1. For a process control system including a source of electricity, a servomotor adapted to drive a load mechanism, and a servomotor controller for energizing the servomotor with current from the source to generate motor torque which overcomes the load mechanism torque and the motor coulomb friction torque to drive the load mechanism through a predetermined and cyclic sequence of movements for carrying out the steps of a process, the level of motor energization being determined as a function of the difference between actual and desired values of a motor operating parameter and a gain factor chosen to result in a desired level of response stability, a method of operation comprising the steps of: monitoring the motor current over at least one cycle of said process and identifying the minimum motor current value occurring therein; comparing the identified current value with a reference value corresponding to the average minimum motor current which occurs when the controller is operated at the desired level of response stability and developing a deviation signal as a function of the difference therebetween to obtain an indication of variation in the motor coulomb friction torque; and adjusting the controller gain factor by a correction amount determined in relation to said deviation signal such that the gain factor is increased with increasing coulomb friction torque and decreased with decreasing coulomb friction torque, thereby to maintain the desired level of response stability despite variation in the motor coulomb friction torque.