Electromagnetic chuck power supply and controller

An electromagnetic chuck power supply and control comprising a magnetic amplifier consisting of a saturable reactor supplying DC power at its output across the electromagnet coils of the electromagnetic chuck. A controllably variable DC current is supplied across the control winding of the magnetic amplifier such that the DC power at the output of the magnetic amplifier is inversely proportional to the DC current flow through the control winding. Variable workpiece holding power is thus obtained at the electromagnetic chuck. At full control current flow, the electromagnetic chuck has no holding power except that due to the residual magnetization of the workpiece. In the event of complete failure of the control current circuit, maximum holding force is applied by the chuck to the workpiece. Automatic de-magnetization of the workpiece is provided by applying a gradually increasing DC voltage across the control winding of the magnetic amplifier and sequentially reversing the current flow at the output of the magnetic amplifier such as to supply alternative current pulses of progressively decreasing amplitude to the chuck electromagnet coils.

BACKGROUND OF THE INVENTION 
The present invention relates to electromagnetic chucks in general and more 
particularly to an electromagnetic chuck power supply and controller. 
Electromagnetic chucks are used on various machine tools such as, for 
example, milling machines, drill presses, lathes and surface grinders for 
holding a workpiece in position while a machining operation is effected 
upon the workpiece. Electromagnetic chucks comprise one or a plurality of 
electrical coils inducing magnetic flux lines in electromagnet cores made 
of a ferromagnetic material such as low carbon, high permeable steel or 
cast iron. The electrical coils are wound in such direction and the 
holding surfaces of the diverse electromagnets are arranged such that 
areas of opposite magnetic polarities are engaged by the workpiece, with 
the result that the workpiece, also made of ferromagnetic material, is 
held on the surface of the electromagnet cores. 
When it is desired to release the workpiece, the chuck coils are 
de-energized, such as to permit the removal of the workpiece from 
engagement with the chuck surface. However, as the workpiece is made of 
ferromagnetic material, generally steel, it becomes magnetized to a 
certain degree and the residual magnetism of the workpiece may be such as 
to make it difficult to remove the workpiece from the chuck surface, 
unless the workpiece is, at least partially, de-magnetized. 
As the coils of electromagnetic chucks are energized with direct current, a 
source of direct current is thus required. The direct current is generally 
obtained from an AC/DC converter, through rectification to direct current 
of the alternating current generally obtainable from the mains. Conversion 
of alternating current to direct current is achieved by way of vacuum 
tubes, diode rectifier bridges, and the like. De-magnetization of the 
workpiece is achieved by reversing the direction of flow of the direct 
current in the chuck coils, such as to knock down the residual magnetism 
in the workpiece. De-magnetization of the workpiece may be accomplished 
manually by operating a reversing switch, and applying direct current, 
generally at a reduced voltage, across the chuck coils for a short period 
of time, of the order of a few seconds or less, for example. However, with 
large workpieces, the holding force from residual magnetism may be so 
great as to prevent removal of the workpiece with a single one-step 
de-magnetization pulse. In order to accomplish full de-magnetization of 
the workpiece, a series of alternating pulses of progressively decreasing 
amplitude is required. 
Adjustably variable holding force is a desirable convenient feature in an 
electromagnetic chuck. It permits the use to controllably adjust the DC 
voltage or current to the chuck such that the magnetic flux and, 
consequently, the holding force may be controllably adjusted. Such a 
feature is convenient if it is desired to maintain a holding force 
sufficient to hold a workpiece on the chuck during machining of the 
workpiece, but not so strong as to cause shape deformation of the 
workpiece. In conventional electromagnetic chucks, variable holding power 
is accomplished for example by using variable or multiple dropping 
resistors, or multi-tapped transformers, or by using phase angle fired 
silicon rectifiers. However, the use of resistors or multi-tap 
transformers requires mechanical devices such as rotary switches manually 
operated or operated by stepping motors. The use of phase angle fired SCR 
to vary the power applied to the electromagnetic chuck is subject to 
fairly erratic performance from surges or variations in the AC line 
voltage. Failure of an SCR eliminates power to the chuck, with the 
resulting safety hazard of complete loss of workpiece holding force, or 
reduction of the holding force to that caused by residual magnetization. 
The present invention provides a controllable power supply for 
electromagnetic chucks which, through the use of a magnetic amplifier 
controlled by a DC current flowing in the control winding of a saturable 
reactor magnetic amplifier, permits to adjustably vary the power supplied 
to the load circuit, the electromagnetic chuck coils, from zero to 100% by 
varying the intensity of the DC control current flowing through the 
control winding of the magnetic amplifier. In addition, in the event of 
failure of the DC control circuit, full output power is applied to the 
electromagnetic chuck coils, as the load power delivered at the output of 
the magnetic amplifier is inversely proportional to the control current 
intensity. Furthermore, the present invention provides automatic 
de-magnetization of the workpiece held by an electromagnetic chuck in 
applications where such a feature is desirable to facilitate removal of 
the workpiece from the chuck. 
