JPH0739068B2 - Method and apparatus for demagnetizing electromagnetic chuck - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a degaussing method and an apparatus therefor used in an electromagnetic chuck for holding a work or the like.

(Prior Art) An electromagnetic chuck has a merit that it holds a work by using an electromagnetic force and does not damage the work, and thus is widely used in machining. However, on the other hand, the work is magnetized, which has a disadvantage that residual magnetism is generated in the work. This residual magnetism makes it difficult to smoothly remove the chuck, and adversely affects subsequent processes. Therefore, it is necessary to demagnetize the work at the end of machining.

Such demagnetization is performed by applying a damping alternating magnetic field to the chuck holding the work.

That is, after the work processing is completed, a voltage whose polarity is inverted a plurality of times is applied to the work and the chuck, and the current flowing through the chuck is gradually reduced at each inversion so that a damping alternating magnetic field is obtained. There is.

As one of the methods, there is a method of gradually shortening the voltage application time for each polarity reversal.

That is, between the degaussing current i, the applied voltage V of the chuck, and the resistance R of the exciting coil, approximately, Therefore, the degaussing current i can be reduced by decreasing the value of the time t. In addition,
α is a rising time constant of the current (hereinafter, referred to as a time constant), and is represented by α = L / R (L: inductance of the exciting coil).

Since the degaussing current i is expressed as a function of the time t as in the above relational expression (a), the method of obtaining the decaying exchange magnetic field by gradually shortening the degaussing current supply time for each inversion is usually a time control. It is called a method.

FIG. 7 is a block diagram showing a schematic configuration of a conventional degaussing device for an electromagnetic chuck that employs such a time control method.

The operation of the apparatus shown in FIG. 7 will be described. First, during the machining of the work, the holding switch 10a is turned on by the manual operation of the worker, and the holding signal is sent from the mode setting circuit 6 to the driver circuit 3. There is. The driver circuit 3 outputs a control signal to the switching circuit 2 and the switching circuit 2
Supplies a holding current for holding the work to the work holding chuck 1. The voltage applied to the work holding chuck 1 is changed from the voltage setting circuit 4 to the driver circuit 3.
Is determined by the voltage setting signal sent to.

Next, when the processing of the work is completed, the worker turns off the holding switch 10a and presses the degaussing switch 10b. Then,
The degaussing signal is sent from the mode setting circuit 6 to the driver circuit 3, the polarity reversing circuit 5, and the oscillation circuit 8 in the degaussing current supply time setting circuit 9. The oscillation circuit 8 sends a clock pulse, which serves as a reference for time setting, to the time ratio setting circuit 7. The time ratio setting circuit 7 supplies the degaussing current supply time each time the polarity is reversed based on a predetermined time ratio. Are set based on this clock pulse.

Then, the polarity reversing circuit 5 sets the polarity of the degaussing current supplied to the workpiece holding chuck 1 through the driver circuit 3 and the switching circuit 2 a plurality of times based on the set time from the degaussing current supply time setting circuit 9. Invert.

FIG. 8 shows a switching circuit 2 which operates as described above.
3 is a circuit diagram showing the configuration of FIG. This switching circuit 2
Is composed of four transistors T _{r1 to} T _r4 and four diodes D connected between their collectors and emitters.

Then, in the case of positive excitation, the transistors T _r1 and T _r4 are turned on, and in the case of reverse excitation, the transistors T _r2 and T _r3 are turned on, and the polarity of the current from the rectifier circuit 11 is alternately inverted. While supplying it to the work holding chuck 1. 12 is a surge voltage absorption circuit. At the time of machining the work, any one of the transistors T _r1 , T _r4 or the transistors T _r2 , T _r3 remains in the ON state, and the holding current for holding the work is supplied to the work holding chuck 1. is doing.

FIGS. 9A and 9B are waveform diagrams of the applied voltage and the degaussing current supplied from the switching circuit 2 to the workpiece holding chuck 1 when the degaussing operation is performed.

As is clear from this figure, the application time of the voltage V (or −V) applied to the workpiece holding chuck 1 is N ₁ × t _s
Seconds, N ₂ × t _s seconds, N ₃ × t _s seconds, and so on (N ₁ > N ₂ > N ₃ ), the maximum demagnetizing current also decreases with each polarity reversal. ing.

