When using a temperature controller to control a heater, it is necessary to convert the control calculation results of the temperature controller into electricity and apply it to the heater. Electromagnetic relays, solid state relays or phase controllers are usually used for conversion. Depending on the conversion method, the output of the temperature controller is different.
There are two types of output methods: time-proportional output mode and continuous output mode. There are two types of time-proportional output methods, one is to output the ON/OFF contact signal as a percentage in a certain output period, and the other is to output the ON/OFF voltage pulse signal in a percentage during a certain output period. In the continuous output mode, the results of the control calculation are directly output to a continuous voltage or current signal, such as a 4-20mA output.
The phase controller has good stability and over-responsiveness in terms of control performance, but it will produce harmonic current and high-frequency noise, and the price is more expensive.
In this article, we will explain the low-cost time-scale output method from the viewpoint of controlling performance and using attention.
When the control calculation result is output as an ON/OFF contact signal, it is usually connected to an electromagnetic relay. At this time, consider the following 2 points to determine the time scale output period.
1. Considering the life of the relay contact, reduce the frequent ON/OFF action as much as possible and increase the cycle.
2. Considering the response speed of the control object, the ON/OFF of the output period does not affect the PV of the measured value during stable control, and the cycle is reduced.
When our temperature controller is used in the contact output method, the initial value of the time-proportional output period is 20 seconds.
For control objects with fast response, the measured PV fluctuates periodically when the initial value is used for stable control. When this happens, you need to reduce the output cycle. However, reducing the output cycle too much will significantly reduce the contact life of the relay. Solid state relays are recommended for this purpose.
Noise also needs to be considered when using relays. The contact output of the temperature controller, which is ON by the percentage of the output during a certain output period, is actually faster than the sampling time of the temperature controller to judge ON/OFF at time intervals, and we don't know where to ON/OFF in the AC power supply. Therefore, there is a possibility that the contact will be OFF in the state of high current, which will inevitably cause noise. For this reason, we recommend implementing noise countermeasures for peripheral devices, including temperature controllers.
When the control calculation results are output as a voltage pulse signal, they are usually connected to a solid-state relay. Unlike relays, there is no need to consider the nodal life. However, there are 2 points to consider to determine the time scale output period.
1. In order to get enough output resolution, increase the period.
2. Considering the response speed of the control object, the ON/OFF of the output period does not affect the PV of the measured value during stable control, and the cycle is reduced.
If the specification of the solid state relay is not a zero-crossing type solid state relay, the output resolution is not considered. However, noise countermeasures need to be considered.
In the case of zero-crossing solid state relays, the output of the temperature controller is not the same as the actual power applied to the heater. In this case, the resolution actually applied to the heater needs to be considered.
For example, consider a power supply frequency of 50Hz, a time-proportional output period of 1 second, and an output of 4.4%.
The half-wave of the AC power supply is 10ms. In a 1-second cycle, 4.4% of the output is an ON time of 44ms. If the output is applied at the moment as shown in the figure below, the contact of the solid state relay is not closed at the time of output of 12V, and the contact is closed when the power supply voltage crosses zero. The contact of the solid state relay is not opened at the moment of output 0V, and the contact is not opened until the power supply voltage crosses zero. The part of the diagonal line in the diagram indicates the closed state.
In fact, the solid state relay has a closing time of 50ms, which is equivalent to 5% of the output applied to the heater.
When using a zero-crossing type solid state relay in this way, the actual output resolution is determined by the power supply frequency and the time-proportional output period, so if the time-proportional output period is set too short, the output resolution will be low, and the stability control performance will be impaired.
When the response speed of the control object is fast and the time-proportional output cycle has to be set for a short time, it is necessary to use phase control with continuous output, or a thyristor power regulator, such as the THV series. In addition, with the REX-F9000 temperature controller, the power feedforward function of the HA series can also solve this problem.
In the THV series (when the input is a continuous output of the temperature controller and the zero-crossing type output) or when the temperature controller with the power feedforward function is used, a function to compensate for the actual output difference has been added, enabling more stable control. Note that there is no compensation function when the input is a zero-crossing output of the voltage pulse output of the temperature controller.
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