Electronic controller with p.i.d. action

Claim 1

An electrical process control system for maintaining the magnitude of a physical parameter at a desired value comprising: measuring means for producing a first signal in accordance with said magnitude; two state switch means having an input and an output, said first signal being applied to said input, a thermal actuator which controls said magnitude and has a relatively slow response, said actuator comprising a container, a material which is disposed within the container and whose volume is dependent on its temperature; an electric heater operatively associated with the material; an output member which is partially disposed within the container and is moved outwardly upon expansion of said material; and a spring which acts on said output member and opposes its outward movement; signal-generating means for producing a second signal; connecting means which connect said signal generating means to said input and which include delay means, said actuator and said signal-generating means being operated by said switch means, said second signal being in the sense to cause said switch means to switch from the state in which it is in so that the said switch means cycles continuously between its two states; and feedback means operatively connected to said measuring means and which, when said output member of said actuator is moved in the direction to cause a change in said magnitude in one sense, changes said first signal in the sense corresponding to a change of said magnitude in the other sense.