Abstract:
A position sensor or controller generates a response signal in existing armature windings of an actuator and detects the response signal to determine the position of the armature. To generate the response signal, the actuator includes a sensor excitation winding near the armature. Two sensor excitation windings can be provided, above and below the armature, to cancel out z components and thus allow for a variable gap. The sensor excitation winding or windings are supplied with an excitation signal to induce the response signal in the armature windings. The response signal is derived by differentially amplifying and frequency filtering a raw output of the armature windings. The response signal is demodulated to determine position. If a position controller rather than a mere sensor is desired, the position signal can be buffered, PID compensated, amplified, and fed back to the armature windings.

Description:
ORIGIN OF INVENTION 
     This invention was made by employees of the United States Government and may be manufactured and used by or for the Government for Governmental purposes without the payment of royalties. 
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     The present invention is directed to a non-contact position sensor and more particularly to a non-contact position sensor for an actuator or for other settings in which an air gap is required to fluctuate. 
     2. Description of Related Art 
     It is often desirable to have a non-contact position sensor when closing a servo loop around an actuator. This becomes difficult in many applications where weight and volume are critical because many non-contact position sensors are implemented using optical devices. If an optical device is not used, then a Hall device is usually chosen. 
     For similar applications, optical devices are usually large and cumbersome to work with and are often impossible to mount without having the sensor become the device which dictates the volume. Optical sensors need a light source, which also requires volume and extra alignment problems. 
     Hall sensors have the disadvantage of nonlinearity and must be precision mounted to obtain accurate measurement of position. The Hall sensors also need another component, usually a permanent magnet, for use in sensing position. Another disadvantage of Hall devices is their reaction to permanent and fluctuating magnetic fields. 
     A final disadvantage of Hall devices and the like is that such devices require a fixed air gap in the motor. In some applications, however, it is required that the air gap fluctuate. For example, one type of actuator for which such devices cannot be used is a linear permanent magnet motor, in which the winding is fixed and the permanent magnet is allowed to move. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to develop a non-contact position sensor which requires a minimum of weight and volume. 
     It is another object of the invention to develop a non-contact position sensor which can be easily implemented with a given actuator, such as a linear permanent magnet motor where the winding is fixed and the permanent magnet is allowed to move. 
     It is a further object of the invention to develop a non-contact position sensor which uses existing actuator windings. 
     It is a still further object of the invention to use electronics to develop a very accurate, low volume, low weight non-contact position sensor. 
     To achieve these and other objects, the present invention is directed to a position sensor for sensing a position of an armature in an actuator, the armature having at least one armature winding, the position sensor comprising: excitation wave generating means for generating an excitation wave; at least one excitation winding for generating an excitation magnetic field from the excitation wave and for applying the excitation magnetic field to at least one armature winding to cause at least one armature winding to generate a response signal; detecting means, connected to at least one armature winding, for detecting the response signal in the armature winding; and demodulating means for determining the position of the armature from the response signal detected by the detecting means. 
     The present invention is further directed to a position controller for sensing and controlling a position of an armature in an actuator, the armature having at least one armature winding, the position controller comprising a position sensor such as that described above and position controlling means, receiving the detected position signal, for generating a position control signal and outputting the position control signal to the at least one armature winding. 
     Linear position can be detected in either a single axis or dual axes simultaneously. The design is simplified because the sensor and actuator are co-located (have the same coordinate frame). Only a single component (the excitation windings) needs to be added to existing actuators. 
     The present invention can be used for vibration isolation for glovebox applications. It can be used for vibration isolation systems using electromagnetic actuators for integrated circuit manufacturing equipment and other sensitive manufacturing machines. It can also be used as a linear position sensor for motors, copiers, antilock brake systems, and the like, and in robotics. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred embodiment of the invention will be set forth in detail with reference to the drawings, in which: 
     FIGS. 1A-1C show an actuator for use with the position sensor of the present invention; 
     FIG. 2 is a schematic block diagram of the electronics used in the position sensor of the present invention; 
     FIG. 3 shows a circuit diagram of the first block of the schematic block diagram of FIG. 2; 
     FIG. 4 shows circuit diagrams of the second through fifth blocks of the schematic block diagram of FIG. 2; 
     FIG. 5 shows circuit diagrams of the sixth through eighth blocks of the schematic block diagram of FIG. 2; and 
     FIGS. 6A and 6B show graphs of sensor linearity. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIGS. 1A-1C show a non-contact actuator for use with the position sensor of the present invention. The design of this actuator is conventional except for the windings added to the end of each permanent magnet; therefore, other parts of the actuator will not be described in detail. The design of the actuator and the armature is described in U.S. Pat. Nos. 4,833,351 and 4,443,743. Of course, other actuators could be used instead of the one set forth here. 
