What Have I Learned? – Cascade Control (Part 2)

In the February 9 entry on “Unexpected Wireless Benefits” we saw how the positive feedback implementation of the integral mode enabled an enhancement of integral action had benefits that extended beyond wireless devices to any loop with appreciable measurement delay. The positive feedback network sends the controller output or an external reset feedback back through a filter and adds the result to the controller output from the proportional mode as shown in PIDPLUS_Results.pdf. The positive feedback network also offers a convenient method of deadtime compensation by just inserting a deadtime block in front of the filter as shown in Advanced Application Note 3.

Additionally the positive feedback network provides a significant improvement for batch control, override control, and cascade control as described in the article “The Power of External Reset Feedback.” In cascade control the use of the secondary process variable (PV) as the input to the filter (external reset feedback) enables the primary controller to deal with a poor (e.g. slow) response in the secondary controller. To set up external reset feedback of the secondary loop PV, the “Dynamic Reset Limit” must be enabled in the primary (outer) loop PID and the secondary (inner) loop PV must be selected for the BKCAL_OUT of the secondary PID as shown in Cascade_Control.pdf. The use of external reset feedback prevents the primary loop from acting faster than the secondary loop can respond, which could occur if the secondary loop has a slow reset setting or has a slow valve. To include the effect of the valve response, the valve position read back must be selected as the PV for the BKCAL_OUT of the AO block. Without this configuration, the primary controller does not know a valve has excessive stroking time, stick-slip, or deadband, a loss of signal, solenoid valve failure, or unexpected de-energization of the solenoid due to a discrete process action, sequence, or interlock. The loop may seem OK for small changes load if it is just a slow valve or slow variable speed drive without external reset feedback, because the velocity limiting (rate limiting) in the final element has little effect for small changes in the controller output. However, a large disturbance or set point change will trigger oscillations when the controller output outruns the response of the final element.

Did you know the peak error for cascade control decreases for an unmeasured disturbance as the size of the secondary process time constant (lag) increases? In the single loop, the secondary process time constant is detrimental because it creates dead time whereas putting the secondary time constant as the largest time constant in the secondary loop is beneficial because it allows a high secondary controller gain and slows down process disturbances entering the secondary loop. The ratio of cascade to single loop peak error goes from about 0.25 to 0.1 as the ratio of the secondary (inner) to primary (outer) loop time constant increases from 0.2 to 1.0 for a 0.6 inner to outer loop dead time ratio as shown in Tuning_and_Control_Loop_Performance_Figure_11.2.pdf. The figure also shows how a smaller deadtime in the secondary loop compared to the deadtime in the primary loop decreases this same peak error ratio. The improvement is even more dramatic when the primary loop has an integrating or a runaway process.