESET command, w hich

causes the controller to clear its

FIFOs a nd b ring to a halt a l l move-

ments in progress in the sys tem.

Controller functionsWe have now described the ma in

functions of the controller: a ccepting

the 16-bit w ide instructions, st oring

them in the FIFO, and synchronously

executing t he commands stored in

the FIFO. Of course, the calculation

of trajectories is not ha ndled by the

microcontroller, since it is not

designed for trigonometric calcula-

tions and w ould be too slow. It

w ould a lso imply programming the

dyna mic characteristics of the sys-tem once a nd for all into the micro-

controller.

Instead, t he movements a re pre-

GENERALINTEREST



4/7

processed in the PC and then sent to

the cont roller in th e form of primitive

instructions such as motor 1 one st ep

count erclockw ise. All movements are

defined in the P C in this w ay. I t is

possible to compute the step infor-

mat ion for any d esired motion andthen send it to the controller : and

thus the microcontroller never needs

to be reprogrammed.

Data format

Now tha t w e have expla ined how

the command da ta are calculated in

the PC a nd tran sferred over the Cen-

tronics interface t o be s ynchronously

processed by the controller, w e can

ta ke a look at to the internal s truc-

ture of the command da ta .The tw o bytes are concat enated

to form a w ord. This w ord contains a

da ta va lue, or operand, a long w ith

an a ddress w hich determines how

the operand is interpreted: in other

w ords, the particular command to be

executed. The number of address

and data bit is variable, the number

of data bits shrinking as more

address bits are required. A com-

mand w ith a short ad dress field can

contain a longer operand , and vice

versa. This kind of addressingscheme can be processed using a

seq uence of branches, decoding the

command using the divide and con-

q uer principle.

This w orks a s follow s: the first

address bit determines w hether w e

are dealing w ith one particular type

of command (w hen the b it is a 1) or

not (w hen it is 0). In the second

case w e sti ll do not know w hat type

of command w e are processing, so

w e examine the second bit. If this is

a 1 , then w e have decoded a sec-ond class of command s, w hile if it is

0, w e proceed to examine t he third

bit, and so on. Of course, as w e con-

tinue to examine bits is this w ay,

there remains less a nd less spa ce for

the operand data .

The advanta ge is tha t w e only

need one address bit for the f irst

command t ype, and so w e have a full

15 bits available for the operand

data . For commands w hose address

field is, say, three bits long w e have

13 bits a vailable. Note tha t a three-bit address field allow s a t most eight

different commands to be separately

decoded.

GENERALIN TEREST


1

1

2

2

3

3

4

4 5

5

6

6 7 8

010024 - 2 - 12

DATA

STROBE

BUSY

1. Quiescent condition:

STROBE from PC to microcontroller is high. BUSY from microcontroller to PC is low.

2. The PC wishes to transfer data as usual, and puts the data byte on the eight data lines.

3. The PC takes STROBE low. The connected microcontroller (or peripheral device) sees

the low state and knows that the data are ready to be read.

4. The microcontroller takes the data byte and sets BUSY high in acknowledgement.

5. The PC takes the STROBE signal high again.

6. When the microcontroller has processed the byte, it indicates this state by taking the

BUSY signal low. This completes the transfer of a byte. The system is ready to transfer

another byte, the control signals (BUSY and STROBE) being once more in their original

states.

1. Quiescent condition:

STROBE from PC to microcontroller is high. BUSY from microontroller to PC is low.

2. The PC wishes to transfer data using the special mode, and puts the data byte on the

eight data lines.

3. The PC takes STROBE low. The microcontroller sees the low state and knows that thedata are ready to be read.

4. The microcontroller now takes the data byte and sets BUSY high in acknowledgement.

5. The microcontroller processes the byte immediately and leaves the BUSY signal high.

6. The PC observes that the BUSY signal is high and therefore that the first byte has been

accepted. It then puts the second byte on the data lines.

7. The PC indicates that the second byte is present by taking STROBE high again. This

causes the microcontroller to accept this byte also.

8. When the microcontroller has finished its processing, it sets BUSY low again.

Figure 1. T he original Centronicsprotocol (above) and asmodified for the drilling machine(below).


