Switching a digital signal on and off produces the equivalent of an analog DC signal. Varying the ratio of on time to off time is what commands a linear actuator to move to a specific position. This technique is called Pulse Width Modulation, or PWM, and it is one of the most widely used control methods in microcontroller-based designs.
For engineers and makers working with Actuonix micro linear actuators, PWM via the RC servo interface is a practical, low wiring count way to achieve precise position control from an Arduino, a flight controller, or any standard RC receiver.
How PWM Works in an Actuator
The RC servo PWM signal runs at a frequency of 50Hz. Each pulse is between 1ms and 2ms in duration. A 1ms pulse commands the actuator to fully retract. A 2ms pulse commands full extension. Pulse widths between those values move the actuator to a proportional intermediate position.
Typically, this signal uses 0-5V logic levels, but our R-series actuators can accept other levels like 0-3.3V. The onboard controller reads each incoming pulse, compares it against the current rod position via the internal potentiometer, and drives the motor until the rod reaches the commanded location. Position is encoded in the pulse width not in the duty cycle percentage.
Which Actuonix Models Support PWM
The R-series models accept RC servo PWM directly. The L12-R and L16-R have internal position controllers and are designed as drop-in replacements for standard rotary RC servos, making them immediately compatible with any RC receiver or microcontroller that outputs a standard servo signal.
The P16-R and P8-P achieve the same result using a P16-P actuator paired with the Actuonix External RC Control Board (Ext-R), and functions identically to the other R-series models for PWM control purposes.
The L12-I also supports the RC servo interface on lead 4. When the actuator powers up, it scans the input leads and self-configures to whichever interface mode it detects first RC servo, 0-5V analog, or 4-20mA. Once configured, the other input modes are disabled until the next power cycle. The I-series is only available in the L12 product family.
S-series and P-series models do not have onboard controllers and cannot accept PWM directly. P-series actuators can be paired with the LAC control board, which accepts a 0-5V PWM input and closes the position loop externally.
Related Article: What Are Linear Actuators?
Understanding Duty Cycle with PWM-Controlled Actuators
When working with PWM controlled actuators, it is important to distinguish between two different uses of the term duty cycle.
The PWM signal duty cycle refers to the on-off ratio of the control signal itself. In RC servo PWM, this is less significant than the pulse width, since position is encoded in pulse duration not in the percentage of time the signal is high.
The actuator's operating duty cycle is a separate and more critical consideration. It is calculated as run time divided by total cycle time. Our brushed DC motor actuators are rated at a maximum operating duty cycle of 20%, determined at each product's peak efficiency point.
That rating is not a fixed ceiling across all conditions it is the starting point. Higher loads increase motor current and heat generation, which reduces the safe allowable run time below 20%. Higher ambient temperatures have the same effect. Running past the rated duty cycle does not break the actuator immediately. It accelerates heat buildup in the motor windings and gearbox, shortening service life over time.
For a robotic joint that moves a few times per minute, a valve that opens and closes occasionally, or a camera platform adjusting periodically, the 20% limit is unlikely to be reached. Where it becomes critical is in test rigs or high-frequency cycling applications.
Related Article: What Is Duty Cycle?
Using PWM with Arduino
The Arduino Servo library generates the RC servo PWM signal natively. Attach the actuator's signal wire to a PWM capable digital pin, include the Servo.h library, and use writeMicroseconds() with a value between 1000 and 2000.
For R-series wiring: white is signal, red is power at 6VDC, black is ground. Keep the actuator's power supply separate from the Arduino logic supply. If the actuator or other servos behave erratically, place a 1 to 4 ohm resistor in series with the red power lead and resolves most noise issues caused by motor current draw.
Related Article: How to Control a Linear Actuator with Arduino
Common Issues to Watch For
Stalling is the most damaging condition for a PWM controlled actuator. If the signal commands a position the actuator cannot reach because the load exceeds its rated force, the motor stalls drawing current while producing no movement. The L16 datasheet states directly that repeated stalling or stalling for more than a few seconds will shorten actuator life significantly.
Signal noise shows up as jitter or erratic movement. It is usually a grounding issue or the result of signal wiring running close to motor power leads. Separating signal wiring from power wiring and ensuring a solid common ground resolves most cases.
Frequently Asked Questions
What does PWM stand for?
PWM stands for Pulse Width Modulation. It is a method of encoding a control signal by varying the on-time of a digital square wave. In linear actuator applications, the pulse width determines the position the actuator moves to.
What PWM frequency do Actuonix actuators use?
Actuonix R-series and I-series actuators use RC servo PWM at a frequency of 50Hz, with pulse widths between 1ms and 2ms. A 1ms pulse commands full retraction and a 2ms pulse commands full extension.
Can I control the speed of an Actuonix actuator using PWM?
RC servo PWM is designed to control position only, not speed. The actuator will move to the commanded position at its natural speed for the given load. If you need to slow down the movement, the practical approach is to send incremental position commands in small steps from your microcontroller rather than jumping to the target position in one pulse.
What is the maximum duty cycle for Actuonix actuators?
Both the L12 and L16 are rated at a maximum operating duty cycle of 20%, determined at peak efficiency. Duty cycle is calculated as run time divided by total cycle time. Higher loads or ambient temperatures reduce the allowable duty cycle below that figure. Exceeding the rated duty cycle consistently accelerates heat buildup and shortens actuator life.
