Magnesense Sensorless Position Measurement

What’s New: We sense just one parameter of solenoid operation, then infer motion from that one parameter plus added knowledge about the system – available knowledge that is typically ignored in controllers.

Why: The goal is to infer the position and velocity of a solenoid from electrical drive signals without using sensors or sensor wiring. This inference is used in servo control for soft landing, thus reducing impact noise, increasing service life, detecting incipient actuator problems, and conserving energy. The simplified hardware is cheaper and more reliable.

How: In our embodiment, we drive a two-sided solenoid with Pulse Width Modulation (PWM). For the purposes of this patent, consider each side of our solenoid as a separate solenoid connected by a pair of wires. We measure the drive current in the wires, and we control the pulse width modulation (PWM) driving the wires. The power supply voltage is known because it is regulated or measured. The PWM setting is known, through time, because it is the output decided upon, step-by-step, by the controller. Because the supply voltage and PWM are both known, the average drive voltage for each time step is known in advance: it need not be measured.[1] By further inference, the total series resistance of the solenoid coil plus drive circuitry is known and updated from one operation cycle to the next, with correction for temperature drift, maintaining an accurate resistance value over time. Thus, using frequent measurements only of electric current, the system determines the resistive voltage loss and the net voltage that remains to cause a change in flux linkage through the solenoid winding. The cumulative change in flux linkage is tracked through time, yielding total flux linkage. Combining the computed flux linkage and measured current with prior knowledge of solenoid characteristics, the controller calculates the position, velocity, and magnetic force of the solenoid armature, moment by moment, using sophisticated software to replace hardware. This information is exactly what a controller needs in order to control magnetic force and thereby control position and velocity. Our “State Space Controller” describes this next step of overall control, incorporating the results of this “Sensorless Position Measurement” patent.

[1] It is difficult to measure a PWM voltage in a traditional sampled data system, since the signal is a pulse train with high frequency transients. If lowpass filtering is used to get rid of the worst “hash” prior to voltage sampling, then the filtered signal is time-delayed, which causes degraded control. Our technique completely avoids this measurement difficulty by eliminating the need for the measurement.