How do sensors and actuators work in robotics?

In robotics, sensors provide information about what is happening, while actuators turn commands into physical movement or force. The controller processes measurements and manages the response. Detecting a part, moving the arm to the right position and gripping with suitable force depend on these three functions working together. The sensor measures, the controller evaluates, and the actuator acts.

What information do sensors provide?

A sensor detects a physical quantity or condition and converts it into a usable signal. Robot applications may require distance, position, speed, force, pressure or image data. Among the main components of a robot, sensors monitor both the surroundings and the robot's own movement.

Sensing the surroundings: An inductive sensor can detect a metal part within its specified range. Photoelectric sensors use light for object detection. An ultrasonic sensor can provide distance information under suitable conditions. A camera supplies images for tasks such as locating parts or inspecting surfaces. Sensors do not perform identically across different materials, distances and environments.

Monitoring the robot's movement: An incremental encoder tracks movement through pulses, while an absolute encoder provides coded position information within its measurement range. A limit switch can indicate that a particular position has been reached. An end-position signal is different from precise position measurement throughout a movement.

What is an actuator, and what does it do?

An actuator converts supplied energy into mechanical movement or force. It may rotate a robot joint, close the fingers of a gripper or move a mechanism forwards and backwards. The resulting motion can be rotary or linear.

Electric actuators use electric motors. With a suitable drive and feedback, servo motors can be used for position, speed or torque control. Stepper motors provide incremental movement in suitable applications; an open-loop stepper system does not necessarily measure whether each commanded step actually occurred. Producing movement and verifying that movement are separate tasks.

Pneumatic actuators use compressed air, while hydraulic actuators use pressurised liquid. In a pneumatic gripper, for example, a cylinder may drive the fingers. The electrical command to a valve directs the airflow; the energy used for the mechanical work comes from the compressed air. A vacuum gripper holds a part through a pressure difference at the suction interface.

Why are controllers and drives needed?

A sensor signal does not replace the system that controls a motor or cylinder. The robot controller manages the target movement and coordinates the axes. A servo drive regulates the motor's electrical supply through the appropriate control loops. In a pneumatic system, valves control the airflow. Command information and the energy producing movement are different things.

A production cell's PLC may coordinate the sequence between the conveyor, stations and robot. The robot program defines task steps such as picking, carrying and placing a part. Sensor outputs must also match the controller inputs: NPN/PNP configuration, supply voltage and wiring need to be compatible. Analogue measurements such as 0–10 V or 4–20 mA are read through a suitable input and scaled into engineering units.

Feedback: Did the commanded movement actually happen?

In feedback control, the target value is compared with the measured value. Their difference is the control error. For example, a target position of 100 mm and a measured position of 98 mm give an error of +2 mm when calculated as target minus measurement. The controller uses that difference to adjust the drive command; the outcome depends on tuning, the load and physical limits.

In open-loop control, the result of the movement is not fed back as a measurement to the relevant control loop. In closed-loop control, measurements are used to regulate the movement. However, feedback does not guarantee zero error under every condition. Backlash, mechanical compliance, sensor resolution and incorrect settings can affect the robot's achievable performance.

Practical example: How does a robot pick a part?

Consider how a robot works during a pick operation. During detection, a sensor or camera indicates that a part is present. During movement, the controller takes the arm to the target, with motors and drives executing the motion. During gripping, the tool picks up the part. During verification, the system evaluates suitable position, force, vacuum or part-presence information.

A “gripper closed” signal does not necessarily mean “part picked successfully”. The fingers can close without a part between them. A vacuum reading alone does not establish that the part was picked in the correct orientation. Success criteria therefore depend on the task. Artificial intelligence may help interpret images or select parts, but sensors and actuators do not require AI to work together.

What matters when selecting sensors and actuators?

For a sensor, consider measurement range, accuracy, repeatability, response time and environmental conditions. For an actuator, consider force or torque, speed, stroke or rotation range, load and duty cycle. The most powerful motor or highest-resolution sensor is not automatically the best choice. Mechanical and electrical compatibility must be assessed together.

For robot safety, an ordinary object-detection sensor cannot substitute for a protective sensing system whose suitability has been validated. The operating area, stopping behaviour and required safety functions follow the cell's risk assessment. Detecting a part and protecting a person are different functions.

Technical references: Festo — sensors and position feedback · Festo — grippers and actuators · maxon — encoder feedback and speed control.

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