1 Understanding Potentiometer Sensors as Position Sensors

Introduction

Position sensors play an important role in various industries, enabling precise measurement and control of object positions. Among the diverse types of position sensors, the potentiometer sensor stands out as a versatile device. In this reading material, we will delve into the fundamentals of position sensors, using the potentiometer sensor as a primary example. Additionally, we will explore common challenges such as loading error, linearity error, and resolution.

Overview of Position Sensors

Position sensors are devices that measure the displacement, rotation, or position of an object and convert it into an electrical signal. This signal can then be used by control systems to monitor movement or provide feedback for precise positioning. Position sensors find applications in numerous fields, including:

• Robotics – Guiding robotic arms for accurate movements.
• Automotive Systems – Monitoring steering wheel angle, pedal positions, and anti-lock braking systems (ABS).
• Consumer Electronics – Volume controls in audio equipment, joysticks in gaming consoles, and trackpads in laptops.
• Industrial Automation – Positioning cutting tools in CNC machines and maintaining conveyor belt alignment.

 Types of Position Sensors

• Resistive Sensors (Potentiometer, Rheostat)
• Inductive Sensors (Inductive Proximity Sensor)
• Capacitive Sensors
• Optical Sensors (Encoder, Photoelectric Sensor)
• Magnetic Sensors (Hall Effect Sensor)

Potentiometer Sensor as a Position Sensor

The potentiometer, a widely used position sensor, offers a simple yet effective design. It consists of a resistive element, typically a long, thin track made of carbon or other conductive material, and a wiper that slides along this track. As the wiper moves, the resistance between the wiper and one end of the track changes. This change in resistance directly corresponds to the position of the wiper, generating a continuously varying voltage output.

Advantages of Potentiometer Sensors:

• Simplicity and Cost-Effectiveness: Potentiometers are relatively inexpensive and straightforward to manufacture.
• Continuous and Smooth Output: They provide a continuous analog output signal that reflects the wiper’s position.
• High Resolution (in certain applications): With a fine-grained resistive element, potentiometers can achieve high resolution for precise measurements.

Applications of Potentiometer Sensors:

• Volume Controls: The familiar volume knob in audio equipment utilizes a potentiometer to adjust the audio level.
• Joysticks and Game Controllers: Potentiometers translate joystick movement into electrical signals for control in games.
• Motor Control Systems: Potentiometers can be used to provide feedback on motor shaft position, allowing for closed-loop control systems.

Below in Fig. 1, is the picture of a potentiometer as a position sensor showing application as a rotary pot and linear motion pot.

Figure 1: Rotary pot and Linear-motion pot (Adapted from “Modern control technology: components and systems”  by Kilian, Christopher T. ,  West Publishing Co., 1996.)

Fig. 2, below, shows a pot that detects the angular position of a robot arm. In this case, the pot body is held stationary, and the pot shaft is connected directly to the motor shaft. 10 volts is maintained across the (outside) terminals of the pot

Figure 2: Robotic arm that detects angular position and its electric circuit (Adapted from “Modern control technology: components and systems”  by Kilian, Christopher T. ,  West Publishing Co., 1996.)

Common Challenges in Position Sensors

While potentiometers are valuable tools, some limitations are essential to consider:

• Loading Error: Loading error occurs when the external load applied to the sensor affects its performance. It can lead to inaccuracies in position measurement. Strategies such as using proper mounting techniques and considering the sensor’s load capacity help mitigate loading errors.

Loading Error = V_NL– V_L

Where,  V_NL= output voltage with no load, and V_L= output voltage with load

Fig. 3, shows a loading error when the pot wiper is connected to a circuit with an input resistance that is not considerably higher than the pot’s resistance. When this happens, current flows out through the wiper arm, robbing current from the lower portion of the resistor and causing the reading to be low.

Figure 3: Loading error circuit

• Linearity Error: Ideally, the output voltage should change linearly with the wiper position. However, imperfections in the resistive element or mechanical design can cause deviations from a perfectly linear relationship. Calibration and careful design can minimize this error.

Linearity Error = (ΔR * 100) / R_tot, Where ΔR = minimum resistance error, and R_pot = total pot resistance

Linearity error refers to deviations from the ideal linear relationship between the physical position and the sensor’s output. Calibration and careful design can minimize linearity errors, ensuring accurate position measurements across the entire range. Fig. 4 shows the linearity error of a pot: ideal vs actual.

Figure 3: Linearity error (Adapted from “Modern control technology: components and systems”  by Kilian, Christopher T. ,  West Publishing Co., 1996.)

• Resolution: The smallest change in position that the sensor can detect is its resolution. Resolution is important for applications requiring high precision. Factors like the physical design of the sensor and the presence of noise can influence resolution.

In digital systems, the resolution usually refers to the value of the least significant bit (LSB) because that is the smallest change that can be reported. For an analog device such as a potentiometer, resolution refers to the smallest change that can be measured. It is usually expressed in percentages.

% Resolution = (ΔR / R_tot) * 100)

Where ΔR = small change in resistance, and Rtot= total resistance

Factors Affecting Resolution: Physical design of the sensor – Analog-to-digital converter (ADC) resolution – Noise in the sensor’s environment

Improving Resolution: Increasing the sensor’s precision – Enhancing signal processing algorithms – Minimizing external noise sources

Deepen your understanding: Watch the accompanying lecture video to delve deeper into the concepts covered in the reading.