Most servo motors expect a specific input signal, typically a PWM with a certain duty cycle. This signal encodes the desired position of the motor.
Observing this signal is critical to understanding how a servo works. This article will demonstrate how to debug this signal using a Wokwi logic analyzer.
Power
Servos are powerful motors that can control an object’s position (linear or angular) with unwavering precision. They can exert forces that conventional motors would struggle to move and are often used in robotics, automation and remote-controlled systems where precise positioning is essential.
A servo is capable of controlling its own motion by applying a constant amount of force against an external load – up to its maximum torque rating. The servo’s torque is determined by the size of its motor and the type of load that it is pushing against.
The way a servo motor is powered and controlled is key to how it behaves. Most servos will require a PWM signal that is sent via a signal wire. This signal wire must be connected to a piece of hardware that can generate a PWM output and set the frequency and pulse width range for that specific servo.
Analog servos operate based on voltage signals that come through the PWM pin. They have a neutral or “zero” position that they return to when not receiving any commands and the angle they can achieve is determined by the pulse width. For example a 1.5 ms pulse width will return the servo to its neutral position while anything longer or shorter will cause the servo to rotate clockwise or counterclockwise.
Pulse Width Modulation
One of the most common ways to control a servo motor is through pulse width modulation. This method involves sending a control signal that varies in width over time, and using the varying signal to change the average voltage applied to the pwm servo motor driver motor. This allows you to adjust its speed and rotational position. Increasing the signal’s width will increase the speed and torque, while decreasing the signal’s width will decrease the speed and torque.
For example, most hobby servos will recognize a pulse that’s 1.25 ms wide as a command to move the shaft into its neutral position. A pulse that’s 2.25 ms will be a command to turn the shaft into its maximum rotational position. These values can vary from servo to servo, though, so make sure you’re familiar with the specifics of your own model.
In order to create these PWM signals, you’ll need either a microcontroller (like an Arduino) or a servo driver module. The latter will often use a monostable multivibrator with an external pot that’s used to set the output pulse width, or duty cycle. This will usually be somewhere between 0 and 255, although you may want to use different settings depending on the motor’s requirements. In some cases, you’ll also need to take into account the motor update rate. If the update rate is too high, the motor might experience oscillation and damage its motor brushes.
Frequency
Besides pulse width and duty cycle, another important factor in controlling a servo is frequency. A servo typically expects to be updated every 20 milliseconds (ms), pwm servo motor driver manufacturer with a single pulse in either direction. The length of this pulse determines the movement. For example, a 1.5ms pulse moves the servo toward its natural 90 degree position, while a 2.5ms pulse moves it away from that position.
A higher PWM frequency reduces the time the transistor is open, and the current chops less frequently. This can also help eliminate acoustic and electromagnetic noise generated by the switching action of the transistor.
However, a servo motor can only tolerate so much pulse width variation. When the pulses are too short or too long, the servo can get stuck in an undesirable position. If the servo is stuck, a manual adjustment of the trim potentiometer can often restore it to its correct position.
If the servo fails to move back to its neutral position after being instructed, the problem may be a low voltage drop across the power wires. This can be caused by too much breadboard jumper wiring, or a lower than expected 5V power supply. In these cases, increasing the current capacity of the power wires or the 5V power supply can resolve the issue. Another possible cause of erratic servo behavior is interference from electrical noise in the circuit. This can be reduced by using shielded cables and proper grounding techniques.
Feedback
A servo motor provides mechanical power to turn the output shaft when instructed. Its internal controller compares the desired position with its current state and turns the servo’s actuator arm on or off to move it to the correct location. The servo’s position is reported back to the control circuit via a feedback sensor. This can be any kind of sensor like a potentiometer, Hall-effect device, tachometer, resolver, linear transducer or encoder.
Most hobby servos don’t provide any feedback, but there are hacks that let you monitor the position of a standard servo by reading the center, or wiper, of its internal positioning potentiometer. The problem is that this requires opening up the servo to violate its warranty.
An alternative is to use an analog feedback servo. This module connects the SCL and SDA lines from your microcontroller to the corresponding pins on its servo. You can supply 5V power to the servo through the screw terminals or a separate connector. You can also connect the OE header pin to the servo’s output terminal to provide feedback to your microcontroller.
To test the module simply load this program into your microcontroller, open the serial monitor and then press the Record button. As the LED lights, twist the servo shaft into different positions. The servo should be able to hold its new position until you stop the Record button. If you then press the Play button the motor should rotate in the same pattern as it recorded.