Technical Field of Invention

The present invention relates to a controller for electro permanent magnet apparatus (referred to herein as EPM, unless otherwise specified, for the sake of brevity and convenience) capable of determining the clamping force between work-piece and working face. More particularly, this invention pertains to an EPM apparatus that operates on half-wave excitation DC pulses and the said apparatus measures the amplitude of the magnetizing current pulses with respect to the reference values for ascertaining the clamping force generated between working face and work piece without using any conventional magnetic flux measuring equipment(s) on the electro permanent magnet itself.

Background of the Invention

Clamping force depends on a number of variables, namely, clamping area, thickness, material of workpiece, air gap between workpiece, working face and the likes. The above factor determines the working capacity of an EPM apparatus, particularly for lifting and/or clamping and/or machining purposes. Inadequate clamping force adversely affects the efficiency of the apparatus, often posing personal hazards to the personnel working on the said set up.

Heretofore considerable attention had been paid to improve the performance and versatility of EPM apparatus and a number of patents have already been granted in this field. Some of the prior patents including an Indian Patent granted to the present Applicant are given below by way of example:

US Patent No. 6,154,353 by Edward Normal Bowers discloses a magnetic work holding apparatus having sensors on the coil to detect the magnetic field of permanent magnet in the object and provide signals to a control circuit, which applies current to the coil to attract the object when the detected field is below a predetermined limit.

US Patent No. 6,104,270 by Gregory E. Elias discloses a work holding apparatus comprising of a magnetic sensor arranged close to the base of the ferromagnetic core and a further magnetic sensor arranged over said permanent magnets so as to measure substantially only the magnetic flux passing through the reversible magnets as well as a safety device for processing the signals transmitted by the magnetic sensor and obtaining the working point of the device on the magnetization curve of the reversible magnet.

US Patent No. 4,237,455 by Philip Beckley discloses a safety device for a work holding apparatus having a sensor means responsive to flux produced by the device in the magnetic circuit including the job, a transducer responsive to the holding tension produced by the device together with a signal translation circuit effective to compare signals from the sensor and from the transducer and to provide an output indicative of a safety margin between holding tension and the field available for holding.

US Patent No. 6,636,153 B1 by Simon C. Barton discloses a sensor system for determining acceptable magnetization between a magnetic clamp and a mold patent for quick mold change system on an injection molding.

Patent granted to the same applicant-cum-inventor vide PCT application WO/2008/032333 illustrates an Electro permanent magnetic work holding system which clamps ferromagnetic work piece(s) and simultaneously senses displacement Although the patent bears similarity to the new application but the difference is that in the earlier patent, the monitoring of induced electric parameters was done after the Magnetization process during operation and corrective action was possible only after the displacement or error has occurred which has been significantly improved in the new system which provides preemptive solution.

This and similar systems are novel, but the complexity of having a number of sensors inside the body of the magnetic apparatus poses a challenge. Moreover, as the size of the magnetic module increases, as more sensors have to be included in the magnetic body.

Brief Description of the Invention

It is an object of the present invention to provide an EPM apparatus capable of determining clamping force without additional sensor on the magnetic module(s) for measuring magnetic flux.

In the above patents, magnetic clamping force measurement has been implemented using conventional measuring devices like search coils/ hall sensor fitted inside the magnet. The above-mentioned system is sensitive and implementing it across multiple / modular apparatuses also becomes very complicated and difficult. The controller used in such apparatuses are custom made for the magnet and measurement circuits are separately integrated to the controller having extra wire(s) going from the magnet to the controller.

The present invention provides an EPM apparatus capable of determining the clamping force of the apparatus without any additional attachments for magnetic flux measurement. It also limits the need for extra wire(s) as the current flowing in the solenoid provides the source of information for analysis of clamping force.
The principal object of the present invention is to provide a system for determining the clamping force between the workpiece and working surface of the EPM apparatus by studying the current characteristics of the main solenoid(s) without use of any additional sensor elements.

A further object of this invention is to provide a convenient and expeditious procedure to determine the clamping forces generated by the same apparatus but subjected to varying conditions like clamping area, thickness, material of workpiece, air gap between workpiece, working face and the like.

