Velocity Feedback Slope and Dead Zone

The feedback component of the motor drive is based on the difference between the requested and the actual (tachometer) velocities.

The lower transfer graph demonstrates that the output is essentially linear, with a velocity error, except for the possible inclusion of a deadzone around 0.

• The slope is designated as eslope on the diagram.

  Typically, the eslope is fairly large in order to achieve a tight velocity servo however, this large value also magnifies the A/D errors.

• The deadzone, between -V and V , is used to minimize motor "chatter" that can result from uncertainty in the LSB of the tachometer voltage samples.

  A small inactive region (dead zone) in the feedback loop, typically two 1 or 2 T-units, eliminates the problem with A/D errors.

The sum of the nominal and feedback terms is clamped within the -100 ... +100 drive unit range and is applied to the D/A converter to produce the motor drive voltage.

Note that the nominal term does not need to be calculated with great accuracy. In traditional, hard-wired, analog velocity servos, this term is unused. In the digital servo, the term:

• Provides and easy way to take motor stall currents into account.
• Helps reduce the mean error that appears in the feedback term necessary to maintain a given velocity.

Every feedback system requires a non-zero error component to maintain control of a non-equilibrium position. By predicting the equilibrium drive requirements, the nominal term helps to ensure that the mean steady-state value of the velocity error is 0.