Current limit system for electrical discharge machining apparatus

A programmable system for controlling maximum machining current. A current limit is entered into the system together with the on time and off time desired. The system operates to use the entered on-time and current values, but the off time will be modified depending on the maximum current limit possible. This will protect the apparatus when an impossible combination has been entered by the operator. The duty cycle is checked against a maximum level such as 87% or a like upper limit.

BACKGROUND OF THE INVENTION 
The field to which the present invention relates is that generally known as 
electrical discharge machining, sometimes hereinafter referred to as EDM, 
in which material is removed from an electrically conductive workpiece by 
the action of electrical gap discharges occurring between a tool electrode 
and a workpiece. A dielectric coolant fluid is circulated and recirculated 
through the gap, usually under pressure, throughout the machining 
operation. An electrode or a workpiece servo feed system is used to 
provide relative movement and thus maintain an optimum gap spacing between 
the electrode and the workpiece as the workpiece material is being 
removed. 
It is important to the process of EDM that the machining power pulses 
provided at the gap are of closely and precisely controllable on-off time 
and frequency to insure repeatability of results and to provide 
appropriate cutting action for the type of operation being carried on. 
Various types of pulse generators which have this capability have been 
developed and are in commercial use for EDM. One commonly used type of EDM 
power supply includes as a principal part of its machining power pulse 
generator an astable multivibrator in which on-off time and frequency are 
controlled and preset by a ganged capacitor and resistor arrangement. One 
example of this type of pulse generator and an associated protection 
system is shown and described in Kurt H. Sennowitz, U.S. Pat. No. 
3,649,802, issued on Mar. 14, 1972 for "Protective System for Electrical 
Discharge Machining Power Supply Circuit", which patent is of common 
ownership herewith. 
A further arrangement for digital type EDM pulse generator is shown and 
described in Oliver A. Bell, Jr., U.S. Pat. No. 3,809,847, issued on May 
7, 1974, for "Method and Apparatus for Electrical Discharge Machining". 
A still further type of digital multivibrator is shown and described in 
Oliver A. Bell, Jr., U.S. Pat. No. 4,071,729, issued on Jan. 31, 1978, for 
"Adaptive Control System and Method for Electrical Discharge Machining". 
This patent shows an on and off time generator which receives inputs from 
a programmable computer and from this general arrangement provides 
machining power pulses to the machining gap. All the above noted patents 
are of common ownership herewith. 
The present invention is particularly designed for use with a digital type 
pulse generator that is controlled by a programmable computer or similar 
input device. Reference is made to our copending U.S. Patent Application 
Ser. No. 068,328 filed on Aug. 21, 1979 for "Programmable Pulse Generator 
for Electrical Discharge Machining Apparatus". 
The prior art has shown a number of current limiting systems for EDM in 
which a current limit is relay set in accordance with the frequency at 
which the pulse generator is operating. The present invention goes further 
and allows on and off times and current limit to be entered. The off time 
is then readjusted to an allowable maximum value for the current limit 
desired. 
SUMMARY OF THE INVENTION 
Our invention provides a current limit system for EDM which operates to 
receive the on time and the off time preset by the operator. The duty 
cycle is computed and checked against a maximum level, for example 87%. If 
it is greater, the off time is increased by the amount required to reach 
that level. The off time is checked against a predetermined value, for 
example 255 microseconds. If it is greater, no current limiting is needed. 
Maximum allowable frequency is checked against a table of predetermined 
values.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows the basic parts of a programmable computer and an associated 
electrical discharge machining apparatus. Gap voltage is sensed by a 
voltage sensing network 10. The gap voltage is passed through an analog to 
digital converter 12 and then to the CPU 14 of the programmable computer. 
The memory 16 of the microprocessor is also shown. One example of a 
microprocessor suitable for use in connection with this invention is the 
microprocessor currently manufactured and sold by the Mostek Corporation, 
1215 West Crosby Road, Carrollton, Tex. 75006 and known as the Model MK 
3880. It will be understood that parts of the microprocessor can be 
reconstructed using architecture of the type of the Texas Instruments 
model 74181 or the 74S181 or the 74LS181 series of four bit slice 
arithmetic logic units. These units are known in the electronic art and 
are used to simplify operations such as add, subtract, find equality, 
negate and still others. 
Also shown in FIG. 1 is a digital pulse generator 18 of the counter type as 
shown and described in our co-pending application Ser. No. 068,328 
referred to above. Keyboard 19 is used for operator input. One counter 18a 
is preset to represent machining pulse on-time. The other counter 18b is 
preset to represent machining pulse off-time. A current limit unit 19 is 
connected in circuit with the gap and associated with the power module 20. 
