If devices contain a function of PID (proportional-integral-derivative) control, it means that it's possible to realize three types of control: P, PI and PID.

**P Control**. Output power is directly proportional to control error. The higher the proportion coefficient, the less the output power at the same control error. Proportional control can be recommended for fast-response systems with a large transmission coefficient. To adjust the propotional controller you should first set the maximum proportion coefficient where in the output power decreases to zero. When the measured value is stabilized, set a specified value and gradually reduce the proportion coefficient and the control error will decrease. If there are periodic oscillations in the system, the proportion coefficient should be increased so that control error is minimal periodic oscillations decrease to the limit.

**PI Control.** Output power equals to the sum of proportion and integration coefficients. The higher the proportion coefficient, the less the output power at the same control error. The higher the integration coefficient, the slower the accumulated integration coefficient. PI control provides zero control error and and is insensitive to interference of the measurement channel. The PI control disadvantage is slow reaction to disturbances. To adjust the PI controller you should first set the integration time equal to zero, and the maximum proportion time. Then by decreasing the coefficient of proportionality, achieve periodic oscillations in the system. Close to the optimum value of the coefficient of proportionality is twice higher than that at which any hesitation, and close to the optimum value of the integration time constant - is 20% less than the oscillation period.

**PID Control.** Output power equals to the sum of three coefficients: proportional, integral and differential. The higher the proportion coefficient, the less the output power at the same control error. The higher the integration coefficient, the slower the accumulated integration coefficient. The higher the differentiation coefficient, the greater the response of the system to the disturbance. The PID controller is used in inertial systems with relatively low noise level of the measuring channel. The advantage of PID is fast warm up time, accurate setpoint temperature control and fast reaction to disturbances. Manual tuning PID is extremely complex, so it is recommended is to use the autotune function.

**PID control autotune in TERA's devices:**

The main thing that determines the quality of PID controller is its ability to achieve a setpoint temperature accurately and fast. For this purpose all modern PID controllers have autotune function. Standard algorithms of auto-tuning PID does not exist, in practice each manufacturer uses its own algorithm. Therefore, when a user purchases the same device named PID controller from different manufacturers, more likely he may receive different results of its application.

The main advantages of auto-tuning algorithm TERA's PID controllers are:

- autotuning and control without overshoot (in standard PID controllers overshoot can reach 50-70% of the set temperature which is not desirable or even prohibited in some technologies)
- autotune duration on the average 2 times shorter than that of other manufacturers (extremely important characteristic for applications with freqently changed properties)

Auto-tuning can be done at any stable state of the controlled system. Furthermore, the greater the difference between the starting and set temperature, the more accurate the coefficients of the PID controller. All PID coefficients are stored in nonvolatile memory.

Autotune must be repeated if::

- actuator power has changed
- physical properties of the controlled system (weight, capacity, heat transfer, etc.) have changed
- control system has been replaced by another non-identical
- significant changes in a set temperature