The components of the basic indexing drive system form two information loops. The

position loop is a closed-loop which consists of the controller, drive, motor, and feedback

device. The controller, continually compares motor position, derived from the feedback

pulses received from the drive, with the desired position calculated within the controller.

Any difference between calculated position and actual position results in a corrective

command signal being sent to the drive. The drive then corrects motor position by

adjusting its velocity loop. The objective of the position loop is to keep the actual position

equal to the commanded position.

The velocity loop is also a closed-loop system. The drive continually compares motor

velocity, derived from the feedback device, with the magnitude of the command signal

generated by the controller. Any difference causes the drive to alter the current to the

motor, which in turn changes the motor velocity. The objective of the velocity loop is to

keep actual velocity equal to commanded velocity.

The position loop and velocity loop are independent loops, but the controller uses the

velocity loop to achieve movement to the desired position. When data is received by the

controller specifying a movement, the controller calculates the time required to accelerate

to maximum speed and to decelerate from maximum speed. It then calculates the time at

maximum speed necessary to complete the movement. This information is then

transmitted to the drive by the controller.