Contents

About A/D Velocity PID Mode

A/D Velocity PID mode makes adjustments to the speed of a motor based on the feedback from an A/D input.  In working with robotics motion control, you need to be able to account for inertia, resistance, and other variables.  Factored together, the terms in the PID formula determine how much PWM is applied over time to reach and maintain the desired position.  Careful selection of the PID gain constants can minimize oscillation and overcompensation. 

The Moto 1.0 Application in A/D Velocity PID mode.

You can manipulate 12 variables in A/D Velocity PID Mode:

Set Point

This mode affects the duty cycle of the PWM signal, which effectively varies the voltage to the motor and varies the speed.  The duty cycle is set using the throttle slider.  The center point of the slider is neutral.  Sliding the set point above the center point moves the motor in one direction; sliding it below the center point moves the motor the other direction.  The stop button returns the throttle to the neutral position and stops the motor. 

P Term

PID Proportional Term — gain constant for the current error. 

I Term

PID Integral Term — gain constant for the accumulated error.  The longer the position varies from the set point, the greater the error. 

D Term

PID Derivative Term — gain constant for the change in the error.  This term affects how quickly the system responds to new input. 

Input Offset

The input offset is added to the encoder input to allow a centering offset adjustment.  With A/D providing the input in this mode, this value is used to set the neutral position.  For the motion control algorithms to work properly, you should adjust this value so that the A/D reading is as close to zero as possible when the set point slider is in the neutral position (PWM off).  The value will then rise above zero in one direction of rotation and fall below zero in the other direction. 

PWM Rail

This setting can vary between 0 and 32767 and puts a limit on the maximum PWM output allowed.  You can use this value to:

• Set a maximum speed
• "Window" the PWM output voltage to keep it within your motor's ratings

The default for this value is no limit or 32767 (the PWM maximum). 

Period

The period is how many timer increments (0.1 ms) pass between A/D measurements.  This A/D measurement represents a velocity value since the back-EMF voltage is proportional to motor velocity.  The range of the values depend on the motor itself and the motor supply voltage. 

Latency

Latency refers to how many time increments (0.1 ms) should be used to float the motor windings before taking the Back-EMF measurement at the end of each period.  Because the A/D reading takes some time, this latency can usually be short, typically between 1 and 5 increments.  If this latency is too large, the motor starts to slow down before the reading is taken. 

PWM Frequency

This value represents the PWM frequency in Hz.  The default is 39,062 Hz, which is a reasonable starting point.  To get maximum efficiency from your system, we recommend a frequency above 20 kHz.  This value is out of the audible range (for humans) so it also provides quiet motor operation.  When entering a new PWM frequency, you may not get the exact value you entered.  The controller generates a fixed number of different PWM frequencies and the program must select a frequency that is the closest match. 

Invert Input

Checking this box inverts the A/D input.  This enables you to match the signs of the PWM output and A/D input.  A positive output should produce a positive input and a negative output should produce a negative input.  If the output and input have opposite signs, the motion control algorithms agressively seek the farthest position from the set point, rather than trying to settle into it. 

Invert Output

Checking this box inverts the PWM output signal.  This reverses the motor direction.  You may want to reverse a motor when using two motors on a differential-drive robot so that positive PWM values mean forward motion for both motors. 

Auto Brake

Enable this feature if the brake needs to be applied in the H-Bridge you are using to "float" the motor for a Back-EMF reading.  The Acroname 3A Back-EMF Bridge requires this check for proper operation in Back-EMF modes. 

The basic logic flow of the A/D Velocity PID mode of operation.

Related Links:

Articles: PID Motion Control Basics

Concepts: Description of Pulse Width Modulation (PWM)

Articles: Back-EMF Motion Control Feedback