SSt-3100-TCx
DSP Brushless Servo Amplifier
The SSt-3100-T is a high bandwidth, fully digital, DSP vector brushless (or brush) torque amplifier that accepts ±10V analog torque commands. Upgrading a conventional current loop "torque" amplifier to an SSt-3100-T will improve tracking, smoothness and settling time.
These performance improvements are possible because conventional torque amplifiers (six-step/trapezoidal or sinewave type) all share a little-known flaw. They servo control current, not torque. Systems that use these amplifiers actually run open-loop with respect to torque! Servo controlling torque requires the synchronized control of all motor phases with respect to the magnetic field of the motor’s rotor. Conventional torque amps control each motor phase with a separate, dissociated current loop servo. This scheme ignores the fact that the motor phases are tied together and thus interdependent. Furthermore, small errors in each phase can combine to create a large vector (i.e., torque) error.
With the conventional method of control, where the torque runs open-loop, torque errors are corrected by the slower velocity and position loops. Errors in motor torque cause velocity and position to deviate from the target, and it is only after these deviations occur that the servo controller can see and react to the error. The controller compensates for the torque error in the only way it can, by adjusting the torque command. Iteratively, the controller eventually converges on a torque command that resolves the error, if there are no further disturbances or trajectory changes. But trajectories do change (acceleration changes to deceleration, etc.) and disturbances occur (imperfect mechanics, other axes moving, etc.), so torque dithering occurs frequently during a move, impairing tracking accuracy, settling time and smoothness. As applications become more demanding, the impairment becomes more and more pronounced.
The SSt-3100-T employs a proprietary torque control method: sinewave, vector feed-forward with DQ decoupling. This method constantly measures all the variables required to accurately calculate the true torque output (and the out-of-phase currents which only heat the motor) and continuously works to servo the torque to the commanded value and the out-of-phase currents to zero. So in the SSt, the torque is truly under closed-loop control. The controller no longer needs to dither the torque command to compensate for torque errors—these errors are preemptively eliminated. Removing the dithering process smoothes axis motion. Additionally, eliminating most of the error in the delivered torque improves axis tracking and settling. So, as a drop-in replacement for a conventional torque amp, the closed-loop SSt-3100-T improves motion quality on all axes, especially demanding ones.