A potentiometer is a simple position sensor that turns mechanical motion into a voltage signal. In robotics, it is often used to measure the angle of a rotating joint, knob, or servo shaft. Because the output depends on the physical position of the wiper, it gives absolute position rather than only changes in position.
This makes it useful for feedback control, calibration, and human input devices.
Inside a rotary potentiometer, a wiper arm slides along a curved resistive track connected between a supply voltage and ground. The wiper taps a fraction of the voltage, so the output voltage changes as the shaft angle changes. A controller reads this voltage with an analog to digital converter and converts it into an angle estimate.
In a servo system, the measured angle is compared with a target angle so the motor can correct its motion.
Understanding Robotics: Potentiometer Position Sensor
The sensor works best when the electronics treat its output gently. A microcontroller input should have very high resistance, so it takes almost no current from the wiper. If a low resistance device is connected directly to the output, it draws current and changes the measured voltage.
This is called loading. A buffer circuit can prevent loading when needed. The reference voltage matters too.
If the sensor is powered from a supply that varies, but the controller assumes a fixed reference, the calculated angle will drift even when the shaft stays still. Many controllers avoid this problem by using the same reference for the sensor supply and the analog measurement.
An analog to digital converter changes the sensor voltage into a number. That number has limited resolution. With a ten bit converter, the full input range is split into one thousand twenty four steps.
If a potentiometer can turn through about three hundred degrees, one step represents a small fraction of a degree. Real measurements are less exact than this ideal estimate. Electrical noise, loose wiring, and tiny changes in the supply can make the number jump between nearby values.
Software often averages several readings or ignores very small changes. Filtering gives a steadier angle value, though too much filtering makes the reading respond slowly when the joint moves.
Calibration connects the electrical reading to the real mechanical motion. A robot joint may not use the entire turning range of the potentiometer. Stops, gears, linkages, or an offset mounting position can limit the useful motion.
During calibration, the robot is moved to known end positions and the controller records the corresponding sensor readings. It can then map readings between those limits to useful joint angles.
This process is important after parts are replaced or adjusted. A sensor mounted slightly off center can produce a correct changing signal while reporting the wrong physical angle.
Potentiometers have physical limits because the wiper rubs on the resistive track. Repeated motion slowly wears the contact area. Dust, vibration, or oxidation can cause scratchy readings, seen as sudden jumps in the reported position.
This is especially noticeable in a robotic arm that pauses at the same common angles. Students can test for this by turning a shaft slowly while graphing the measured value.
A smooth motion should create a smooth trend. Sudden spikes suggest contact trouble, electrical interference, or a poor connection.
These sensors appear in classroom robot arms, radio control transmitters, volume knobs, throttle pedals, and many hobby servos. They are useful when a system must know its position immediately after power returns. An incremental encoder may need movement before it knows where it is, while a potentiometer can report a position as soon as the voltage is read.
Its main tradeoff is limited rotation and mechanical wear. When designing a robot, pay attention to the sensor range, the safe mechanical stops, stable reference voltage, clean wiring, and calibration data. Those details often determine whether feedback control feels precise or unstable.
Key Facts
- A potentiometer acts as a voltage divider when its ends are connected to Vref and ground.
- Ideal output relation: Vout = Vref(Rlower / (Rupper + Rlower)).
- For a linear rotary potentiometer over its usable angle range: Vout = Vref(theta / theta_max).
- Angle estimate from voltage: theta = theta_max(Vout / Vref).
- A potentiometer measures absolute position because each shaft angle corresponds to a voltage.
- Servo feedback uses error = target position - measured position to decide motor correction.
Vocabulary
- Potentiometer
- A variable resistor with a movable contact that can produce a voltage related to position.
- Wiper
- The moving contact that slides along the resistive track and carries the output voltage.
- Resistive track
- The fixed conductive material whose resistance is spread along the motion path of the wiper.
- Voltage divider
- A circuit that produces an output voltage that is a fraction of an input voltage using two resistances.
- Analog to digital converter
- A circuit that changes a continuous voltage into a digital number a microcontroller can process.
Common Mistakes to Avoid
- Connecting the wiper directly to power or ground, which removes the variable output and can cause excessive current. The two end terminals should usually connect to Vref and ground, with the wiper as the signal.
- Assuming every potentiometer can rotate through a full 360 degrees, which is often false. Many rotary potentiometers have a limited electrical travel such as 270 degrees.
- Ignoring the reference voltage, which makes angle calculations wrong. The output voltage is proportional to Vref, so a changing supply changes the measured value unless the converter uses the same reference.
- Treating the signal as perfectly smooth and exact, which can lead to unstable control. Real potentiometers have noise, friction, dead zones, and wear, so filtering and calibration may be needed.
Practice Questions
- 1 A 270 degree rotary potentiometer is connected between 5.0 V and ground. If the wiper is at 135 degrees and the track is linear, what is Vout?
- 2 A robot joint uses a 300 degree potentiometer with Vref = 3.3 V. The controller reads Vout = 2.2 V. What joint angle does this represent, assuming a linear track starting at 0 degrees?
- 3 A servo uses a potentiometer for feedback. Explain why this sensor can tell the controller the shaft position immediately after power is turned on, while a sensor that only counts changes in motion cannot.