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Level controllers monitor, regulate, and control liquid or solid levels in a process. There are three basic types of control functions that level controllers can use. Limit control works by interrupting power through a load circuit when the level exceeds or falls below the limit set point. A limit controller can protect equipment and people when it is correctly installed with its own power supply, power lines, switch and sensor. Advanced or non-linear control includes dead-time compensation, lead/lag, adaptive gain, neural networks, and fuzzy logic. Level controllers can be used for either liquid or powder or other dry material applications.Linear level controllers can take many different styles. Feedforward control offers direct control or compensation from the reference signal. It may be open loop or in conjunction with PID control. Proportional, integral, and derivative (PID) control is an intelligent I/O module or program instruction, which provides automatic closed-loop operation of process control loops. Proportional plus integral (PI) control has the error signal integrated and is used for eliminating steady state or offset errors. It may also be called automatic reset/bias/offset control.
Proportional plus derivative (PD) control has the error signal differentiated to get the rate of change. This type of control is used to increase controller speed of response, but can be noisy and make the system less stable. In proportional (P) control, the control signal is proportional to the error between the reference and feedback signals.
Level controllers differ in terms of specifications, user interface, and features. Specifications include the number of inputs, control outputs and control feedback loops. Control loops may be linked to improve control performance and/or stability. The control output is usually analog current, voltage or a switched output. These controllers can have discrete or TTL I/O as well and can handle high power switching needs. The user interface for level controllers may be analog, digital or computer controlled. Displays for level controllers can be analog meters, digital numerical readouts, or video display terminals. Another possible type of display is a strip chart or circle chart. When connecting to a computer host, level controllers can use the standard serial, parallel or SCSI interfaces or can be networkable via Ethernet, CANBus or a number of other network protocols. Features that are sometimes optional for level controllers include sensor excitation current or voltage, built-in alarms or indicators and washdown or waterproof ratings. Other features can include programmable setpoints, autotune or self-tuning functions and signal computation functions or filters.