     FIG. 1A shows armature  1  of the actuator. FIG. 1B shows permanent magnet bracket  3  of the actuator. Permanent magnet bracket or backiron  3  is formed of permanent magnet portions  5  and  7 , which have sensor excitation windings or coils  9  and  11 , respectively, disposed on end portions thereof. FIG. 1C shows armature  1  and permanent magnet backiron  3  assembled into actuator  13 . 
     Armature  1  has two sets of wires or windings  15  and  17 , which are wound 90° out of phase from each other. This allows actuator  13  to have an x-y movement. 
     As noted above, windings  9  and  11  are wound around both the top and bottom permanent magnet portions, namely, permanent magnet portions  5  and  7 , respectively. As will become more readily apparent below, this winding configuration renders the sensor insensitive to z motions, thus allowing for a variable air gap in the motor. Therefore, the final result is an accurate, low weight, low volume, non-contact x-y position sensor insensitive to z motions. 
     FIG. 2 is a schematic block diagram of the electronics used in the position sensor. In FIG. 2, the schematic is blocked into functions purely for ease of explanation. The electronics of the position sensor do not have to be physically provided in the form of these blocks, but instead can be provided in any suitable level of integration, from purely discrete electronic elements to a single integrated circuit or any form of integration in between, and can be implemented in many ways by those skilled in the art who have reviewed this disclosure. First, the functions of the blocks will be set forth; then, one possible circuit configuration for implementing the functions will be set forth in detail. 
     Sensor excitation windings  9  and  11  are excited with a 20-kilohertz (or other suitable frequency) sine wave excitation signal generated by the sine generator with buffer of BLOCK 1  and output through terminal TERM 1 . In actuator  13 , sensor excitation windings  9  and  11  and armature windings  15  and  17  together act as a transformer which transformer-couples the 20-kilohertz sine wave to actuator windings  15  and  17  as the relative displacement between sensor windings  9  and  11  and armature windings  15  and  17  goes back and forth across the range of relative movement. The signal which is transformer coupled to armature windings  15  and  17  is a 20-kilohertz sine wave modulated signal whose amplitude varies with the relative position between permanent magnet portions  5  and  7  and armature windings  15  and  17 . Because sensor windings  9  and  11  are provided both above and below armature  1 , sensor windings  9  and  11  induce z components which cancel each other out. Thus, the detected position is z-invariant and so is not affected by fluctuations in the air gap. 
     This modulated signal is retrieved through terminals TERM 2  and TERM 3  of BLOCK 2  and signal-conditioned in BLOCK 2 , BLOCK 3 , and BLOCK 4 . BLOCK 2  is a differential amplifier which senses the voltage on armature windings  15  and  17 . BLOCK 3  is a band-pass or high-pass filter which filters out any low frequencies (e.g., less than 200 hertz) and any high frequencies (e.g., greater than 100 kilohertz) which appear on armature windings  15  and  17 . The high frequencies are filtered out for noise purposes, and the low frequencies are filtered because armature windings  15  and  17  also have low-frequency voltages which are associated with the actuator and not the sensor. In fact, a filter can be designed which can pass only the 20-kilohertz excitation frequency. The latter type of filter is better if cost is not an overriding concern, but for demonstration purposes and when cost has to be kept down, the band-pass or high-pass filter of BLOCK 3  is sufficient. 
     BLOCK 4  is a demodulator circuit which takes the modulated 20-kilohertz signal and demodulates it, with reference to the same excitation signal output on TERM 1 , into a DC signal which is directionally sensitive and whose amplitude is proportional to the relative position between permanent magnet portions  5  and  7  and armature windings  15  and  17 . The signal developed at the output of BLOCK 4  is significant because it represents the system position. 
     BLOCK 5  through BLOCK 8  represents the actuator controller. BLOCK 5  is simply a high-impedance buffer circuit (with gain equal to one) used to keep the position output of BLOCK 4  from getting loaded. BLOCK 6  is a proportional-integral-derivative (PID) compensation circuit which develops signal P proportional to position, signal I which is the integral of position, and signal D which is the time derivative of position. 