5/7

GENERALINTEREST


PIC16C64

OSC2

IC13

OSC1

MCLR

RB0

RB1

RB2

RB3

RB4

RB5

RB6

RB7

RE0

RE1

RE2

RD0

RD1

RD2

RD3

RD4

RD5

RD6

RD7

RA0

RA1

RA2

RA3

RA4

RA5

RC0

RC1

RC2

RC3

RC4

RC5

RC6

RC7

13 14

33

11

12 31

32

34

35

36

37

38

39

40 10

19

20

21

22

27

28

29

30

15

16

17

18

23

24

25

26

1

8

9

2

34

5

6

7

X1

16MHz

C63

15p

C64

15p

+5V

R41

10k C60

100n

+5V

JP1

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

K21

R23

1k

C39

820p

R24

1

!

1W

C40

820p

R25

56k

K15

+5V +30V

C38

100n

C41

100n

R26

1k

C43

820p

R27

1

!

1W

C44

820p

R28

56k

K16

+5V +30V

C42

100n

C45

100n

R17

1k

C31

820p

R18

1

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1W

C32

820p

R19

56k

K13

+5V +30V

C30

100n

C33

100n

R20

1k

C35

820p

R21

1

!

1W

C36

820p

R22

56k

K14

+5V +30V

C34

100n

C37

100n

SEL

R11

1k

C23

820p

R12

1

!

1W

C24

820p

R13

56k

K11

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C22

100n

C25

100n

R14

1k

C27

820p

R15

1

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C28

820p

R16

56k

K12

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C26

100n

C29

100n

R35

1k

C55

820p

R36

1

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1W

C56

820p

R37

56k

K19

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C54

100n

C66

100n

R38

1k

C58

820p

R39

1

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C59

820p

R40

56k

K20

+5V +30V

C57

100n

C67

100n

R29

1k

C47

820p

R30

1

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C48

820p

R31

56k

K17

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C46

100n

C49

100n

R32

1k

C51

820p

R33

1

!

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C52

820p

R34

56k

K18

+5V +30V

C50

100n

C53

100n

SEL STEPA1

PBL3717A

PHASEIC7

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC8

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC5

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC6

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC3

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC4

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC11

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC12

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC9

RSNS

IN0

V+A

CIN

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REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

PBL3717A

PHASEIC10

RSNS

IN0

V+A

CIN

IN1

REF V+B

16

11

12

14

13 10

PT

OB OA

15

9

6

4 5

87

3

2

1

D7

D5

D3

D1

D0

D2

D4

D6

STROBED0

D1

D2

D3

D4

D5

D6

D7

R44

10k

+5V

ERROR

PE

SELECT

STEPA2

STEPB2STEPA3

STEPB3

STEPA5

PL

SELECT

PE

IC2

16V8

GAL

19

20

10

I0

I1

I2

I3

I4

I5

I6

I7

I8

F7

11I9

12F0

13F1

14F2

15F3

16F4

17F5

18F6

1

2

3

6

7

8

4

5

9

+5V

D5

C15

220n

C14

100n

+15V

T5

R7

100!

K7

D1

C7

220n

C6

100n

+15V

T4

R3

100!

K3

D6

C17

220n

C16

100n

+15V

T6

R8

100!

K8

D2

C9

220n

C8

100n

+15V

T3

R4

100!

K4

D7

C19

220n

C18

100n

+30V

T7

R9

100!

K9

D3

C11

220n

C10

100n

+30V

T2

R5

100!

K5

D8

C21

220n

C20

100n

+30V

T8

R10

100!

K10

D4

C13

220n

C12

100n

+30V

T1

R6

100!

K6

ADR1

DRILL

ADR0

PWM

SPARE1

SPARE0

R1

10k

R2

10k

R43

10k

R42

1k

BU4B U1 B U2 B U3

+5V

STEPB5

CLK

STEPB4

STEPA4

STEPB1

STEPA1

SEL

SPARE0

SPARE1

PWM

ADR0

ADR1

DRILL

STROBE

ERROR

SOUT

8x 47k1

2 3 4 5 6 7 8 9

R48

+5V

BO1

BO2

BO3

BO4

BO5

R47

560!

T11 T10 T9

R46

560! R45

560!