Note that N ₁ , N ₂ , N ₃ , ... Are numerical values stored in the time ratio setting circuit 7 for each inversion, and t _s is the cycle of the clock pulse output from the oscillation circuit 8. Further, t ₀ in FIG. 9 indicates the off time.

[Problems to be Solved by the Invention]

However, in the above-mentioned relational expression (a), the time constant α depends on the shape and size of the chuck and the work,
The values are greatly different. Therefore, as shown in FIG. 9 (b), the degaussing currents i _a , i _b , and i _c have large differences when the time constant α is small, moderate, and large. There is. That is, the conventional degaussing device has a problem that the degaussing performance greatly changes due to the variation of the time constant.

On the other hand, the inventor of the present application has already proposed, prior to the present application, an invention that attempts to secure a certain degaussing performance regardless of variations in the time constant by feedback-controlling the degaussing current in consideration of such problems. (Japanese Patent Application No. 63-1
81307).

However, the value of the degaussing current has a range of several tens of times to 100 at the start of the degaussing operation and the end of the degaussing operation. It is necessary to consider when the current is about 1/10. Therefore, the difference between the maximum detection current and the minimum detection current in the feedback control reaches about 1000 times at the maximum. Therefore, high-precision current detectors and comparators must be used, and the use of high-precision current detectors makes them susceptible to noise.

The present invention has been made in view of the above circumstances, and an object thereof is to prevent fluctuations in degaussing performance due to variations in time constants without requiring a highly accurate current detector or the like. An object of the present invention is to provide a demagnetizing method for an electromagnetic chuck and an apparatus therefor.

[Means for Solving the Problems]

As a means for solving the above problems, the present invention measures the time until the degaussing current reaches a predetermined value when the degaussing current performs the first reversal, and based on the measured time, The degaussing current supply time is set for each inversion after the second time.

[Action]

According to the above configuration, the current measurement only needs to be performed during the first inversion, a high-precision measurement circuit is not required, and the excitation time for each polarity inversion actually causes the chuck to move the workpiece. It corresponds to the rising time constant of the current flowing through the chuck in the held state. Therefore, regardless of the magnitude of the time constant, the peak value of the current at each reversal becomes a value according to a predetermined attenuation pattern, and it is possible to prevent the degaussing performance from changing due to the different magnitude of the time constant.

〔Example〕

Hereinafter, an embodiment of the present invention will be described, but before that, how the peak value of the current for each inversion becomes constant regardless of the magnitude of the time constant will be described.

Now, let α _{1 be} the time constant when holding a small workpiece and α _{2 be the} time when holding a large workpiece, and let i ₁ and i ₂ be the instantaneous values of the respective currents.

On the other hand, the chuck coil resistance is R [Ω] and the applied voltage is V
[V], the steady state current i _{0 of the} chuck is i ₀ = V / R
[A] and the time until the degaussing currents i ₁ and i ₂ reach the specific ratio j of the steady current i ₀ is T ₁ and T ₂ , the following relationship holds.

Ji ₀ = i ₁ = i ₂ ...... ,, from the formula That is, the time taken for the degaussing current to reach a certain ratio of the steady current is proportional to the time constant.

According to the present invention, the time until the current reaches a predetermined value (for example, J = 0.9) is measured during the first reverse excitation, and this time is proportional to the above time constant. There is. Therefore, if the voltage application time for each inversion after the second time is determined so as to be proportional to this time, the peak value of the degaussing current for each inversion will have a predetermined attenuation pattern regardless of the magnitude of the time constant. It becomes the value which followed.

Next, the present embodiment will be described based on FIGS. 1 to 6. FIG. 2 is a block diagram showing a schematic configuration of the degaussing device according to this embodiment, and FIG. 1 is a circuit diagram showing a circuit configuration of a main part of FIG.

In FIG. 1, reference numeral 15 is a measuring circuit composed of a degaussing current detection circuit 13 and a time measuring circuit 14, and 16 is a degaussing current supply time setting circuit composed of a pulse generating circuit 16A and a time ratio setting circuit 16B. Reference numeral 17 is a degaussing operation command circuit.
The detection resistor R _D in the degaussing current detection circuit 13 is provided at the position shown in FIG.

