A noncontact position sensor has been designed for use with a specific two-dimensional linear electromagnetic actuator. To minimize the bulk and weight added by the sensor, the sensor has been made an integral part of the actuator: that is to say, parts of the actuator structure and circuitry are used for sensing as well as for varying position.
The actuator (see Figure 1) includes a C-shaped permanent magnet and an armature that is approximately centered in the magnet gap. The intended function of the actuator is to cause the permanent magnet to translate to, and/or remain at, commanded x and y coordinates, relative to the armature. In addition, some incidental relative motion along the z axis is tolerated but not controlled. The sensor is required to measure the x and y displacements from a nominal central position and to be relatively insensitive to z displacement.
The armature contains two sets of electromagnet windings oriented perpendicularly to each other and electrically excited in such a manner as to generate forces in the x,y plane to produce the required motion. Small sensor excitation coils are mounted on the pole tips of the permanent magnet. These coils are excited with a sine wave at a frequency of 20 kHz. This excitation is transformer-coupled to the armature windings. The geometric arrangement of the excitation coils and armature windings is such that the amplitudes of the 20-kHz voltages induced in the armature windings vary nearly linearly with x and y displacements and do not vary significantly with small z displacements. Because the frequency of 20 kHz is much greater than the maximum frequency characteristic of the actuation signals applied to the armature windings, there is no appreciable interference between actuator and sensor functions of the armature windings.
The voltages across the armature windings are fed as inputs to the circuitry depicted in simplified form in Figure 2. First, the voltages are band-pass filtered at the 20-kHz sensor excitation frequency to minimize lower frequency actuation components and higher-frequency noise components. The filtered voltages are processed through a differential amplifier and a demodulator to obtain voltages proportional (in both magnitude and sign) to the x and y displacements. These voltages are fed, through a buffer, as inputs to a proportional + integral + derivative (PID) control circuit. The output of the PID controller is summed with a position- command voltage to obtain a control signal that is fed as input to a current amplifier. The output of the current amplifier, characterized by frequencies much below 20 kHz, is applied to armature coils to control the x and y displacements.
This work was done by David E.Howard and Dean C. Alhorn of Marshall Space Flight Center.
This invention has been patented by NASA (U.S. Patent No. 6,246,228). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
MSFC Commercialization Assistance Lead
Refer to MFS-31218.