+5V

R49

1k

D12

PL

K1

K2

C3

3300 50V

C4

3300 50V

C5

3300 50V

2A T

F2

2A T

F1

C1

100n

C2

100n

7805

IC1

B1

B2

+30V

+5V

+15V

SOUT

PL

CLKC1[LOAD]

74HC165

IC14

SRG8

C2/

10

11

12

13

14

"1

2D

1D

1D

1D

15

7

9

3

4

56

2

1

C65

100n

IC14

16

8

+5V

C62

100n

C61

100n

SPARE

SEL

SEL

SEL SEL

SEL

SEL

SEL

SELSTEPB1

STEPA2

STEPB2

STEPA3

STEPB3

STEPA4

STEPB4

STEPA5

STEPB5

DRILL4

DRILL3

DRILL2

DRILL1

COIL4

COIL3

COIL2

COIL1010024 - 2 - 11

COIL1/DRILL1

COIL2/DRILL2

COIL3/DRILL3

COIL4/DRILL4

TARGETADR0 ADR1

0

1

0

1

0

0

1

1

D1 ... D8 = 1N4001

T1 ... T8 = BUZ11

GBU6B

GBU6B

Figure 2. T he drilling machine controller board: lotsof brawn, and a little bit of brains!

The variable-length address field also

allow s a higher priority to be as signed to

more common or more urgent comma nds. For

a ra rely used command , or one that initiates

a lengthy act ion, we ca n a l low the address

decoding t o ta ke a little longer.

The command s w hich are most commonlyused, and w hich sent to the controller at the

fas tes t ra te , a re w i thout doubt the s t epper

motor commands; they also require a long

operand , w hich is provided by t he highest

priority comma nd format . The format of the

commands for sending st epping pulses to the

motors is s et out in Table1.The second comma nd cla ss is the d rilling

command . Three address b its ca n be used for

this command w ithout ca using any d ifficulty

because its execution time is much

longer than the motor step com-

mands, and because i t requires a

shorter operand . The mea ning of the

va rious bits in the d rilling comma nd

is set out in Table2.

These tw o comman ds s uffice forbasic operation of the PCB drilling

machine. In fact there is a range of

other commands w hich are less rel-

evant t o our discussion and w hich

are not d escribed here.

Brawn and brains

At fi rst glance t he circuit diagram in

Figure 2may look like an impene-

trab ly complex piece of electronics,

but on closer inspection w e see that

the b rains of the circuit are a ctua lly

very s imple. The intelligence is con-

cent rated in a ty pe PIC16C64 micro-

cont roller from Microchip, clocked a t

20 MHz. This microcontroller boa ststhe ma gnificent tota l of 33 individu-

a lly a dd ressa ble I/O-ports , but only

2 K of EPROM progra m memory an d

128 bytes of internal SRAM, here

used for (among other things) the

FIFO memory. The PIC16C64 is

equipped w ith peripherals such as a

real-time clock, timer/coun ter a nd

ca pture/compa re inputs . In this

ap plica tion w e do not use the 3-w ire


6/7

sy nch ronous seria l SPI/I2C bus.

The modified Centronics interface uses

port D (data signals), RC6 (active-low

STROBE), RE1 (SELECT) and RC7 (ERROR).

The BUSY sig na l from t he microcont roller is

not w ired to the sta nda rd connection on pin

11, but to t he PAPER EMP TY signa l (PE) onpin 12. There are s everal reas ons for this: fi rst,

it prevents the PCB drilling machine from

springing into action w hen a print job is

sta rted under Window s, and s econd, it

allow s a printer to operat e in parallel w ith the

ma chine. Further, pulses on t he STROBE a nd

BUSY signals appear to af fect the internal

interrupt processing of Window s: a nd t hat is

best left w ell alone.

R44 is the pull-up resistor referred to above

tha t pulls STROBE high in the event of a sy s-

tem crash, thus deactivat ing the signa l.

The microcontroller determines the config-uration of the drilling mac hine via t he d rilling

head limit sw itches connected to Bo1-Bo4. I fa d rilling arm is fit ted a nd the hea d is in the

up position, the corresponding s w itch is

closed. If an a rm is not equipped w ith a limit

sw itch, the corresponding conta cts remain

open.