In practice the EPM apparatus is operated using industrial alternating current source (AC). The controller of the above apparatus has provision to receive magnetization/ demagnetization commands. Once the controller accepts the magnetizing command, the controller gives a predetermined number of half wave rectified DC pulses in required direction. On receipt of demagnetization command, the controller gives a predetermined number of half wave rectified DC pulses in the reverse direction. It is observed that the amplitude of the current increases gradually (due to inductive nature of the circuit) and then saturates both in magnetization and demagnetization operation.
The common factors that determines the amplitude of the saturated magnetization/ demagnetization current depends on the circuit resistance, line voltage/ frequency and magnetic reluctance of the system in operation.

During demagnetization operation, as the magnetic circuit is completed internally, the amplitude of current attaining saturation is independent of both the work piece and also the air gap between the work piece and the working face.

However, during magnetization process, the work piece becomes integral part of the magnetic circuit and therefore saturated amplitude of current also depends on air gap and magnetic characteristics of the work piece.

By accurately measuring the amplitude of current flowing through the magnetization pulse, and comparing it with reference value, the magnetic clamping condition can be easily ascertained.

To facilitate the same, the EPM apparatus has an accurately current monitoring circuit and analyzing circuit. It determines the peak amplitude of the current flowing through the solenoid during Mag / Demag operations. When compared with the data of reference value already stored in the equipment, it is possible to estimate the clamping force.

The measurement could also vary if the AC input power supply is stable but lower or higher than the one for which the reference values has been stored. To compensate the difference in power supply condition, we can also derive similar results by taking ratio of the saturated amplitude of the current measured during the demagnetization pulse with respect to the saturated amplitude of the current measured during the magnetization pulse.

To facilitate the same, as soon as the magnetization command is received by the controller, it passes demagnetization pulse(s) and then immediately starts the magnetization pulses. As stated earlier, the saturated amplitude of current of the demagnetization pulse is independent of the air gap / workpiece on the EPM apparatus. By comparing the ratio of stable amplitude of the demagnetization pulse with respect to the stable amplitude of the magnetization pulse, it is possible to estimate the magnetic clamping force generated.

Sometimes, however, there is also a possibility that the line connected to the EPM apparatus is not ideal. This can be due to unstable or fluctuating voltage in the supply.

To cater to such working conditions, the controller can also be equipped with an input voltage monitor. As soon as the magnetization / demagnetization process starts, the controller starts monitoring the voltage in the circuit and compensates the amplitude of the stable magnetization/ demagnetization pulse accordingly, if there is variation in line voltage.

Brief Description of the Drawings

Fig (1) shows the EPM apparatus in demagnetized condition. The magnetic lines of forces (7) are internally completed.

Fig (2) shows the pulse pattern during the demag operation. 16 pulses found requisite for subject EPM apparatus are applied in the negative direction and in this case from 12th pulse, the amplitude is saturated. The pulses after that nearly remain constant.

Fig (3) shows the EPM apparatus in magnet in contact with a work piece having minimal air gap and the magnetic lines (9) flow out to the work piece as shown in the drawing.

Fig (4) shows the pulse pattern during the Fig (3) condition. 16 Pulse are applied in the positive side and in this case the pattern is similar to the one as shown in Fig (2) but in opposite (positive) direction.

Fig (5) shows the EPM apparatus in magnet in contact with a work piece having significant air gap (10) and the magnetic lines (11) flow out to the work piece as shown in the drawing. It is seen that the intensity and the extent of lines of force is significantly reduced in comparison to Fig (3) condition.

Fig (6) shows the pulse pattern during the Fig (5) condition. 16 pulses are applied in the positive side and in this case the pattern shows a clear reduction in the saturated amplitude with respect to the Fig (4), indicating that air gap has significant effect on the amplitude.

Detailed Description of the Drawings

The invention will now be illustrated with the help of drawings accompanying the specification as shown in Figures 1 to 7: -

EPM apparatus used in the present instance comprises of a controller unit and the EPM module(s). The controller unit operates the EPM module(s) by generating and providing necessary electrical pulses, equipped with an additional feature for analysis of electrical parameters., wherein (1) is EPM module, (2) denotes working face, (3) denotes at least one magnetic pole, (4) indicates reversible magnet(s), (5) stands for solenoid(s), (6) shows irreversible magnets, (8) is the work piece and (10) indicates the air gap between the working face and work piece. The magnetic lines of forces in demagnetized condition of apparatus is indicated by (7), magnetized with minimal air gap condition is (9) and with significant air gap condition is (11). The power circuit of the controller (12) provides requisite half wave DC pulses which are momentarily applied to the solenoid(s) to operate the EPM apparatus and also carrying out the necessary monitoring activities.

