Lowest Total Servo Phase Delay
In the discussion of the PIV compensator, you have seen how quickly and accurately the SSt produces torque after it calculates the required amount—this is the Torque Response Time. Now let’s examine the Total Servo Phase Delay (TSPD)—the total time from the moment the position feedback is read to the time that torque begins to be updated.
TSPD is not only dependent upon the time it takes for each individual step in the servo process to be completed, but it is also influenced by the degree of synchronization of these steps. If elements of the system operate asynchronously, as is the case with the traditional servo architecture (where one or more servo loops are closed by the controller and the remainder by the amplifier), this lack of synchronization will cause uncertainties which, on average, will drastically increase the TSPD.
Teknic chose a step and direction interface architecture not only for noise immunity and as a convenience for upgrading from stepper motor systems, but also because this architecture allowed the SSt servo drives to close all the servo loops with one processor. Eliminating the delays and uncertainties associated with the asynchronism of the traditional architecture is a big advantage of any skillfully implemented step and direction servo.
Teknic went several steps further to reduce TSPD. We dedicated a DSP to each axis, designed a true digital torque controller and developed proprietary algorithms optimized specifically for reducing TSPD. The result of these efforts can be seen in the figures below. Low TSPD, combined with improved Torque Response Time, strengthens the weak links found in other servos and results in state-of-the-art servo bandwidth.
-
- Figure 1: Response time in traditional servos
The traditional servo architecture (i.e., position controller with separate velocity/torque amplifier) has many delays and uncertainties because of asynchronism between the servo controller and the amplifier, and because one processor controls multiple axes.
-
- Figure 2: Response time with position mode, sinewave commutated servo drive
A well designed step and direction interface, sinewave-commutated drive eliminates many of the problems associated with the traditional architecture, but still has delays due to sinewave lookup and current loop delay.
-
- Figure 3: Response time in the SSt servo drive
SSt servo drives reduce TSPD to a minimum. Combined with rapid torque response, this results in excellent servo performance.