OptSpare is an opt ical input tha t, like thesw itch input Spare, is not used in the present

design. OptBase is an optical input used t ocalibrate the reference positions of arm 1,

arm 2 and the rota t ing tab le . OptAdd does

the same job for arms 3 and 4. How exactlythe ca libration is ca rried out w ill be des cribed

in a la ter a rticle in this series. The LED can

be used t o indicat e tha t the ma chine is oper-

at ing: it lights during drilling a nd d uring the

calibrat ion process.

It ma y s eem t hat 33 I/O-ports are plenty,

but unfortunately are not enough for all the

signals just described. We therefore use a

shift register type 74HC165 (IC14) to combine

the bits into a single byte read serially by the

microcontroller over a single port bit RA4

(SOUT). A para llel load p ulse load s th e bits

into an intermediat e register where they a rebuffered b efore being clocked out us ing th e

CLK sig na l (RB2).

The final item of informa tion read by the

controller is the st op sign al from the sole-

noid l imit sw itch. These sw itches are con-

nect ed t o BU1-BU4. Since the current must be

sw itched off quickly, it is not fea sible to read

these signa ls out s lowly via a shif t register.

Instead, the stop signal drives the interrupt

input RB0 low. Since only one d rilling hea d is

act uated at a time, the four limit sw itches can

be c onnected in pa rallel.

The rest of the circuit consists of the fouridentical output drive stag es for the solenoids

(T1, T2, T7 a nd T8), the d rill m ot ors (T3-T6)

and ten identical integrated stepper motor

GENERALIN TEREST


Table 1: M ot or pu lse com m and

MSB Byte1 Byte2 LSB

1 Z Z Z D T T T Z Z Z Z Z Z Z Z

Addressing:bit 7, byte 1 = 1.

Z (11 bits): Time

The time value specifies how long AFTER the execution of the motor pulse com-

mand the controller must w ait before executing the next instruction. If the value

is zero, then the next command (assuming it is also a motor pulse command) is

executed immediately and synchronously w ith the present one.

T (3 bits): TargetThese three bits select the stepper motor drive stage affected by the co mmand.

The order runs from Target 1 (000), the o utput stage driving connectors K11

and K12, through to Target 5 (100), the o utput stage driving connectors K19 and

K20.

Target addresses 6, 7 and 8 (101, 110 and 111) are reserved.

D (1 bit): Direction

This bit specifies the direction in which the motors are to turn. D= 1 specifies

clockwise rot ation, D = 0 counterclockwise.

Table 2: Dr i l l i ng com m and

MSB Byte1 Byte2 LSB

0 T T T 1 1 B B B B B D D D D D

Addressing:bit 7, byte 1 = 0; bits 2 and 3, byte 1 = 11.

T (3 bits): Target

These three bits select the output stage for the drilling command. The machine

can be fit ted w ith up to four arms, and each of these arms has its own drill and a

dedicated driver stage on t he controller board. The targets are numbered from

left to r ight, start ing with Target 1 (000) at connect or K10, where the solenoid

for the first arm is connected. The corresponding drill motor is connected to K8.

Target 4 (011) is driven from connectors K5 (solenoid) and K3 (drill mot or). Tar-gets 5 to 8 (100 to 111) are reserved.

D (5 bits): Drilling force

The force applied to the drill by the solenoid can be controlled by this five-bit

data value. The value sets the amuont of energy delivered to the solenoid, and

can be set to any number in the range 0 to 31.

B (5 bits): Braking control

When the drilling head is withdrawn the solenoid cannot simply be switched off,

since this would cause long-term w ear on the mechanism. To prevent undue

stress on the motor guide spring, the magnetic field in the solenoid is gradually

reduced, so that the drilling head is withdrawn gently.

In principle this value can be set in advance for a particular weight of drilling head.Here, however, we have the possibility of optimising the value. If, for example,

you use a more powerful drill motor or a 3-jaw chuck instead of the collet chuck,

the weight of the drilling head may be significantly different.


7/7

drivers type PBL3717A from ST Microelec-

tronics. The da ta sheet for this IC can read ily

be found on STs w ebsite a t w w w .s t .com.