SUMMARY OF THE INVENTION 
The principal object of the present invention is to provide a novel 
electromagnetic chuck power supply and control having the advantages of 
enabling the use to controllably use full holding power, variable holding 
power, no holding power, except that due to residual magnetism for removal 
of the workpiece, and automatic de-magnetization of the workpiece.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawing and more particularly to FIG. 1, the magnetic 
chuck 10 of a machine tool, such as a surface grinder for example, is 
provided with electromagnet coils 12 having a source of DC electrical 
current connected across its inputs 14 and 16 through a reversing relay 
switch 18 operable by a solenoid 20. The input of the reversing relay 
switch 18 is connected to the output terminals 22 and 24 of a magnetic 
amplifier 26. 
The magnetic amplifier 26 comprises a saturable reactor 28 having a pair of 
power windings 30 and 32 and a DC control winding 34. The power windings 
30 and 32 of the saturable reactor 28 are connected across a first 
secondary winding 36 of a power transformer 38 via a full-wave rectifier 
bridge comprising diodes 39, 40, 41 and 42, such as to provide DC current 
across the output terminals 22 and 24 of the magnetic amplifier 26. The 
power transformer 38 has a primary winding 44 connected across a source of 
alternating current, and a second secondary winding 46 providing a 
relatively low voltage DC current through the magnetic amplifier control 
circuit 47, after rectification through a full-wave rectifier bridge 48 
connected across the input terminals 50 and 52 of the control windings 34 
through respectively lines 54 and 56. A control potentiometer 58 is 
connected in the control circuit 47 of the control winding 34 through a 
normally closed relay switch 60 operable to an open position by a solenoid 
62. A normally closed relay switch 64 shunts the potentiometer 58. The 
relay switch 64 is operable to an open position by a solenoid 66. 
A workpiece automatic de-magnetization unit 68 is connected in parallel to 
the series circuit formed by the control potentiometer 58 and the relay 
switch 60. A normally open relay switch 70, operable by a solenoid 72, 
controllably places the workpiece automatic de-magnetization unit 68 in 
the control circuit. 
A forth normally closed relay switch 74, operable to an open position by 
energizing a solenoid 75, is connected in series in the DC control circuit 
47 in the line 56, as shown, or, alternatively in the line 54, such as to 
controllably open the control circuit 47, irrespective of whether current 
flows through the circuit branch including the automatic de-magnetization 
unit 68 or the circuit branch including the potentiometer 58. 
The electromagnetic chuck 10 is operated from a control panel placed at the 
disposal of the operator of the machine tool on which the electromagnetic 
chuck 10 is installed. The control panel, not shown, has an on-off switch 
for operating the main relay switch 77 connecting or disconnecting the 
primary winding 44 of the power transformer 38 across a source of 
alternating current, and a rotary switch 76 for manually controlling the 
modes of operation of the electromagnetic chuck. 
A common terminal 78 of the rotary switch 76 is connected through a line 79 
to the relatively low +B DC voltage at the output of the rectifier bridge 
48. The rotary switch 76 has at least four function terminals 80, 82, 84 
and 85 for the operation modes of the electromagnetic chuck 10, which, 
simultaneously with controlling the functional mode, energize one of the 
function indicator lights 86, 88, 90 or 98. 
When the indicator light 86 is on as a result of the rotary switch 76 being 
operated to place the terminal 80 at the +B voltage, "full" power is 
delivered to the electromagnetic chuck from the output terminals 22-24 of 
the magnetic amplifier 26. Such a function is accomplished as a result of 
the solenoid 75 being energized through a line 92 such as to open the 
control circuit shut-off relay switch 74, thus opening the DC current 
control circuit 47 of the magnetic amplifier control winding 34. As no DC 
current flows through the control winding 34, the core saturation effect 
caused by the control winding 34 is nil, and full power is delivered at 
the output teminals 22-24 of the magnetic amplifier 26, limited only by 
the self-saturation of the reactor 28 caused by the current flowing 
through the power windings 30 and 32. 
Full power is also delivered to the electromagnetic chuck 10 anytime the 
control circuit 47 of the magnetic amplifier 26 is open, thus ensuring 
that, in the event of malfunction of the low voltage control circuit 47, 
full power is delivered to the electromagnetic chuck 10 such as to hold 
any workpiece, not shown, mounted on the chuck with full holding force, 
and thus preventing accidental or premature loosening of the workpiece 
from the chuck surface. 
