Connections to Controls Concepts: Balance Arm
Prof. R. H. Bishop
University of South Florida
The balancing arm assembly provides insight into two feedback control system concepts: (i) closed-loop stability and
(ii) transient and steady-state performance—the ability of closed-loop control to provide a quick response and to
reduce steady-state tracking errors.
Plant Balance arm
Sensor Infrared (IR) range finder
Actuator Servo motor
Performance Command following in the presence of disturbances and stability
Design objectives Tune the control system by adjusting PID gain constants & stabilize the ball at the given
set-point
Reference inputs Set-points
The balancing arm and one standard servomotor are the plant and actuator, respectively. The sensor is an IR range
sensor and provides the feedback signal on the location of the ball relative to the sensor from which we can compute
the offset from the set-point. The commands enter the control system via the host computer through front panel
commands sent over the USB Ethernet connection to the myRIO, which hosts the PID controller code.
Closed-loop stability
A stable feedback control system displays a bounded response to a bounded input. For example, if the balancing
arm feedback control system is stable, then when we place the ball at the setpoint and gently push it, the actuated
arm will rotate according to commands from the controller to return the ball to the setpoint. If the system is unstable,
when the ball is pushed, it will not return to the setpoint, but instead will continue to deviate farther and farther away
from the setpoint until likely coming to rest against the stop. If the system is marginally stable, the ball will roll away
and come rest at some distance from the setpoint in a controlled fashion. Stability is a key objective in the design of
feedback control systems. Closed-loop feedback systems that are unstable are, in general, particularly undesirable.
Disturbance rejection and performance
The gentle push can be viewed as an external impulsive disturbance. The stable closed-loop balancing arm control
system demonstrates an ability to reject these disturbances by returning the ball back to the setpoint. A key element
of the control design process is not only to achieve closed-loop stability, but also to have the ball return to the desired
setpoint with a specified performance. In other words, we want the feedback control system to meet our design
specifications that typically include settling time, percent overshoot, time to peak, time to rise, and steady-state errors
in response to a specific set of test input signals, such as impulses, steps,
120 Balancing Arm Assembly
120 Balancing Arm Assembly
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