Demagnetizing initially by current pulses before magnetization is just one of the embodiments of the present invention, but an exclusive study of magnetization current could also be made in this regard keeping other operating variables constant.
Another electrical parameter that can be used for detecting the presence of air gap is the inductance of the solenoid(s). In-situ measurement of solenoid's inductance in the magnetized and demagnetized states and analysis of the values can also be used to estimate the air gap.
Yet another parameter that can determine the clamping force is the number of cycles the magnetization cycles required before the amplitude of the current pulse attains saturation. Lesser the number, lower is the clamping force.

These electrical parameters are also affected by the thickness of the work piece and effectively indicate the amount of magnetic flux linking the EPM module(s) to the work piece. These values can be analyzed to determine the safe lifting capacity between the EPM module(s) and the work piece and also the safe limit of air gap for satisfactory operation, which assumes importance for ensuing safety of the operators.

Study of current characteristics has been found to reveal the presence of air gap, which is an important embodiment of the present invention. The EPM apparatus usually operates on half wave DC pulses and the current gradually with progressive magnetization of the system. Normally 8 to16 pulses are needed and current characteristics with varying air gaps have been graphically illustrated in the drawings accompanying this specification depicting three different states, namely, demagnetized state as shown in Fig (1) along with the current characteristics Fig (2), magnetized state Fig (3) with minimal air gap along with the current characteristics (4), magnetized state with air gap as indicated in Fig (5) and Fig 6. However, demag current remains the same despite presence of air gaps.

An electro permanent magnetic holding apparatus (1) having a working face (2) for holding work pieces (8) magnetically at least one magnetic pole (3) comprising at least one non-reversible permanent magnet (6), at least one reversible permanent magnet (4), and at least one electrical winding (5) for reversing the reversible permanent magnet (4) between an ON state for clamping a workpiece (8) and air gap (10) . The magnetic lines of forces in demagnetized condition of apparatus is (7), magnetized with no air gap condition is (9) and with air gap condition is (11) and controller is (12). A flow chart of a typical working procedure has been shown in Fig (7) of the drawing.

It is to be noted that the nature and pattern of the curves are specific for the apparatus used even with different EPM apparatus, the values may change but the pattern of curves remains the same.
This invention assumes particular importance as it has a direct bearing on the associated question of safety of workmen. It has been observed that presence of air gap greatly deteriorates lifting characteristics of EPM module(s), which may lead to undesirable consequences. The present invention provides an effective means for detection of air gaps and consequent adoption of remedial measures.

Additional modifications and improvements of the present invention may also be apparent to those skilled in the art. Thus, the particular combination of parts described and illustrated herein is intended to represent only one embodiment of the present invention and is not intended to serve as limitations of alternative devices and features within the spirit and scope of the invention.

Although the preferred embodiment of the present invention has been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as recited in the claims appended hereafter.

CLAIMS:

1. An EPM apparatus comprising of magnetic module (1) and controller (12) for determining clamping force between the workpiece (8) and working surface (2) of EPM apparatus that operates on half-wave DC excitation pulses provided by the controller (12) and the said apparatus measures the amplitude of the magnetizing current pulses with respect to the reference values for ascertaining the clamping force generated between working face (2) and work piece (8) without using any conventional magnetic flux measuring equipment(s) fitted on the EPM module (1) itself.

2. The EPM apparatus as claimed in Claim 1, wherein by comparing the ratio of stable amplitude of the demagnetization pulse with respect to the stable amplitude of the magnetization pulse, we can estimate the magnetic clamping force generated as illustrated in Fig (2), Fig (4) and Fig (6) of the drawings accompanying this specification.

3. The EPM apparatus as claimed in Claims 1 to 2, with additional input AC line voltage monitor circuit which monitors the voltage in the circuit and compensates the amplitude of the stable magnetization/ demagnetization pulse accordingly.

4. The EPM apparatus as claimed in Claims 1 to 3, in which the number of pulses required to saturate the amplitude during magnetization is less than the reference values which is indicative of less clamping power as depicted in Fig (4) and Fig (6) of the drawing.

5. The EPM apparatus as claimed in Claims 1 to 4, wherein inductance of solenoid(s) in the magnetic module (1) magnetized state may be analyzed to determine and estimate the air gap when compared with the stored reference values.