By actuating the rotary switch 76, such as to connect its terminal 82 to 
the common terminal 78 connected to the +B end of the low voltage DC 
source, the indicator light 88 is energized, thus indicating that 
workpiece "variable" holding force is available at the electromagnetic 
chuck 10. Workpiece variable holding mode is achieved via a line 94 
connecting the rotary switch terminal 82 to the solenoid 66 of the relay 
switch 64. When the solenoid 66 is energized, the normally closed relay 
switch 64 opens, thus causing the DC current flowing through the magnetic 
amplifier control winding 34 to pass through the potentiometer 58. By 
adjusting the position of the slider of the potentiometer 58, the machine 
tool operator regulates the intensity of the current flowing through the 
control winding 34 of the magnetic amplifier 26 from full value to a 
minimum value. At full DC current flow through the control winding 34, the 
saturable reactor 28 of the magnetic amplifier 26 is fully saturated and 
negligible current, if any, is supplied across the magnetic amplifier 
output terminals 22-24. The magnetic amplifier output voltage and current 
increase as an inverse function of the current flowing through the control 
winding 34, such that, at the maximum resistance setting of the 
potentiometer 58, with practically no current flow through the magnetic 
amplifier control winding 34, full power is delivered at the magnetic 
amplifier output terminals 22-24 to the coil or coils 12 of the 
electromagnetic chuck 10. 
By operating the rotary switch 76 such as to connect the rotary switch 
terminal 84 to the +B common terminal 78, the indicator light 90 is 
activated, thus indicating that the workpiece is held on the 
electromagnetic chuck 10 by "residual" magnetic force. As the series relay 
switch 60 in the control circuit 47 of the magnetic amplifier 26 is 
normally closed, and the shunt relay switch 64 of the series potentiometer 
58 in the control circuit is also normally closed, full control DC current 
flows through the magnetic amplifier control winding 34, with the result 
that no power is delivered at the output terminals 22-24 of the magnetic 
amplifier 26. The workpiece held by the electromagnetic chuck 10 is 
consequently held only by the residual magnetic force caused by the 
residual magnetization of the workpiece and any residual magnetization of 
the cores of the electromagnetic chuck 10. If the residual magnetic force 
is not strong enough to effectively hold the workpiece on the surface of 
the electromagnetic chuck 10, the workpiece may be manually removed. 
In order to operate the magnetic chuck 10 in the automatic de-magnetization 
mode, the machine tool operator operates the rotary switch 76 to the 
appropriate position placing the terminal 85 of the rotary switch in 
connection with the +B voltage terminal 78. The +B voltage is thus 
applied, through the "release" indicator light 98, via a line 100 across 
the solenoid 72 of the normally open relay switch 70, closing the relay 
switch 70 and placing the automatic de-magnetization unit 68 in the DC 
control circuit 47 of the magnetic amplifier control winding 34. The line 
100 being also connected to the solenoid 62 of the normally closed relay 
switch 60, the solenoid 62 is simultaneously energized, thus causing the 
relay switch 60 to open for disconnecting the circuit branch comprising 
the potentiometer 58 from the magnetic amplifier DC control circuit 47. 
Simultaneously therewith, the line 100 applies the +B voltage to an input 
102 of the automatic de-magnetization unit 68, such as to enable the unit 
for starting the automatic de-magnetization sequence. The workpiece 
automatic de-magnetization unit 68 automatically increases the DC current 
flowing through the control winding 34 of the saturable reactor 28 of the 
magnetic amplifier 26 from a zero value to a maximum value, by specific 
steps, as illustrated at FIG. 3 at ideal curve A, with the result that the 
corresponding current flow in the power windings 30 and 32 of the 
saturable reactor 28 is progressively decreased by consecutive steps, 
represented by ideal curve B, as a function of time. Simultaneously, the 
workpiece automatic de-magnetization unit 68 supplies at an output 104 
through a line 106 a series of timed pulses at twice the frequency of the 
step variation rate of the current flow through the control circuit to 
actuate the solenoid 20 of the reversing switch 18, such that the coil, or 
coils, 12 of the magnetic chuck 10 is supplied with alternating DC pulses 
of gradually decreasing amplitude, as illustrated at C at FIG. 4, as a 
function of time. 