     These three signals are summed together using the summer circuit of BLOCK 7 . The output of BLOCK 7  is a PID-compensated position signal which commands the current amplifier of BLOCK 8 , which outputs signals over terminals TERM 4  and TERM 5  to drive the actuator. 
     In this particular arrangement, the signals output from BLOCK 8  are input to the same two connections on armature windings  15  and  17  as the two connections going into terminals TERM 2  and TERM 3  of BLOCK 2  which senses position. Thus, the position sensor drives the actuator to a null position. On the other hand, many applications require that a position command be added to the circuit so that the actuator can be controlled to any commanded position. To do this, one simply has to add another adder circuit such as that of BLOCK 7  (shown in FIG. 2 as BLOCK 7 A with a dashed outline) between BLOCKS and BLOCK 6 . The adder circuit of BLOCK 7 A, if used, may include inverters; those skilled in the art who have reviewed this disclosure will readily understand the use and placement of such inverters. The inputs to the adder circuit of BLOCK 7 A are the output of BLOCK 5  and position command POS. 
     Constructions of BLOCK 1  through BLOCK 8  will now be described in detail with references to FIGS. 3-6. 
     FIG. 3 shows one possible construction of BLOCK 1 . BLOCK 1  is based on ICL8038 circuit  101 . The first pin of this circuit is connected to the wiper arm of 100K potentiometer  103 , which has one terminal connected through 10 kΩ resistor  105  to −15V source  107  and the other terminal connected to +15V source  109 . These sources are also connected through 100K potentiometer  111  and 10 kΩ resistor  113 . The second pin is used as an output, as will be explained below. The third pin is allowed to float. The fourth and fifth pins are connected through 8.25 kΩ resistors  115 ,  117  to the terminals of 1K potentiometer  119 , whose wiper arm is connected to +15V source  109 . The sixth pin is connected to +15V source  109 . The seventh and eighth pins are connected to each other. The ninth pin is connected to +15V source  109  through 10 kΩ resistor  121 . The tenth pin is connected to −15V source  107  through 0.033 μF capacitor  123 , while the eleventh pin is connected to this source directly. The twelfth pin is connected to the wiper arm of potentiometer  111 . The thirteenth and fourteenth pins are allowed to float. 
     The second pin of circuit  101  is connected through 5.11 kΩ resistor  125  to the negative input (fifth pin) of PA 10 A amplifier  127 . The −VS (sixth) pin of amplifier  127  is connected to −15V source  129 . This source is also connected through 0.1 μF capacitor  131  to ground  133  and through 8.7 μF capacitor  135  to ground  137 . The positive input (fourth pin) is connected between capacitor  1   5   131  and ground  133 . +15V source  139  is connected to the +VS (third) pin directectly, through 0.1 μF capacitor  141  to ground  143  and through 8.7 μF capacitor  145  to ground  147 . The output from the output (first) pin is the signal output at TERM 1 . This output is also fed back through resistor  149  to the CL− (eighth) pin, through resistor  151  to the CL+ (second) pin and through 15 kΩ resistor  153  and 270 pF capacitor  155  in parallel to the negative input. The FO (seventh) pin is allowed to float. TERM 1  and ground  157  are connected to BLOCK 4  in a manner to be described below. 
     FIG. 4 shows possible constructions for BLOCK 2 , BLOCK 3 , BLOCK 4 , and BLOCK  5 . 
     BLOCK 2  is based on INA105AM circuit  201 . The first (reference) pin of this circuit is connected to ground  203 . The second and third (−In and +In) pins receive the inputs from the actuator applied at TERM 2  and TERM 3 . The fourth (−Vcc) pin is connected directly to −15V source  205  and through 1 μF capacitor  207  to ground  209 . The fifth and sixth (Sense and Output) pins are connected to form the output to BLOCK 3 . The seventh (+Vcc) pin is connected to +15V source  211  and through 1 μF capacitor  213  to ground  215 . The eighth (NC) pin is allowed to float. 
     BLOCK 3  is based on LF356H amplifier  301 . The negative input of amplifier  301  receives the output from the fifth and sixth pins of circuit  201  of BLOCK 2  through 3300 pF capacitor  303 . The positive input of amplifier  301  is connected to ground  305 .The V+ pin of amplifier  301  is connected directly to +15V source  307  and through 0.1 μF capacitor  309  to ground  311 . The V− pin of amplifier  301  is connected directly to −15V source  313  and through 0.1 μF capacitor  315  to ground  317 . The BAL pins of amplifier  301  are allowed to float. The output of amplifier  301  is supplied to BLOCK 4  in a manner to be described below and is also fed back to the negative input through 100 kΩ resistor  319  and 150 pF capacitor  321  in parallel. 
