Glossary Of PID And Temperature Controller Terms

An explanation of some common terms and abbreviations used in PID temperature and process control:

PID - Proportional, Integral, Derivative. Three values which comprise the algorithm used to control temperature to high levels of accuracy

PV - Process value, the current measured temperature

SV/SP - Setpoint, the target temperature or process value

RTD - a type of temperature sensor also known as thermistor. A resistance which changes as temperature rises and falls

TC - abbreviation for thermocouple. A sensor type with a small voltage output which reacts to changes in temperature. Type J and K are commonly used

PT100/1000 - platinum type RTD which measures 100/1000 ohms at 0 degrees C.

MV - With a Novus PID controller this refers to % of control output applied. In Automatic mode this is calculated by the PID algorithm. In Manual mode it can be adjusted

AT - auto tuning, a means of establishing the correct values of PID for the process. Most modern controllers contain an autotune function

Soft Start - some Novus controllers allow limiting of the control output (MV) during the initial phase of control. This can help avoid sharp changes in temperature or high startup current drawn by the temperature control system

Loop Break Detection - LBD looks for a change in the process value (temperature) when the control output is active. If the process value is not responsive after an adjustable time period the controller will alarm out. This highlights any faults with the circuit under control e.g. open circuit heater

SSR - solid state relay, a device commonly used to switch power to an element. SSRs can operate very quickly to hold a process to a fine tolerance. They are better suited to PID processes than electromechanical relays and contactors which require longer time periods for the contacts to open and close  

Single Loop - refers to a single setpoint which is maintained indefinitely. For example when the controller is turned on it reaches 100 deg. C setpoint and will maintain this until switched off. The N1030 family are examples of single loop controllers

Ramp And Soak - a method of holding a setpoint for a predetermined time period before rising/falling under control. A simple example would be reaching 100 deg. C setpoint and automatically maintaining this for 10 minutes before switching off the control output. The N1050 family are examples of ramp and soak controllers

Auto Adaptive - with a standard PID controller the PID parameters are set by autotuning and can only be altered manually or by subsequent autotuning. Auto adaptive PID responds to changes during the process to maintain efficiency. In simple terms PID values are changed automatically. The N1200 family are examples of auto adaptive controllers

Segment - segment refers to a single stage of a multistage controller. Ramp and soak controllers comprise multiple segments linked together to create a program or profile

Hysteresis - is referenced in all Novus PID devices as they can function in thermostatic control mode. In a heating system the hysteresis value is the amount the measured temperature will fall below the set point before the output energises to raise the temperature. A low value of hysteresis can cause the output to ‘chatter’ which is to be avoided when using relays to switch the load. Hysteresis control can also be applied to any alarm conditions which are required