There are also many applications where experienced operators can make manual corrections faster than a feedback controller can. The simplest way to terminate such unstable oscillations is to break the loop and regain control manually. If the controller then overcompensates for its overcompensation, the pressure may end up lower than before, then higher, then even lower, then even higher, etc. For example, a particularly aggressive pressure controller may overcompensate for a drop in line pressure. Operator intervention is generally required when a feedback controller proves unable to maintain stable closed-loop control. A sensor may fail to generate the feedback signal or an operator may take over the feedback operation in order to manipulate the controller’s output manually.
An open-loop controller may still measure the results of its commands: Did the door actually open? Did the motor actually start? Is the pump actually off? Generally, these actions are for safety considerations rather than as part of the control sequence.Įven closed-loop feedback controllers must operate in an open-loop mode on occasion. They apply a single control effort when so commanded and assume that the desired results will be achieved. Open-loop controllers do not use feedback per se. Waiting to see how each one turns out before trying another simply takes too long. It has to anticipate the cumulative effects of its recent corrective efforts and plan future efforts accordingly.
#ROBOTC PID CONTROL SERIES#
Inertia tends to complicate the design of a continuous control loop since a continuous controller typically needs to make a series of decisions before the results of its earlier efforts are completely evident.
#ROBOTC PID CONTROL DRIVER#
The driver controlling the car gets instantaneous results after turning on the lights, whereas the cruise control sees much more gradual results as the car slowly speeds up or slows down. No further adjustment is required until the next triggering event such as the end of the trip.įeedback loops for discrete processes are generally much simpler than continuous control loops since discrete processes do not involve as much inertia. If she decides that it’s too dark to see well, she turns on the car’s lights. For example, the human controller driving the car uses her eyes to measure ambient light levels at the beginning of each trip. Common actuators for manipulating such conditions include heating elements, valves, and dampers.įor a discrete process, the variable of interest is measured only when a triggering event occurs, and the measure-decide-actuate sequence is typically executed just once for each event. These are all quantities that can vary constantly and can be measured at any time. Other common process variables include temperatures, pressures, flow rates, and tank levels. The car is the process, the speedometer is the sensor, and the accelerator is the actuator. If the car is traveling too quickly, the controller lets up on the accelerator. If the car is traveling too slowly, the controller instructs the accelerator to feed more fuel to the engine. The magnitude and duration of the error signal then determines the value of the controller’s output or manipulated variable which in turn dictates the corrective efforts applied by the actuator.įor example, a car equipped with a cruise control uses a speedometer to measure and maintain the car’s speed. The controller subtracts the latest measurement of the process variable from the setpoint to generate an error signal.
For a continuous process, a feedback loop attempts to maintain a process variable (or controlled variable) at a desired value known as the setpoint. Familiar examples include using a thermostat controlling a furnace to maintain the temperature in a room or cruise control to maintain the speed of a car.Ĭontinuing the analysis, it is clear that all closed-loop operations are not alike.
This measure-decide-actuate sequence-known as closed-loop control-repeats as often as necessary until the desired process condition is achieved. In a closed-loop control system, information flows around a feedback loop from the process to the sensor to the transmitter to the controller to the actuator and back to the process. An actuator functioning as the final control element that applies a corrective effort to the process per the controller’s instructions.A controller that reads the transmitter’s signal and decides whether or not the current condition of the process is acceptable, and.A transmitter that converts the measurement into an electronic signal.An instrument with a sensor that measures the condition of the process.It consists of five fundamental elements: Arguably the most ingenious tool of the control engineering profession is the feedback loop shown in the Basic Feedback Loop graphic.
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