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Entry type: FAQ Entry ID: 109757695, Entry date: 05/15/2018
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TPI control with electrothermal or electromotoric 2-point actuators

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This article compares TPI-control at a fixed cycle time with TPI-control at a variable cycle time regarding the actuator life time.

What is TPI?

TPI stands for ‘Time proportional and integral’ and is a way to control a control path (e.g. 2-point actuator, boiler, etc.) in a more precise manner in order to reduce energy consumption. Other terms also used instead of TPI are PWM (Pulse width modulation), PDM (Pulse duration modulation) or PID6 / PID12 (proportional integral derivative algorithm).

How does TPI work compared to classical 2-point control?

The following pictures serve as a demonstration and are not completely correct technically.


For classical 2-point control a temperature band (e.g. ±1°C) is defined around the setpoint. In the example of a boiler application, the boiler is switched on, as long as the room temperature is below the upper threshold temperature of the setpoint band. Thereafter the boiler is switched off until the room temperature reaches the lower threshold temperature of the setpoint band. This behavior then is repeated continuously.



TPI control reduces the temperature swings by modulating the pulse width of the control signal, as soon as the room temperature is within the setpoint band.



 

According to EN 15500-1:2017 Chapter 5.4, lower temperature swings lead to a lower setpoint by the user and thus to energy savings.



 

This control pattern leads to more switching cycles, which can be a disadvantage for certain control paths (e.g. thermal or motoric actuators).

How does a variable TPI cycle duration increase life time in comparison to a fixed TPI cycle duration?

In order to change the duty cycle, the following modulation variants are common.
- Fixed cycle duration, variable pulse duration.
- Variable cycle duration, fixed pulse duration*.
*Pulse duration of On-pulse at controller output < 50%, pulse duration of Off-pulse at controller output > 50%.



If the life cycles of an actuator are available, the actuator life time can be estimated for a fixed cycle duration (precondition especially for thermal actuators: Pulse duration > Opening time), that the pulse duration is longer. For a variable cycle duration the life time will be higher in comparison. But it is harder to estimate life time, since certain parameters are not predictable (e.g. disturbance variable, set point changes due to different comfort requirements).

An average cycle duration can be used to estimate actuator life time for a variable cycle duration. The minimum cycle duration times two is a good guide value.



 

*Depending on installation dimensioning, process variables and control parameters.

So, if variable cycle duration is used, the actuator life time will be approximately doubled under the precondition, that the period duration of fixed TPI (red graph in picture above) equals the minimal period duration of variable TPI (black graph at 50% in picture above).


If you have any specific questions regarding this subject, please contact your local BT Customer Support.
Responsible for the content of this article is the Customer Support team in Zug (BLK).

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