Servo Motors – Information, Usage and Control
An introductory article about servo motors: What are servo motors? What is an RC Servo motor? Servo motors usages in robotics applications. Advantages and disadvantages of servo motors, comparison between servo motors and stepper motors. Servo motor control. Servo motor voltage, current, and wiring.
What is an R/C Servo Motor?
A Servo Motor is a motor that has an internal gear-based transmission system and electronic control
of the position of the motor’s head.
RC Servo Motors are radio-controlled servo motors that use direct-current, and are able to move to a certain angle, usually between 0 to 180 degrees. The are called RC servos because they are usually connected to radio-control, although they can be used without it.
Servo motors work in a closed loop, meaning that they can measure the exact position of the motor head, and by controlling the motor fix the gaps from the desired position.
Usage of Servo Motors in Robotics Applications
Servo motors are being used in a wide variety of robots, machines, and general robotic applications, including: robotic arms, radio-controlled toy-cars, air-planes and helicopters, industrial machinery and many many more applications. There are many reasons that make servo motors so common – their ease of control, the low energy requirements (efficiency), the high torque, TTL voltage level control, and even the physical properties – servo motors are relatively small sized and have a low weight.
You can see a few demonstrations of servo motors in the next two videos:
Advantages and Disadvantages of Servo Motors
Servo motors are used in many robotics applications, due to many reasons:
- Servo motors usually have a small size
- Servo motors have a large angular force (torque) comparing to their size
- Servo motors operate in a closed loop, and therefore are very accurate
- Servo motors have an internal control circuit
- Servo motors are electrically efficient – they required current is proportional to the weight of the load they carry.
Comparison between Servo motors and Stepper Motors
- The most significant difference between servo motors and stepper motors is the fact servo motors operate in a closed loop while stepper motors operate in an open loop. This means servo motors have an internal feedback – they are able to measure their position, the difference between the actual position and the desired position, and to fix the gap by controlling the motor. Stepper motors, on the other hand have no feedback and thus are more error-prone.
- RC Servo motors are limited to 0-180 degrees of movement and require physical and electrical modification in order to be able to move in 360 degrees. Stepper motors do not have this limit.
- Stepper motors are usually cheaper than servo motors
- Stepper motors lose torque in high rotational speeds, while servo motors do not.
How does a servo motor work?
Servo motor control
Controlling a servo motors is achieved by sending a digital signal to the motor’s control wire. The general idea is sending a square wave signal to the motor, where the wavelength of the wave sets the angle to which the motor will move.
In example, when sending a 1ms pulsed square wave to the motor, it will move to its minimal angle – 0 degrees.
When sending a 1.5ms pulsed square wave to the motor, it will move to its middle angle – 90 degrees.
When sending a 2ms pulsed square wave to the motor, it will move to its ‘largest’ angle – 180 degrees.
Schematic diagram:
In the video below you can see an example of controlling a servo motor. Pay attention to the wavelength of the wave shown in the oscilloscope, and to the motors angle that changes accordingly.
Servo voltage
Servo motors operate in various voltage levels, but RC servo motors usually operate between 4.8 to 6 volts. The reason for using these voltage levels are their proximity to TTL voltage levels (5volts) which most micro-controllers use. So, what voltage is recommended to use? the answer is simple – using the maximal allowed voltage specified for the motor will yield the highest torque.
Servo current
Servo Wiring
Servo motors have 3 wires:
A brown or black wire, which is negative voltage supply (ground, -)
A red wire which is positive voltage supply (+)
A yellow, orange or white wire that carry the control signal.
In example, in the common HiTec \ Futaba servos:
Building a Servo Motor Controller
Servo motor control is done by generating a PWM (pulse width modulation) signal. Generating such a singal is CPU intensive as it requires a lot of Interrupt calls. Generating several PWM signals at the same time (if several motors are controlled) is an even harder tast for a computer — it the best case it will cause a high system load, and in the worst case it will cause a wrong PWM signal generation, and hence an erroneous movement of the motor. This is why, besides the ease of use, an dedicated servo controller is recommended for interfacing servo control to a PC.
You can read more about the subject in the USB Servo Controller page.