Next, the operation of FIG. 1 will be described with reference to the time chart of FIG. Now, assuming that the work holding chuck 1 is holding a work for processing the work, a voltage drop occurs in the detection resistor R _D in the degaussing current detection circuit 13, and the amplifier 18 sets the level of this voltage drop. The voltage signal corresponding to is output to the comparator 19. Since the level of the voltage signal at this time is equal to or higher than the set voltage of the comparator 19, that is, the set value, the output (point A) of the comparator 19 becomes L level. However, when the work processing is completed and the worker turns off the holding switch (not shown), the level at point A becomes H level (see FIG. 3).

Next, when the worker turns on the degaussing switch (not shown) to start the degaussing operation, the degaussing signal is input to the latch circuit 20 in the degaussing operation command circuit 17, and its output (point B).
Becomes H level. At this time, since the output (point C) of the flip-flop circuit 21 in the time measuring circuit 14 is at the H level, the AND condition of the AND gate 22 is satisfied, the potential at the point D becomes the H level, and the H signal becomes It is sent to the polarity reversing circuit 5. As a result, the polarity inversion circuit 5 outputs the first inversion signal to the driver circuit 3.

At this time, the L signal is sent to the reset terminal of the counter 24 via the inverter 23, whereby the counting of the number of clock pulses input to the clock terminal is started. This clock pulse has a period of 2 _n × t _s seconds. That is, in the pulse generation circuit 16A, an oscillation circuit (corresponding to the oscillation circuit 8 in FIG. 7) including the resistor R _s , the capacitor C _s , and the Schmitt inverter 25 is provided, and from the point S, t A pulse with a cycle of _s seconds is output.
The cycle of the pulse from the point S is expanded to 2 _n × t _s seconds by frequency division by the counter 26 and input to the clock terminal of the counter 24 via the NAND gate 27. The pulse from the point S is also output to the clock terminal of the frequency dividing counter 28, but at this point, the H signal from the point C is input to the reset terminal,
No pulse has yet been output from the output terminal of the frequency division counter 28.

As described above, the H signal is output from the AND gate 22 to the polarity inverting circuit 5, so that the switching circuit 2 (see FIG. 4) starts the first inverting operation and the detection resistor R _D
A voltage drop corresponding to the degaussing current begins to occur at both ends of the. Then, when the output of the amplifier 18 becomes equal to the set value of the comparator 19 (that is, when the demagnetizing current of the first inversion reaches 90% of the rated current), the level at the point A changes from the H level. It changes to the L level, and the level at point C also changes from the H level to the L level.

When the level at the point C is changed to the L level, first, the counting of the pulse of the 2 ⁿ × t _s second cycle in the counter 24 is stopped (because the condition of the NAND gate 27 is not satisfied), and the frequency ^dividing counter 28 Starts the frequency division operation with a period of _ts seconds (since the L signal is input to the reset terminal of the frequency division counter 28). Further, the condition of the AND gate 22 is not satisfied either, so that the H output to the polarity inversion circuit 5
The signal also changes to the L signal, and the first inversion operation ends.

The counter 24 stops counting pulses at a cycle of 2 ⁿ × t _s seconds, and at the same time T _a (from the time point B becomes H level to the time point C becomes L level) Output the number of pulses M counted during
Output from Q _{0 to} Q ₃ . And the number of pulses M and time T _a, T _a
= M · (2 ⁿ × t _s ). The counter 24 keeps the pulse number M while the point B maintains the H level, that is, while the degaussing signal is input to the latch circuit 20.
The signal of is output from terminals Q _{0 to} Q ₃ to the input terminal of divider 28
Keep sending to D ₀ ~ D ₁ .

The frequency division counter 28 divides the pulse signal of the t _s second cycle into 1 / M based on the signal of the pulse number M, and the t _s second cycle is M · t _s.
Seconds or T _a / 2 ⁿ seconds. Then, when this pulse of _Ta / ²ⁿ second cycle is given from the output terminal OUT to the time ratio setting circuit 16B, the time ratio setting circuit 16B changes the degaussing current supply time in the second and subsequent inversions. And outputs a pulse corresponding thereto to the polarity reversing circuit 5. The polarity reversing circuit 5 thereby reverse-controls the switching circuit 2 so that the degaussing current after the second time reaches a predetermined ratio of the rated current.

