Understanding the CAL Derivative and DAC Parameters

-         Derivative power is in the range -100% to +100%. It is combined with proportional power in PD controllers, and with proportional and integral power in PID controllers to affect the overall value of the correcting variable.

-         NOTE: CAL controllers only apply derivative power when the process is within the "Derivative Band" which is derived from the proportional band multiplied by the value of the DAC parameter.

-         Derivative Approach Control (dAC)

-         Increasing the value of DAC increases the Derivative Band, which means that derivative is applied further away from the setpoint, reducing the level of over/undershoot as the setpoint is approached. DAC in level 1 can be adjusted from 0.5 to 5, making the derivative band 0.5 to 5 times the width of the proportional band, and positioned symmetrically around the setpoint.

-         E.g. if BAND = 20 and DAC = 2.5, the derivative band is 50 wide, and usually 25 above, and 25 below the setpoint.

-         *but this can be changed by the Offset parameter.

-         Derivative Time (dEr.t) and Action

-         Within the derivative band (see above), 100% of derivative power occurs when the PV changes by the proportional band width in half of the derivative cycle time*.

-         E.g. if reverse acting control, and the PV rises by the width of the Pb over 1/2 the derivative time, derivative action would contribute -100% power, cutting back the overall power, so slowing down the temperature rise.

-         If the process is stable, the rate of change is zero, so derivative power contribution is 0%

-         *but this can be changed by the Derivative Sensitivity in level 4.      

-         With a time proportioned (Relay or SSR Driver) control output, changes to the derivative contribution is only actually implemented at the start of the next proportional cycle, but if while within the derivative band, a new derivative calculation produces a value of >100% (because of a rapid fall in PV) the output is immediately turned fully ON until another new calculation is less than 100%. This allows a quicker response to very rapid PV movement.

-         Similarly, a new derivative calculation with a value of <-100% (because of a rapid rise in PV) the output is immediately turned fully OFF   

-         see also Solution "Understanding CAL"Offset" Parameter, and Solution 186 "Understanding the CAL "Derivative Sensitivity" Parameter (dEr.S)          

-         Applies to CAL3200, 3300, 9300, 9400 and 9500