*By Clarence Klassen*

**Adjustable Speed Drives (ASD)**need to have the at least the torque and velocity regulators tuned. The tension regulator if used will also need to be tuned. Tuning can be done with Auto-Tune or done manually. Tuning is a process of setting regulator gains to give a desired response. In web handling, tuning applies a

**(sudden increase) to the setpoint. The desired response is a curve that rises rapidly and then more slowly approaches the new setpoint. The desired response follows an exponential curve. The diagram above shows several typical responses to a step.**

*step function*

*Less desirable responses include:**Instability**Oscillatory*(hunting)*Over-Shoot**Accuracy**Steady State Error**Dead Time*

There are industry accepted measurements and calculations that can be used to describe the undesirable traits and to characterize the desired exponential response.

The only useful measurement for

*instability*and

*oscillatory*systems is the

*Period*(seconds) or

*Frequency*(Hz= 1/Period),

*Angular Frequency*(2π*Frequency radians/second).

The desired response is measured by its

*time constant*in seconds. The time constant is the time to reach 63% of the setpoint. The time constant can be converted to

*bandwidth*in Radians/second.

*Bandwidth*and

*time constant*are inversely related.

*Over-Shoot*is measured as a percentage of the step size. Measure the highest peak and compare with the setpoint value. A

*Critically-Damped*system is the fastest exponential response without over-shoot.

*Over-Damped*systems look identical to the Critically-Damped response, except they have a greater time constant/lower Bandwidth.

*Under-Damped*systems have overshoot.

*Dead time*is undesirable and always limits regulator response.

The chart above shows typical responses. The response may be of a torque regulator, speed regulator or tension regulator. The chart shows an ideal response with time constant of 0.5 seconds, an under-damped response with 30% overshoot and an over-damped response.

**Important assumptions**

The discussion above assumes the system is a single loop, second order, type 1 (single integrating). This applies to drives used in web handling. The ideal is a true approaching exponential curve defined by this equation: 1-e-t/τ