April 10, 1997

µC uses simple tool for angle measurements

Lev Freidin, Patriot Sensors & Controls Corp, Simi Valley, CA

The circuit in Figure 1 uses a 2V, 2250-Hz resolver as an angle sensor. The excitation for the resolver is irregular, using two 90°-phase-shifted, 900-Hz signals. (The stator is the primary winding.) The output signal’s phase changes from 0 to 360° with shaft rotation. The signal amplitude is constant, provided that the stator’s magnetic field (proportional to input current) remains constant. Because the stator’s impedance changes during shaft rotation, it’s preferable to use constant-current excitation. The phase shift equals the ratio of the I/O delay to the period.

To obtain the resolver’s excitation frequency, counter IC₂ divides the frequency of the E signal from mC IC₁ down to the reference frequency. You thus know the period, and the reference frequency is 1,843,200÷2048=900 Hz (the lower the frequency, the higher the resolution). Exclusive-OR circuit IC_3A provides the 90° -phase-shifted signal. R₁ and R₂ provide the excitation in the form of a nearly constant current. C₁ and C₂ filter the high-frequency-component spikes from the square-wave excitation. To obtain a high excitation impedance, the resonant frequency of the tank comprising C₁, C₂, and the resolver’s input impedance must be at least triple the reference frequency.

C₃ and the rotor impedance form a resonant tank for the reference frequency. IC_3B and IC_3C form the amplifier for the output signal. R₃ sets the digital gates into an active analog mode. R₄-C₄ and R₅-C₅ smooth the signal for a monotonic presentation to the mC. To measure the phase shift, you use the two input-capture timers in the M68HC811E2 mC or any other mC having two input-capture timers. Listing 1 shows how the method calculates the angle. In this case, 360° corresponds to 2048 counts for 11-bit resolution.

Nonlinearity amounts to approximately 1°. The maximum deviation from a straight-line transfer function (from 0 to 360°) occurs at 22.5(2n+1)°, where n is the number of bits (resolution). Increasing the excitation impedance (by lowering the values of C₁ and C₂) improves linearity, but in doing so, be careful to check the signal fronts for monotonicity. You could also use software corrections for linearity. The minimum recommended value for R₁ and R₂ is 1.5 kilohms. (DI #2010)

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