Welcome to Analog position control closed-loop position servo. Controlling the position of a
physical object is achieved by a closed-loop feedback system. The servo mechanism shown
below is an application of position control. It uses pulleys belts gears and a motor to
drive a rack horizontally in both directions. Let's read about the elements of the analog
closed-loop position servo system. Actuator: the actuator is the muscle of the system it provides
the physical power that drives the load. In this example a DC motor is the actuator.
Controlled variable: the controlled variable is the actual variable being monitored and maintained
at a desired value. In this example, the position of the rack is the controlled variable. Measured
variable: to monitor the status of the controlled variable it must be measured. The condition of
the controlled variable is referred to as the measured variable. In this example it is the
position of the rack. Measurement device: the measurement device is the eye of the system.
It senses the measured variable and produces an output signal and represents the status
of the controlled variable. In this example a potentiometer connected through belts and
pulleys and shafts is the measurement device. As the motor turns the potentiometer wiper arm
rotates and provides a feedback signal that represents the position. Feedback signal:
the feedback signal is a 0 to negative 15 DC voltage produced by the potentiometer. Set point:
the set point is the input signal applied to the closed-loop system. It indicates the
desired condition of the controlled variable. In this example it is a 0 to positive 15 DC voltage
from a potentiometer that provides the desired setpoint position of the gear rack. Controller:
the controller is the brain of the system it produces an appropriate output by comparing
the status of the feedback signal to the setpoint. In this example a summing op-amp is the controller
it produces a DC voltage that is amplified before it drives the actuator. The operation:
when the gear rack is all the way to the right it is in the home position which can also be
considered the starting or the reference position. In this situation the set point
potentiometer is at 0 volts and the feedback position is at 0 volts. Moving the gear rack
left. By moving the set point potentiometer up to positive 10 volts the following conditions occur:
the feedback signal is 0 volts, a positive 10 volt setpoint and 0 volt feedback at the
summing op-amp inputs cause its output to become negative 10 volts. The power amp amplifies and
inverts its inputs to a positive output. The DC motor begins to turn clockwise the rack is forced
to move left the feedback potentiometers wiper rotates in the clockwise direction and as it
does it provides a negative feedback signal that increases from 0 volts. When the wiper arm of the
feedback potentiometer reaches the negative 10 volt position it cancels out the positive
10 volts applied to the top input of the summing op-amp. The result is that the op-amp output goes
to 0 volts, the motor shuts off, and the gear rack stops moving. Moving the gear rack right.
By decreasing the setpoint potentiometer down to positive 5 volts the following conditions occur:
the feedback signal is negative 10 volts, a positive 5 volt set point and a negative 10
volt feedback at the summing op-amp inputs cause its output to become positive 5 volts,
the power op-amp amplifies and inverts its input to a negative output then the DC motor begins to
turn counterclockwise, the rack is forced to move right, the feedback potentiometer wiper rotates in
the counterclockwise direction, and as it does the voltage decreases from negative 10 volts.
When the wiper arm of the feedback potentiometer reaches the negative 5 volt position it cancels
out the positive 5 volts applied to the top input of the summing op-amp. The result is that
the op-amp output goes to 0 volts, the motor shuts off, and the gear rack stops moving.
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