     BLOCK 4  includes AD 790  amplifier  401 . Amplifier  401  receives the output from TERM 1  through its positive input (second) pin. The negative input (third) pin is grounded at grounds  157  and  403 . The first (+VCC) pin is connected directly to +15V source  405  and through 0.1 μF capacitor  407  to ground  409 . The fourth (−VCC) pin is connected directly to −15V source  411  and through 0.1 μF capacitor  413  to ground  415 . The fifth ({overscore (LATCH)}) pin is connected through 10 kΩ resistor  411  through +Vcc source  413 , while the Vlogic (eighth) pin is connected directly to source  413 . The sixth (GND) pin is connected to ground  415 . 
     The output from the seventh pin of amplifier  401  is applied to the first (IN 1 ) pin of DG201A circuit  417 . The IN 2 -IN 4  (sixteenth, ninth, and eighth) pins and the S 1 -S 4  (third, eleventh, fourteenth, and sixth) pins are grounded to ground  419 . The D 2 -D 4  (fifteenth, tenth, and seventh) pins are allowed to float. The V+ (thirteenth) pin is connected directly to +15V source  421 , while the V− (fourth) pin is connected directly to −15V source  423 . Sources  421  and  423  are connected through 0.1 μF capacitors  425  and  427 , respectively, to ground  429 . The GND (fifth) pin of circuit  417  is also connected to ground  429 . 
     The output of the second (D 1 ) pin of circuit  417  is used in the next stage of BLOCK 4 , which is based on AD840SQ circuit  431 . Circuit  431  receives, at its fourth (−VIN) pin, the output of amplifier  301  of BLOCK 3  through 20 kΩ resistor  433 . The output of amplifier  301  is also passed through 20 kΩ resistor  435 , connected with the output of the D 1  pin of circuit  417 , and applied to the +VIN (fifth) pin of circuit  431 . The third and twelfth (BAL) pins are allowed to float. The eleventh (+VCC) and sixth (−VCC) pins are connected to +15V source  436  and −15V source  438  respectively, and are connected through 0.1 μF capacitors  437  and  439 , respectively, to ground  441 . The output from the VOUT (tenth) pin is fed back to the −VIN (fourth) pin through 20 kΩ resistor  443 . This output is also applied to BLOCK 5  in a manner to be described below through 0.7 mH inductor  445 . Between inductor  445  and BLOCK 5 , part of the output is picked off and sent through 1.0 μF capacitor  447  to ground  449 . 
     BLOCK 5  is based on LF356H amplifier  501 . The output of BLOCK 4  is applied to the positive input of amplifier  501 . The V+ pin is connected directly to +15V source  503  and through 0.1 μF capacitor  505  to ground  507 . The V− pin is connected directly to −15V source  509  and through 0.1 μF capacitor  511  to ground  513 . The BAL pins are allowed to float. The output is fed back to the negative input and is also supplied to BLOCK 6 . 
     FIG. 5 shows possible constructions for BLOCK 6 , BLOCK 7 , and BLOCK 8 . If BLOCK 7 A is used, BLOCK 7 A can have a construction similar to that shown for BLOCK 7 . 
     In BLOCK 6 , the output of BLOCK 5  is split three ways. The output is applied through 3.0 μF capacitor  601  to the negative input of LF356H amplifier  603 . The V+ pin of amplifier  603  is connected directly to +15V source  605  and through 0.1 μF capacitor  607  to ground  609 . The V− pin of amplifier  603  is connected directly to −15V source  611  and through 0.1 μF capacitor  613  to ground  615 . The positive input of amplifier  603  is connected to ground  617 . The output of amplifier  603  is fed back to its negative input through 5.11 kΩ resistor  619 . The BAL pins of amplifier  603  are allowed to float. 
     The output of BLOCK 5  is also applied through 15 kΩ resistor  621  to the negative input of LF356H amplifier  623 . The V+ pin of amplifier  623  is connected directly to +15V source  625  and through 0.1 μF capacitor  627  to ground  629 . The V− pin of amplifier  623  is connected directly to −15V source  631  and through 0.1 μF capacitor  633  to ground  635 . The positive input of amplifier  623  is connected to ground  637 . The output of amplifier  623  is fed back to its negative input through 4.4 μF capacitor  639 . The BAL pins of amplifier  623  are allowed to float. 
