Accuracy of Potentiometer Feedback: How Good Is the Position Sensor?

In the world of robotics, RC hobbies, and precision automation, the micro servo motor is a titan of tiny motion. From guiding a robotic arm’s delicate grip to steering a model airplane with hair-trigger responsiveness, these compact devices are marvels of engineering. At the heart of their closed-loop control—the very feature that defines a servo—lies a humble, often overlooked component: the potentiometer. This position sensor is tasked with telling the motor’s control circuit, “You are here.” But just how good is this feedback? How accurate, reliable, and precise can a simple variable resistor truly be? Let’s dive deep into the world of potentiometer feedback, exploring its surprising capabilities and inherent limitations within the context of the ubiquitous micro servo.

The Micro Servo: A Closed-Loop Powerhouse

Before we judge the sensor, we must understand the system it serves. A standard micro servo (like the ubiquitous SG90 or MG90S) is not just a motor; it’s an integrated system.

Core Components: The Trinity of Control

• The DC Motor: Provides the rotational force.
• The Gear Train: Reduces the high-speed, low-torque output of the motor to a slow, powerful movement suitable for control surfaces, joints, or actuators.
• The Control Circuit & Feedback Sensor: This is the brain. It compares the desired position (from the Pulse Width Modulation, or PWM, signal) with the actual position (from the potentiometer) and drives the motor to minimize the difference, or error.

This last component is where the potentiometer earns its keep. Without its continuous feedback, the servo would be nothing more than a geared motor blindly spinning to a command.

The Potentiometer: Anatomy of an Analog Sensor

A potentiometer, or “pot,” is a fundamentally analog device. In a servo, it’s typically a rotary potentiometer mechanically linked to the servo’s output shaft.

How It Works: A Voltage Divider in Action

Physically, it consists of a resistive track (often made of carbon, cermet, or conductive plastic) and a wiper that moves along it. The track is connected between a reference voltage (e.g., 5V) and ground. The wiper, connected to the output shaft, taps off a voltage proportional to its angular position. If the shaft is at 0 degrees, the wiper might output 0.5V; at 90 degrees, 2.5V; at 180 degrees, 4.5V. This analog voltage is the continuous, real-time report of the servo’s position.

The Path of Feedback: From Resistance to Correction

1. Signal In: The receiver sends a PWM pulse (e.g., a 1.5ms pulse typically means “center position”).
2. Comparison: The servo’s internal IC converts the PWM width and the potentiometer’s voltage into comparable values.
3. Error Detection: It calculates the difference between where it’s told to be and where the pot says it is.
4. Motor Drive: It applies power to the DC motor in the direction that reduces this error to zero.
5. Continuous Monitoring: This happens hundreds of times per second, creating the stiff, holding torque and precise movement we associate with servos.

Quantifying Accuracy: What Does "Good" Mean for a Potentiometer?

Accuracy in this context is multifaceted. It’s not just about hitting a single point, but about consistency, resolution, and repeatability over time and temperature.

Theoretical Resolution vs. Practical Limits

In theory, an analog voltage offers near-infinite resolution. The control circuit’s analog-to-digital converter (ADC) determines the practical limit. A 10-bit ADC on a 180-degree range offers a theoretical resolution of 180°/1024 ≈ 0.18 degrees per step. This is superb for most micro servo applications. However, this is a theoretical limit. Real-world resolution is degraded by electrical noise, mechanical slop in the gears and pot wiper, and the quality of the resistive track.

Key Metrics of Potentiometer Performance

• Linearity: Does a 10% change in command result in a 10% change in actual position? Poor linearity means the servo will be accurate at some points (like center and endpoints) but consistently off in between.
• Repeatability: Can the servo return to the exact same physical position when given the same command multiple times? This is often where decent pots perform acceptably, with repeatability within a few tenths of a degree.
• Hysteresis: This is a lag caused by mechanical slack. If you approach a position clockwise, does it settle in the same spot as when you approach it counter-clockwise? Hysteresis is a primary source of inaccuracy in low-cost servos.
• Deadband: The smallest change in command pulse the servo will actually respond to. A smaller deadband means finer control.

The Inevitable Trade-Offs: Why Not All Servo Pots Are Created Equal

The accuracy of the potentiometer is the single biggest differentiator between a bargain-bin micro servo and a high-end, digitally controlled model.