The workpiece automatic de-magnetization unit 68 may take any appropriate 
circuit form, capable of providing a DC current increasing over a finite 
adjustable period of time, from a fraction of a second to two or three 
seconds for example, applied across the control winding 34 of the 
saturable reactor 28 of the magnetic amplifier 26, or a DC current 
increasing by steps or increments from a zero value to the maximum value 
required for full saturation of the cores of the magnetic reactor 28, 
while simultaneously energizing the control solenoid 20 of the reversing 
relay switch 18. The circuit of the workpiece automatic de-magnetization 
unit 68 may consist exclusively of electromagnetic devices, such as a 
plurality of relays disconnecting from the control circuit 47, in 
sequence, each of a plurality of voltage dropping resistors or current 
limiting resistors, or it may consist of an appropriate logic integrated 
circuit and electrical elements. 
An example of such a circuit is illustrated at FIG. 2 wherein four 
resistors 110, 112, 114 and 116 are shown connected in the magnetic 
amplifier control circuit in series with a switch in the form of a 
transistor 118. The emitter collector circuit of a transistor 120 shunts 
the resistor 110, the emitter collector circuit of a transistor 122 shunts 
the resistors 110 and 112, the emitter collector circuit of a transistor 
124 shunts the resistors 110, 112 and 114, and a fourth transistor 126 is 
connected such that its emitter collector circuit shunts the full string 
of resistors 110-116. All the transistors are normally biased to a 
non-conductive state. A multiplexer 128 has each of a plurality of outputs 
connected to the base of each of the transistors 118-126. When a signal, 
such as the +B voltage through the line 100, is applied through the 
terminal 102 to an input of the multiplexer 128, voltage levels are caused 
to change sequentially at each of the outputs of the multiplexer. The 
voltage level for the multiplexer output applied to the base of the 
transistor 118 biases the transistor 118 to its conductive state and a 
limited current flows through the string of resistors 110 through 116, 
corresponding to the first step of the current flow curve A of FIG. 3. 
When the voltage level at the output of the multiplexer 128 connected to 
the base of the transistor 120 changes, the transistor 120 is biased to 
its conductive state and therefore shunts the resistor 110, with the 
result that the current flow through the string of resistors 112, 114 and 
116 is increased, corresponding to the second step of curve A of FIG. 3. 
Subsequently, and in sequence, voltage level changes at the remaining 
outputs of the multiplexer 128 biases the transistors 122, 124 and 126, 
sequentially, to their conductance state, such as to shunt sequentially 
resistors 110 and 112, resistors 110, 112 and 114, and the whole string of 
resistors 110 through 116, thus incrementally increasing the current 
flowing through the magnetic amplifier control circuit 47 to full current 
flow. Consequently, the current through the control circuit 47 is 
increased by consecutive steps from a minimum value, cut-off, to a maximum 
value when the whole string of resistors 110 through 116 is shunted. In 
time synchronization with the operation of the transistors 120-126 from a 
non-conductive to a conductive state, a signal at twice the frequency of 
transistor bias changes appears at another output of the multiplexer 128. 
The signal, after amplification by an amplifier 130, is applied through 
the automatic de-magnetization unit 68 to the output 104 and the line 106 
to the actuating solenoid 20 of the reversing relay switch 18, FIG. 1, at 
the input of the electromagnetic chuck 10. The reversal of the 
incrementally decreasing current flow through the coil or coils 12 of the 
electromagnetic chuck 10 causes a series of alternating pulse, as ideally 
represented by curve C of FIG. 4, to flow through the coil or coils 12 of 
the electromagnetic chuck 10, thus progressively de-magnetizing the 
electromagnetic chuck cores and the workpiece on the chuck surface. 
The time basis for the operation of the multiplexer 128 is obtained from a 
potentiometer 132, installed on the electromagnetic chuck control panel 
and accessible to the machine tool operator. The adjustable voltage across 
the potentiometer 132 widens or narrows the periods of the voltage 
increase steps in the magnetic amplifier control circuit 47. 
It will be readily appreciated by those skilled in the art that digital 
units other than multiplexers, such as monostable multivibrators operating 
up/down counters or flip-flops, for example, may be used in conjunction 
with appropriate semiconductor switches for progressively shunting a 
string or series of dropping resistors in the magnetic amplifier control 
circuit, and that any or all of the relay switches in the circuit of FIG. 
1, which have been represented by electro-mechanical switches, may be in 
the form of semi-conductor switches. It will be further appreciated by 
those skilled in the art that, if so desired, an auxiliary manual 
de-magnetization mode may be provided in the arrangement of the invention 
by simply connecting via a line 134, shown in dashed line at FIG. 1, the 
actuating solenoid 20 of the reversing relay switch 18 to the +B voltage 
source through a normally open switch 136, manually closed for momentarily 
reversing the current flow through the magnetic chuck coil 12, while 
manually increasing the current flow through the control circuit 47 from a 
minimum to a maximum by way of the potentiometer 58. Such a manual 
de-magnetization mode becomes convenient in the event of malfunction of 
the automatic de-magnetization unit 68.