First let us consider the drill motors and

solenoids. These are not driven directly, but

rather via a programma ble logic device type

GAL 16V8. You ca n read t he reason s for thisin the box A universal control ler board.

Address signals ADR0 (RC3) and ADR1 (RC4)

select from the a t most four motors a nd s ole-

noids in t he order show n. The level on DRILL

(RC5) select s eithe r the mot or (w hen RC5 is

low ) or the s olenoid (w hen RC5 is high). In

this w ay w e make sure that in the case of a

failure in the c ircuit the drill w ill not turn a nd

the d rilling hea d w ill be up. When the circuit

is sw itched on, the PICs ports are in a high

impedance state , and resistors R1 and R2

hold the signals at suitable levels to ensure

that the motors and solenoids are safelyturned off.

The controller activates the selected

motor/solenoid p air us ing port pin RC2. The

pin does not just t urn on and off, but provide

a current c ontrol using a PWM signa l, different

for the s olenoid a nd for the motor.

The solenoids a re controlled using a rather

complicat ed current w av eform. The timing

depends on the instanta neous position of the

drilling head (w hich is determined from the

tw o sw itches on the head g uide), the value

specif ied in the dril l ing command, and on

w hether an error s ituation a rises d ue, forexample, to a timeout resulting from a

jam med or dirty g uide. The importa nt fa ct is

that the PWM control value is calculated a t

each point in time to actua te the g uide sole-

noid as smoothly as poss ible.

The solenoid coils are rated for continuous

opera t ion a t 12 V, and t hey are dr iven

(althoug h only moment arily) from the

s a me + 30 V s upply a s the s te pper

motors. This produces an enormous

magnetic field to force the drilling

head dow n. As soon as the appro-

priat e Bo sw itch opens, the PIC can

deduce tha t the solenoid ha s movedand it then proceeds to regulate the

current to t he value specified in the

drilling command using the PWM

signal. Although the solenoid is ini-

tially overdriven, in continuous oper-

ation it is only driven at effectively

12 V w ith a n 80%dut y cy cle. This is

w ithin the sp ecified limits. Overall,

w ith typical activity, the duty cycle

is only a few percent, and in pract ice

the coils only get w arm to the touch.

The drill motors are controlled

during sw itch-on and running usinga PWM ramp that reaches 100%duty

cycle d uring drilling .

The stepp er motors are ac tiva ted

using the STEP s ignals . Ea ch

PBL3717A integrated driver controls

one motor w inding, and tw o drivers

(STEPAx and STEPBx) are allocated

to each motor. The drivers are

responsible for producing the d rive

pulses and for conver t ing the + 5 V

logic levels to + 30 V, the operat ing

voltage of the stepper motors. The

microcontroller act ivates the d rivervia the level (repsectively from

STEPAx and STEPBx) at the PHASE

input, and a current then fl ow s from

OA to OB. Input s IN0 a nd IN1 have a

special function: they allow control

of the output sta ge d rive current.

This is norma lly a chieved us ing a

retriggerable timing circuit for each

output stage. If no more motion is

required of the motors, the pulses

stop a nd t he current is reduced in

the mot ors. Here, this is realised via

the s igna l from RB7 (pin 40) con-

nected to the IN1 pins of all thePBL3717s. When motion is required,

all the motors a re driven w ith full

current (even those w hich are not to

move), by setting IN1= 0. Shortly

aft er the motion stops, IN1 is set t o

1 again, and the current drive is

redu ce d to 19%.

Why do w e increas e the current

to the motors not involved in a

motion? Partly because in this w ay

the pow er supply is loaded to the

same extent independent of the

motion, and partly because, as aresult of various resonances in the

sys tem, those mot ors driven at 19%

of the maximum current can be

jogg ed out of position. Althoug h this

is rath er unlikely, it w ould lead t o

errors almost impossible to track

d ow n.

(010024-2)

In t he next instalm ent of thi s seriesw e get dow n to t he constr uct ion of

t he contr ol ler board as well as t he

nut s and bolt s of the project. The var-

ious parts of the PCB dri l l ing

machine w i l l be brought t ogeth er,

w ith il lustrati ons and lots of glue

and g rease!

GENERALINTEREST


pcb drilling machine 2

Documents

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ca n w e

output w aveform

w hich meansthat

w hen printing

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