The Cost & Quality Spectrum

• Low-Cost Carbon Film Pots: Found in sub-$5 servos. They suffer from poorer linearity, higher noise, and significant wear over time. Their accuracy can drift, and they may develop "jitters" or dead spots.
• Cermet or Conductive Plastic Pots: Used in mid-range "hobby-grade" servos. They offer better linearity, lower noise, and much longer rotational life (often 50,000 to 100,000 cycles). Accuracy and repeatability are significantly improved.
• Precision or Multi-Turn Pots: Used in industrial or instrumentation servos. These are designed for high linearity (e.g., 0.5% or better) and extreme longevity.

Environmental Enemies of Accuracy

• Temperature: The resistance of the track changes with temperature, causing drift. The control circuit may compensate to some degree.
• Wear and Tear: Every movement of the wiper causes microscopic wear on the track. Over thousands of cycles, this can create non-linearities or increased electrical noise, manifesting as jitter or loss of accuracy.
• Mechanical Shock and Vibration: Can cause momentary loss of contact or, over time, loosen the mechanical coupling between the output shaft and the pot.

The Digital Revolution: Encoders as the Challenger

To understand the potentiometer’s place, we must look at its competitor: the digital optical or magnetic encoder.

Encoder Advantages: Where the Pot Falls Short

• Absolute Accuracy: Encoders provide a direct digital readout of position. They are not subject to analog drift or non-linearity from the sensor itself.
• Longevity: With no physical contact (in optical models), there is no wear on the sensing element.
• Multi-Turn Capability: A standard pot only knows its position within one rotation. An encoder can track total rotations, enabling more complex control schemes.

Why the Potentiometer Endures in Micro Servos

Despite the encoder’s advantages, the potentiometer remains king in the micro servo market for compelling reasons: * Cost: A pot costs pennies. A reliable encoder costs significantly more. * Simplicity: It provides an absolute analog voltage directly proportional to position. No complex interpolation or additional circuitry is needed for basic function. * Integrated Form Factor: The compact, cylindrical shape of a rotary pot fits perfectly into the standard servo housing. It is mechanically simple to couple to the output shaft. * Adequacy: For the vast majority of hobbyist applications—steering an RC car, controlling the tilt of a sensor, animating a robot—the accuracy of a mid-grade potentiometer is more than sufficient. The 1-2 degree potential error is lost in the mechanical slop of the linkages and the requirements of the task.

Pushing the Limits: Calibration and Advanced Techniques

Hobbyists and engineers have developed methods to squeeze the best possible performance from potentiometer-based servos.

The Critical Role of Calibration

Factory calibration is often minimal. Precision users will often: 1. Command the servo to its extreme positions. 2. Physically measure the actual output arm angle. 3. Program their controller to map the command pulse range to this measured physical range, correcting for the pot’s endpoint inaccuracies.

Modding and Upgrading

A common upgrade is to replace a worn or low-quality carbon pot with a higher-grade conductive plastic potentiometer. This can breathe new life and improved accuracy into an otherwise jittery servo. Furthermore, the rise of digital servo controllers (which still use an analog pot) allows for features like programmable deadband, smoothing, and endpoint adjustment, effectively managing the pot’s raw feedback to create more refined motion.

The Verdict on the Vernacular Workhorse

So, is the potentiometer a good position sensor for a micro servo? The answer is a nuanced but largely affirmative "It depends, but often yes."

Its accuracy is a blend of theoretical potential and practical compromise. For a standard hobby servo costing a few dollars, expect workhorse-level performance: repeatability within a couple of degrees, noticeable hysteresis, and a lifespan of thousands of cycles before degradation becomes problematic. This is perfectly matched to the demands of its primary market.

The potentiometer’s genius lies in its elegant simplicity and perfect suitability for the application. It provides just enough accuracy, at just the right cost, in just the right package, to enable the revolution of accessible robotics and RC technology. While encoders will continue to advance and claim the high ground of precision robotics and industrial automation, the humble potentiometer, faithfully reporting its analog voltage from the heart of the micro servo, is likely to remain a cornerstone of maker and hobbyist projects for years to come. Its accuracy is not laboratory-perfect, but it is, in a very real sense, good enough to have moved mountains—or at least, countless model airplanes, race cars, and robot arms.