The test setup in Fig 1 determines the ac frequency response of a mechanical system. In this case, the mechanical system includes a dc motor, switch-mode driver, and the motor's mechanical load.
The test setup in Fig 1 determines the ac frequency response of a mechanical system. In this case, the mechanical system includes a dc motor, switch-mode driver, and the motor's mechanical load.
The HP3562A dynamic signal analyzer generates a 1V p-p sinusoid whose frequency sweeps from 1 Hz to 1 kHz. The signal also has a 2V-dc offset. This signal feeds an LM319 comparator, IC1, functioning as a PWM, IChas some hysteresis for noise rejection. An HP8116A signal generator feeds a 20-kHz, 0 to 5V triangular wave to the PWM's other input.
The signal generator's frequency determines the PWM's frequency. Further, the dc level of the signal analyzer's sinusoid together with the amplitude of the signal generator's triangular wave determine the quiescent duty cycle of the PWM. Set this quiescent point so that the mechanical system is always rotating, thus avoiding nonlinear operation. The analyzer's sinusoidal sweep changes the PWM's duty cycle linearly around this quiescent point.
Fig 1 shows the PWM feeding a motor driver via a 75447. The motor driver is only an example. Use your application's motor driver in your setup. However, you must have some means of monitoring your motor's current. Note the 1Í resistor, which develops a voltage signal proportional to the motor's current. Differential amplifier ICand lowpass filter ICrecover the average value of the current through the motor. The average current through the motor is approximately
where àis the angular acceleration of the motor, Iis the fixed viscous friction loss in the system, Kis the torque constant of the motor, and J is the total inertia of the mechanical system, including the motor.
The mechanical system acts as a filter for the driving signal and responds to the driving signal's average energy. The lowpass filter's cutoff frequency must filter out the PWM's frequency but must be high enough to pass the mechanical system's response, which is usually less than 100 Hz. The components associated with ICin Fig 1 yield an 894-Hz cutoff frequency.
The filter's output feeds back to the signal analyzer. The analyzer can produce a graph of motor current vs the frequency of the signal modulating the PWM's duty cycle.
You can interpret this graph as the power spectrum of DeltaIMOTOR/DeltaPWM DC in the frequency range from 1 Hz to 1 kHz, where δIis the change in the average current through the motor and DeltaPWMis the change in the duty cycle of the PWM.
This expression corresponds to ào(Í)/ài(Í)]AC, where ào(Í) is the angular acceleration of the driven element and ài(Í) is the angular acceleration of the input (both in the Fourier domain).
Therefore, measuring Iyields the frequency response of Iand the motor's angular acceleration. You can deduce velocity and position from acceleration.