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Each output can be used to do several things including control a process such as turning on a heating or cooling source , initiate an alarm, or to retransmit the process value to a programmable logic controller PLC or recorder. Typical outputs provided with temperature controllers include relay outputs, solid state relay SSR drivers, triac, and linear analog outputs. The controller energizes the relay coil, providing isolation for the contacts. This lets the contacts control an external voltage source to power the coil of a much larger heating contactor. It's important to note that the current rating of the relay contacts is usually less than 2A.
The contacts can control a heating contactor with a rating of 10—20A used by the heater bands or heating elements. Another type of output is an SSR driver.
SSR driver outputs are logic outputs that turn a solid-state relay on or off. Most solid-state relays require 3 to 32VDC to turn on. A typical SSR driver turn-on signal of 10V can drive three solid-state relays. A triac provides the relay function without any moving parts. It is a solid state device that controls currents up to 1A. Triac outputs may allow some small amount of bleed current, usually less than 50mA. This bleed current doesn't affect heating contactor circuits, but it may be a problem if the output is used to connect to another solid-state circuit such as a PLC input.
If this is a concern, a standard relay contact would be a better choice. It provides absolute zero current when the output is de-energized and the contacts are open. Analog outputs are provided on some controllers which put out a 0—10V signal or a 4—20mA signal.
These signals are calibrated so that the signal changes as a percentage of the output. Temperature controllers have several other parameters, one of which is a setpoint. Basically, a setpoint is a target value set by an operator which the controller aims at keeping steady. Another parameter is an alarm value. This is used to indicate when a process has reached some given condition.
There are several variations on types of alarms.
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For instance, a high alarm may indicate that a temperature has gotten hotter than some set value. Likewise, a low alarm indicates the temperature has dropped below some set value. For example, in a temperature control system, a high fixed alarm prevents a heat source from damaging equipment by de-energizing the source if the temperature exceeds some setpoint value. A low fixed alarm, on the other hand, may be set if a low temperature could damage equipment by freezing.
The controller can also test for a broken output device, such as an open heating element, by checking the amount of output signal and comparing it to the amount of detected change in the input signal. This feature is known as Loop Alarm. Another type of alarm is a deviation alarm. This is set at some plus-or-minus value from the setpoint. The deviation alarm monitors the process setpoint. The operator is notified when the process begins to vary some preprogrammed amount from the setpoint.
A variation on the deviation alarm is the band alarm. This alarm will activate either within or outside a designated temperature band. Typically, the alarm points are half above and half below the controller setpoint. Another common set of controller parameters are PID parameters. PID, which stands for proportional, integral, derivative, is an advanced control function that uses feedback from the controlled process to determine how best to control that process.
All controllers, from the basic to the most complex, work pretty much the same way. Controllers control, or hold, some variable or parameter at a set value. There are two variables required by the controller; actual input signal and desired setpoint value. The input signal is also known as the process value. The input to the controller is sampled many times per second, depending on the controller. This input, or process, value is then compared with the setpoint value.
If the actual value doesn't match the setpoint, the controller generates an output signal change based on the difference between the setpoint and the process value and whether or not the process value is approaching the setpoint or deviating farther from the setpoint. This output signal then initiates some type of response to correct the actual value so that it matches the setpoint. Usually, the control algorithm updates the output power value which is then applied to the output.
The control action taken depends on the type of controller. It works by setting up a hysteresis band. For instance, a temperature controller may be set to control the temperature inside of a room. When an analog output type is used, the output drive is proportional to the output power value. However, if the output is a binary output type such as a relay, SSR driver, or triac, then the output must be time proportioned to obtain an analog representation. A time proportioned system uses a cycle time to proportion the output value.
As long as the power value doesn't change, the time values wouldn't change. All things being equal, a shorter cycle time is desirable because the controller can more quickly react and change the state of the output for given changes on the process. Due to the mechanics of a relay, a shorter cycle time can shorten the life of a relay, and is not recommend to be less than 8 seconds. For solid state switching devices like an SSR driver or triac, faster switching times are better. Longer switching times, no matter what output type, allow for more oscillation in the process value.
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The general rule is that, ONLY if the process will allow it, when a relay output is used, a longer cycle time is desired. Controllers can also have a number of additional optional features. One of these is communication capability. A communication link lets the controller communicate with a PLC or a computer. This allows data exchange between the controller and the host. An example of typical data exchange would be the host computer or PLC reading the process value.
A second option is a remote setpoint. This feature allows a remote device, such as a PLC or computer, to change the controller setpoint. Niazi is a licensed practitioner of patent law, has published numerous books and papers, and has been recognized with several awards for his contributions to science and literature.
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