It includes a plurality of relays, six in number, which corresponding to 
their actuation switch a set of resistors in series with the gap lead to 
actually limit machining current. The six relays in binary fashion give a 
total of 64 current limit positions. This will be explained more 
completely hereinafter. The EDM power output module is indicated by the 
numeral 20 and further includes the main DC source and an output power 
switch that is turned on and off by the pulse generator 18 to provide 
machining power pulses to the machining gap. The machining gap is defined 
between a tool electrode 22 and a workpiece 24. The gap voltage signal is 
shown at the left hand side of the drawing. It is used in the pulse 
generator 18 to control the off time in a manner that will be further 
explained in the section "Description of Operation", hereinafter. It will 
further be clarified by inclusion of an exemplary program for the 
microprocessor. 
FIG. 2 is a block diagram of the major elements in the CPU 14. These 
include the CPU Control 30, internal register 32, data bus control 34, 
arithmetic logic unit 36, CPU registers 38, and address control 40. The 
several interfaces and data buses are also shown in the drawing. While the 
described embodiment of the invention includes elements of a 
microprocessor, the invention is not limited to this type of device. A 
variety of different programmable computers can be used. 
DESCRIPTION OF OPERATION 
The current limit, on time and off time data are entered in the keyboard 19 
by the operator. The entry is made into the CPU 14 and the pulse generator 
18. The on-time and current limit value will always be entered as preset 
by the operator. However, the off time is subject to modification 
depending on the duty cycle and frequency. An impossible machining 
combination of frequency and current limit will not be allowed to operate. 
In the absence of control by the current limit system, the workpiece, 
electrode and EDM circuitry could be damaged. 
Basically, our system receives the on and off time and derives the duty 
cycle. If it exceeds 87%, which represents the maximum safe level, then it 
will be reduced by increasing the off time until the duty cycle is 87% or 
less. After that operation is performed, we check the total period to see 
if it is greater than 255 microseconds. If it is, no current limiting 
action is necessary since any period greater than 255 with an 87% duty 
cycle will permit any of the current limit combinations. 
If the period is less than 255, then we will subtract the number 20 from 
it. If the result is greater than 20, we retire our routine. If the period 
desired is less than 20, we must start checking our look up table to 
verify we don't have a frequency combination that is too great for the 
current limit desired. The look up table, illustrated in the manner it is 
used in an exemplary program, is set forth below and entitled, "Current 
Limit Table". 
______________________________________ 
CURRENT LIMIT TABLE 
______________________________________ 
02888 CLTAB MBYTE 3,8 
109C 03 + BYTE 3 
109D 03 + BYTE 3 
109E 03 + BYTE 3 
109F 03 + BYTE 3 
10A0 03 + BYTE 3 
10A1 03 + BYTE 3 
10A2 03 + BYTE 3 
10A3 03 + BYTE 3 
02889 MBYTE 4,2 
10A4 04 + BYTE 4 
10A5 04 + BYTE 4 
02890 MBYTE 5,2 
10A6 05 + BYTE 5 
10A7 05 + BYTE 5 
02891 MBYTE 6,2 
10A8 06 + BYTE 6 
10A9 06 + BYTE 6 
02892 10AA 07 BYTE 7 
02893 MBYTE 8,2 
10AB 08 + BYTE 8 
10AC 08 + BYTE 9 
02894 MBYTE 9,2 
10AD 09 + BYTE 9 
10AE 09 + BYTE 9 
02895 MBYTE 10,3 
10AF 0A + BYTE 10 
10B0 0A + BYTE 10 
10B1 0A + BYTE 10 
02896 MBYTE 11,3 
10B2 0B + BYTE 11 
10B3 0B + BYTE 11 
10B4 0B + BYTE 11 
02897 MBYTE 12,3 
10B5 0C + BYTE 12 
10B6 0C + BYTE 12 
10B7 0C + BYTE 12 
02898 MBYTE 13,2 
10B8 0D + BYTE 13 
10B9 0D + BYTE 13 
02899 MBYTE 14,4 
10BA 0E + BYTE 14 
10BB 0E + BYTE 14 
10BC 0E + BYTE 14 
10BD 0E + BYTE 14 
02900 MBYTE 15,3 
10BE 0F + BYTE 15 
10BF 0F + BYTE 15 
10C0 0F + BYTE 15 
02901 MBYTE 16,4 
10C1 10 + BYTE 16 
10C2 10 + BYTE 16 
10C3 10 + BYTE 16 
10C4 10 + BYTE 16 
02902 MBYTE 17,3 
10C5 11 + BYTE 17 
10C6 11 + BYTE 17 
10C7 11 + BYTE 17 
02903 MBYTE 18,6 
10C8 12 + BYTE 18 
10C9 12 + BYTE 18 
10CA 12 + BYTE 18 
10CB 12 + BYTE 18 
10CC 12 + BYTE 18 
10CD 12 + BYTE 18 
02904 MBYTE 19,13 
10CE 13 + BYTE 19 
10CF 13 + BYTE 19 
10D0 13 + BYTE 19 
10D1 13 + BYTE 19 
10D2 13 + BYTE 19 
10D3 13 + BYTE 19 
10D4 13 + BYTE 19 
10D5 13 + BYTE 19 
10D6 13 + BYTE 19 
10D7 13 + BYTE 19 
10D8 13 + BYTE 19 
10D9 13 + BYTE 19 
10DA 13 + BYTE 19 
02905 10DB 14 BYTE 20 
______________________________________ 
If the position in the table has a frequency greater than the frequency 
that has been calculated for the current on-off time settings, then no 
action is required. If it is less than the current frequency desired, then 
we must make an adjustment of the off time to meet that frequency. As an 
example, if you have current limit 10 and an on time 8 and off time 2, the 
resultant frequency would be 100 KHZ and no change to off time would be 
required. 