That is, the period of the pulse output from the frequency division counter 28, that is, the coefficient T _a / 2 ⁿ is a numerical value corresponding to the time constant, and therefore the degaussing current supply time It has become an appropriate time corresponding to.

In this way, when the degaussing current supply time setting circuit 16 outputs the setting signal for the final inversion to the polarity reversing circuit 5, the degaussing current supply time setting circuit 16 outputs a degaussing end signal to the latch circuit 20. . As a result, the level at point B becomes L level and the level at point C becomes H level, and the degaussing operation for the work is completed.

Meanwhile, T _b in Figure 3, all inversion operation from the first round of reverse operation ends indicates the time until the end. In this case, even if the level of the point A is changed to the H level after time T _a elapses, whereby the point B, there is no possibility that the level of point C is changed. In the waveform diagram showing the level of the point A in FIG. 3, the part indicated by the chain double-dashed line shows such an originally unnecessary level change.

The switching circuit of the present invention, instead of the switching circuit 2 including the transistor T _r1 through T _r4 such as FIG. 4, FIG. 5, as shown in FIG. 6, a switching circuit using a relay contact 2A may be adopted. However, in this case, the voltage adjustment cannot be performed by the on / off control of the transistors T _{r1 to} T _r4 as in the switching circuit 2. Therefore, the voltage regulator 30 is added in FIG. 5, and the voltage regulator 30 is added in FIG. It is necessary to add a transistor T _r5 for adjustment.

Further, the mounting position of the detection resistor R _D is not limited to the position shown in FIGS. 4 to 6, but may be mounted at any position E, F, G, H in these drawings. it can.

Further, in the above-described embodiment, the case where the ratio of the first degaussing current is 90% of the rated current has been described, but of course, this ratio can be freely changed according to the specifications.

〔The invention's effect〕

As described above, according to the present invention, the signal corresponding to the time constant is obtained by measuring the time until the degaussing current in the first reversal reaches the predetermined value, and the second time is determined based on this signal. Since each degaussing current supply time in the subsequent reversal is set, the degaussing performance due to the variation in the time constant can be achieved without using a high-precision current detector or the like that is required when performing feedback control. Fluctuation can be prevented.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a configuration of a main part of an embodiment of the present invention, FIG. 2 is a block diagram showing a schematic configuration of a degaussing device according to FIG. 1, and FIG. 3 explains an operation of FIG. FIG. 4 to FIG. 6 are circuit diagrams showing a configuration example of the switching circuit in FIG. 2, FIG. 7 is a block diagram showing a schematic configuration of a conventional degaussing device, and FIG. FIG. 9 is a circuit diagram showing a configuration example of a switching circuit, and FIG. 9 is a waveform diagram for explaining the characteristics of the degaussing device of FIG. 1 ... chuck for holding work, 5 ... polarity reversing circuit, 15
…… Measuring circuit, 16 …… Demagnetizing current supply time setting circuit.

Claims (2)

[Claims] 1. A degaussing device for an electromagnetic chuck, wherein a degaussing current whose polarity is reversed a plurality of times is supplied to a chuck holding the work piece at the end of machining of the work piece, and the degaussing current supply time for each reversal is gradually reduced. In the method, when the degaussing current performs the first reversal, a time until the degaussing current reaches a predetermined value is measured, and based on the measured time, the demagnetization current is changed for each of the second and subsequent reversals. A degaussing method for an electromagnetic chuck, characterized in that a degaussing current supply time is set. 2. A degaussing device for an electromagnetic chuck, which is configured to supply a degaussing current whose polarity is reversed a plurality of times to a chuck holding the work piece at the end of machining of the work piece and to gradually reduce the degaussing current supply time for each reversal. In the device, when the degaussing current undergoes the first reversal, a measuring circuit for measuring the time until the degaussing current reaches a predetermined value, and each reversal of the second and subsequent reversals based on the measured time. A degaussing current supply time setting circuit that sets the degaussing current supply time for each of them.