     The output of BLOCK 5  is further applied through 15 kΩ resistor  641  to the negative input of LF356H amplifier  643 . The V+ pin of amplifier  643  is connected directly to +15V source  645  and through 0.1 μF capacitor  647  to ground  649 . The V− pin of amplifier  643  is connected directly to −15V source  651  and through 0.1 μF capacitor  653  to ground  655 . The positive input of amplifier  643  is connected to ground  657 . The output of amplifier  643  is fed back to is negative input through 1.1 MΩ resistor  659 . The BAL pins of amplifier  643  are allowed to float. 
     The outputs of amplifiers  603 ,  623 , and  643  are passed through 20 kΩ resistors  661 , 663 , and  665 , respectively, and, in BLOCK 7 , are connected and applied to the negative input of LF356H amplifier  701 . The V+ pin of amplifier  701  is connected directly to +15V source  703  and through 0.1 μF capacitor  705  to ground  707 . The V− pin of amplifier  701  is connected directly to −15V source  709  and through 0.1 μF capacitor  711  to ground  713 . The positive input of amplifier  701  is connected to ground  715 . The output of amplifier  701  is fed back to its negative input through 20 kΩ resistor  717 . The BAL pins of amplifier  701  are allowed to float. 
     The output of amplifier  701  is output to BLOCK 8 . In BLOCK 8 , this output is connected through 51.5 kΩ resistor  801  to the negative input (fifth pin) of PA 10 A amplifier  803 . The −VS (sixth) pin of amplifier  803  is connected directly to −15V source  805 , through 0.1 μF capacitor  807  to ground  809 , and through 8.7 μF capacitor  811  to ground  813 . The positive input (fourth pin) of amplifier  803  is connected between capacitor  807  and ground  809 . The +VS (third) pin of amplifier  803  is connected directly to +15V source  815 , through 0.1 μF capacitor  817  to ground  819 , and through 8.7 μF capacitor  821  to ground  823 . The output of the output (first) pin of amplifier  803  is output on TERM 4 . The output is also fed back through resistor  825  to the CL− (eighth) pin of amplifier  803 , through resistor  827  to the CL+ (second) pin of amplifier  803  and through 1 μF capacitor  829  and 1.0 MΩ resistor  831  to the negative input of amplifier  803 . TERM 5  is connected through 2.5 kΩ resistor  833  to the output of BLOCK 7  which has passed through resistor  801  and is also connected through 1.0 Ω resistor  835  to ground  837 . The FO (seventh) pin of amplifier  803  is allowed to float. 
     As noted above, armature  1  has two sets of windings  15  and  17 , one for each axis. To control both sets of windings, it is necessary to implement the circuit of FIG. 2 for each axis. However, a single set of excitation windings  9  and  11  on permanent magnet bracket  3  suffices for both axes. Thus, with a single set of windings  9  and  11 , the x and y positions can be sensed and fed back to control these positions. 
     FIGS. 6A and 6B are graphs of data showing the linearity of the sensor according to the present invention. FIG. 6A shows the sensor linearity in the full range of movement of one embodiment, namely, ±1 cm (±10 mm). FIG. 6B is a magnification of the portion of the graph of FIG. 6A for position values between 0 and ±2 mm. As can be seen, the raw data and the linear fit are nearly identical. 
     The sensor according to the present invention has several advantages over the previous techniques used for non-contact position sensing. First, the sensor is highly linear over the normal operating range of ±1 cm. Second, the sensor is insensitive to permanent or fluctuating magnetic fields. Third, the sensor is co-located with the actuator geometry. Fourth, the sensor uses the actuator windings for sensor coils and therefore requires no extra volume. Fifth, the sensor is insensitive to motions in the z axis, thus allowing for a variable air gap in the actuator. 
     The non-contact sensor according to the present claimed invention employs existing actuator geometry and windings to perform its function, while not requiring significant extra volume for the sensor. 
     While a preferred embodiment of the present invention has been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the present invention. For example, any or all of the blocks shown in FIG. 2 can be implemented in many ways. There are many different ways to implement buffer circuits, adder circuits, comparator circuits, current amplifiers to drive motors, differential amplifiers, demodulator circuits, and analog switches. Also, the present invention can be implemented as a position sensor only with no actuator capability. Another embodiment of this invention can use a square wave, a triangular wave, or any other suitable wave in place of the twenty-kilohertz (or other frequency) sinusoidal excitation signal which goes to the winding on the permanent magnet.