Servos & Simulation's Blog

Why is Frequency Response of the Servo Critical to Motion Base Platform Design?

Posted by Rachel Baker on Jan 24, 2017 5:26:57 PM

Why is frequency response of a servo critial to motion base design? Why is it even required? The fact is, the better the frequency response of the servo, the better performance of the motion base platform. 

In the simpliest terms, the motion base must follow the input properly. So if the motion base is to follow the commands from a Host computer in a full-flight simulator, and the pilot response is about 2Hz or less, the motion base needs to have >5˚ at 2Hz of phase shift so that the motion base accurately tracks the signal from the Host Computer flight model. It is required as the phase loss of the motion base affects the stability of the pilot's control loop.

The Engineering Team at Servos & Simulation, Inc., analyzed the performance of one of our motion base designs. The plots for the performance show the maximum position as a function of frequency, the maximum velocities as a function of frequency, and the maximum accelerations as a function of frequency. The servo has an inner rate loop using the encoder on the motor for feedback allowing the servo to have a bandwidth of more than 40 Hz. The outer loop is a position loop with a bandwidth of 4 Hz. The position loop gain is limited by the gain available in the drive amplifier (power amplifier that drives the motor). Servos’ engineers add electronics to the servos and increase the gain by at least a factor of 4 which increases the position loop bandwidth to 16 Hz.

The structure of the motion base is stiff enough that it does not limit the bandwidth of the position loop. This includes the motor/gearbox. Since the structure is stiff, the bandwidth does not need to be limited to less than 16 Hz.  The backlash in the gearboxes has been reduced to almost zero.  This is necessary since at high frequency the motor movement is quite small. If the gearboxes had any significant backlash, the gearbox output shaft would not move.  Servos’ engineers reworked the stock gearmotors to successfully remove the backlash from the gearmotors.

4hz displacements.png4Hz Velocities.png

Another set of plots shows the motion base performance with a 16 Hz position loop bandwidth.  As can be seen from the plots, the high frequency response is much better with a 16 Hz bandwidth than with a 4 Hz bandwidth.

16Hz Displacement.png16Hz Velocities.png16Hz accleration.png

For both cases, the roll velocity is limited to 16.47 deg/sec, and the roll acceleration is limited to 230 deg/sec^2.  Pitch and yaw are similarly limited.  The Longitudinal velocity is limited to 15 inches/second and the acceleration is limited to 188 inches/second^2.  Lateral and vertical are similarly limited.   

The velocity and acceleration limits can be increased within limits by changing the motors and gear ratios in the gear boxes. Below is video of this motion base platform in action. If you would like more information on servo frequency response, please email us.

710-6-2000 motion.gif

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Topics: motion control, frequency response of a motion base platform servo, motion platform software