ES2C6 – Electromechanics 1 – Control
Control
Control is a mechanism which exhibits authority over a physical system. It is also a controlled
variable, or output, which must be held as close as possible to desired value, despite disturbances.
Open-Loop
In open-loop control there is no defined feedback. This is in systems such as an engine throttle, in
which the driver controls the system without feedback or intervention from the pedal. It can also be
a light switch in which there is no intermediate state – just “on” or “off” – and there is not feedback
from the switch itself. In open-loop control, the system is usually either on or off, however there can
be settings in between. The system will not give the user feedback though, such as if a water pipe is
blocked – the user will have to determine that themselves when the tap does not work.
Closed-Loop
A closed-loop system gives feedback back into the system to gain the required output. An example is
the negative feedback from a ball-cock in a cistern which gives feedback to stop a valve when the
water level rises. This gives proportional control, where the input changes based on the current
condition of the output.
Elements
A closed-loop has a setpoint – the value required – such as the input or demand; an output – the
value developed; feedback – a measure of the actual value developed; error – the difference
between the value required and that returned; and gain – how much change in the output is
requested for a given error.
Advantages
• Corrects for any disturbances in the input.
• Compensates for system non-linearities.
• Has greater accuracy – especially in transient and steady-states.
• Has greater stability.
• Gives better dynamic performance.
• Can be a faster system – if it anticipates ahead it can reduce lag.
• Facilitates automation.
Disadvantages
• Needs to be designed and set up properly.
• Is more complex and can require more maintenance.
• Could oscillate if the feedback is lost.
• Doesn’t always provide optimal control.
Bang-Bang Control
A bang-bang, or on-off, control system uses a threshold as the single value control element. When
the control value is higher than the current value, it will turn on, and when it is passed, it will turn
off.
If a controller is constantly turning on and off at a high frequency it has a high chance of damaging
itself. To counter this, hysteresis and deadbands are used to deliberately lag the system and stop
oscillations. Hysteresis is the difference in output for a given input due to direction. A deadband is
the change in an input required to obtain a change in an output.
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Control
Control is a mechanism which exhibits authority over a physical system. It is also a controlled
variable, or output, which must be held as close as possible to desired value, despite disturbances.
Open-Loop
In open-loop control there is no defined feedback. This is in systems such as an engine throttle, in
which the driver controls the system without feedback or intervention from the pedal. It can also be
a light switch in which there is no intermediate state – just “on” or “off” – and there is not feedback
from the switch itself. In open-loop control, the system is usually either on or off, however there can
be settings in between. The system will not give the user feedback though, such as if a water pipe is
blocked – the user will have to determine that themselves when the tap does not work.
Closed-Loop
A closed-loop system gives feedback back into the system to gain the required output. An example is
the negative feedback from a ball-cock in a cistern which gives feedback to stop a valve when the
water level rises. This gives proportional control, where the input changes based on the current
condition of the output.
Elements
A closed-loop has a setpoint – the value required – such as the input or demand; an output – the
value developed; feedback – a measure of the actual value developed; error – the difference
between the value required and that returned; and gain – how much change in the output is
requested for a given error.
Advantages
• Corrects for any disturbances in the input.
• Compensates for system non-linearities.
• Has greater accuracy – especially in transient and steady-states.
• Has greater stability.
• Gives better dynamic performance.
• Can be a faster system – if it anticipates ahead it can reduce lag.
• Facilitates automation.
Disadvantages
• Needs to be designed and set up properly.
• Is more complex and can require more maintenance.
• Could oscillate if the feedback is lost.
• Doesn’t always provide optimal control.
Bang-Bang Control
A bang-bang, or on-off, control system uses a threshold as the single value control element. When
the control value is higher than the current value, it will turn on, and when it is passed, it will turn
off.
If a controller is constantly turning on and off at a high frequency it has a high chance of damaging
itself. To counter this, hysteresis and deadbands are used to deliberately lag the system and stop
oscillations. Hysteresis is the difference in output for a given input due to direction. A deadband is
the change in an input required to obtain a change in an output.
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