Robots do not move perfectly on the first try because motors, gears, loads, and surfaces all introduce small errors. Feedback control is the method robots use to notice the difference between where they are and where they should be. In a robot arm, this can mean reaching past a target spot, measuring the mistake, and moving back until the gripper is aligned.
This idea matters in factories, drones, self-driving cars, and medical robots because accurate correction makes machines safer and more useful.
A feedback control loop starts with a desired position and compares it with the actual position measured by a sensor. The difference is called error, and the controller uses that error to decide how strongly and in which direction the motor should move. If the arm overshoots the target, the sign of the error changes, so the controller commands a correction in the opposite direction.
Good feedback control reduces error quickly without causing endless wobbling or unstable motion.
Key Facts
- Error = target position - actual position
- Control signal = controller output sent to the motor or actuator
- Negative feedback means the correction acts to reduce the error.
- If error = 0, the robot is at the desired position and no correction is needed.
- Proportional control can be written as u = Kp e, where u is control signal, Kp is gain, and e is error.
- Sensors close the loop by measuring actual position and sending that information back to the controller.
Vocabulary
- Feedback control
- A method where a system measures its output and uses that measurement to correct future action.
- Setpoint
- The desired value a system is trying to reach, such as a target position for a robot arm.
- Error signal
- The difference between the desired value and the measured actual value.
- Controller
- The part of a system that calculates what action to take based on the error signal.
- Sensor
- A device that measures a physical quantity, such as position, speed, angle, or force.
Common Mistakes to Avoid
- Thinking the robot knows it made a mistake without a sensor is wrong because feedback requires a measurement of the actual position.
- Using error = actual - target without checking the sign is wrong because the controller may command motion in the opposite direction from what was intended.
- Making the controller gain too large is wrong because a strong correction can cause overshoot, oscillation, or instability.
- Confusing open-loop control with feedback control is wrong because open-loop control sends commands without checking whether the robot reached the target.
Practice Questions
- 1 A robot arm has a target position of 50 cm and its sensor reads 57 cm. Calculate the error using error = target position - actual position. State whether the arm overshot the target.
- 2 A proportional controller uses u = Kp e with Kp = 4. If the error is -3 cm, what is the control signal u? What does the negative sign mean for the motor direction?
- 3 A robot arm keeps moving back and forth across a target instead of settling on it. Explain what might be wrong with the feedback control loop and name one change that could improve its motion.