Hysteresis and PWM Controller

Controllers are the heart of the any feedback system which is used to minimise the error so that system can work on the desired state. Example: Suppose we have a Permanent Magnet Synchronous Motor (PMSM) drive system which should be run at the speed of 2000 rpm and Torque of 8 N-m. The task of the controller will be to maintain the PMSM at desired speed and maintain the torque at desired level. Number of controllers are there But two most commonly used controllers are Hysteresis and PWM Controller.

Hysteresis Controller:

This is a simplest controller, only having upper limit and lower limit. The band between this limit is known as hysteresis band in which error is bound.

Error = Reference value - Actual value

Upper Limit:

When error cross upper limit i.e. actual value is less than reference value then switch of the supply to the system is becomes ON.

Lower Limit:

When error cross lower limit i.e. actual value is more than reference value then switch of the supply to the system is becomes OFF.

On the basis of this limits ,a switching sequence is generated and its frequency depends upon the hysteresis band.

Pulse Width Modulation (PWM) Controller:

This is a mostly used controller, in which a sawtooth wave is generated with predefined frequency. This wave is compared with error signal. There are two conditions:

Condition I: Error is greater than sawtooth i.e. actual value is less than reference value then switch of the supply to the system is becomes ON.

Condition II: Error is smaller than sawtooth i.e. actual value is more than reference value then switch of the supply to the system is becomes OFF.

On the basis of error signal a switching sequence is generated and its frequency is same as of sawtooth wave.

Reference:

Paramjeet Singh Jamwal and Sanjeev Singh; “Comparative Analysis of Hysteresis and PWM Current Controllers for PMSM Drive,” National Conference on Advances in Power & Control (AIPS-2015), pp. 24-28, 2015.