If we input an on time of 2 and an off time of 1, the total period would be 
3 microseconds and the frequency 333 KHZ. Referring to the current limit 
table above, with current limit 7, we allow for only 3 microseconds 
period. With current limit 9, we allow only 4 microseconds period. The 
progression is in this manner from a current limit of 1 on top to a 
current limit of 64 on the bottom of the table. 
An example will now be taken which will require corrective action and 
change to off time. We have an operator input of 2 microseconds on time, 2 
microseconds off time and a current limit of 25. We find in the table in 
the 25th position, the number 11, which indicates that 11 microseconds is 
the smallest allowable period for current limit 25. Since we have set 2 on 
and 2 off, this is an impossible combination. It is now necessary to 
modify the off-time so that the total of the off-time and the on-time will 
equal 11 or more. We then leave the value of the on time as it was preset, 
take the period and subtract it from the 11 to give 7. The 7 is added back 
to the off time so to set the correct off time for the operation. The 
current limit will be output through an appropriate port to operate the 
right relays in current limit unit 19 and set the proper current limit for 
the output from power module 20. At the same time, the on-time and 
off-time are entered from the CPU 14 to counters 18a and 18b respectively 
in the pulse generator 18. As a result, we have an on and off setting and 
a current limit that have been properly checked and are all protective of 
the EDM apparatus and of the workpiece 24 and tool electrode 22. We have 
not violated the operator's parameters for finish and overcut since he has 
been given the exact current limit and on-time that he asked for. To 
further clarify the mode of operation of the invention, an exemplary 
program showing the current limit protection system is set forth below. 
__________________________________________________________________________ 
Current Limit Program 
__________________________________________________________________________ 
01377 04A0 2A0320 &gt; OFFIX 
LD HL, (OFTIM) 
GET OFFTIME 
01378 04A3 EB EX DE,HL PUT OFFTIME IN D & E 
01379 04A4 2A0520 &gt; 
LD HL,(ONTIM) 
GET ON TIME 
01380 04A7 44 LD B,H PUT THE ONTIME IN B & C 
01381 04A8 4D LD C,L BC = ONTIME 
01382 04A9 CDC503 &gt; 
CALL CAL CORRECT FOR 87% DUTY CYCLE 
01383 ZCR H IF PERIOD IS - 255 THEN NO ACTION 
04AC 24 + INC H 
04AD 25 + DEC H 
01384 04AE C0 RET NZ H HAD A NUMBER SO PERIOD - 255 
01385 04AF 7D LD A,L CHECK LOWER 8 BITS 
01386 04B0 D614 
SUB 20 SEE IF PERIOD IS - 20 
01387 04B2 D0 RET NC NO OVERFLOW SO PERIOD - 20 
01388 04B3 3A0A20 &gt; 
LD A,(CTWR) 
GET CONTROL WORK (CURRENT LIMIT,CYST BIT, POL 
BIT) 
01389 04B6 E63F 
AND 3FH MASK OFF POLARITY BIT AND CYCLE START BIT 
01390 04B8 55 LD D,L SAVE PERIOD IN D 
01391 04B9 219C10 &gt; 
LD HL,CLTAB 
HL POINTS TO TOP OF CURRENT LIMIT TABLE 
01392 04BC 0600 
LD B,O CLEAR B 
01393 04BE 4F LD C,A BC CONTAINS THE CURRENT LIMIT 
01394 04BF 09 ADD HL,BC OFFSET TABLE POINTER 
01395 04C0 46 LD B,(HL) GET ALLOWABLE PERIOD 
01396 04C1 7A LD A,D GET DESIRED PERIOD 
01397 04C2 90 SUB B SUBTRACT MAX ALLOWABLE FREQ 
01398 04C3 D0 RET NC NO OVERFLOW INDICATES ALL OK 
01399 04C4 78 LD A,B GET CORRECT PERIOD 
01400 04C5 92 SUB D GET DIFFERENCE 
01401 04C6 B7 OR A CLEAR CARRY 
01402 04C7 8B ADC A,E ADD OLD OFFTIME 
01403 04C8 320320 &gt; 
LD (OFTIM),A 
STORE NEW OFFTIME 
01404 04CB C9 RET DONE 
__________________________________________________________________________ 
It will thus be seen that we have provided by our invention, an improved 
current limit system that controls operator input to the degree necessary 
to protect the equipment. The system operates precisely and reliably 
without interferring with parameters affecting the important factors of 
finish and overcut. These remain as selected by the operator.