Distinguishing stepper and servo motors requires one to be familiar with what a motor is. An electric motor uses either a Direct Current (DC) or an alternating current (AC) to drive a machine. It has several parts including a commutator, axle, stator, brushes, power supply, and rotor. There are different types of motors but we will expand on the specific difference between a stepper and a servo motor.
Stepper Motors Explained
A stepper motor is a brushless DC motor with multiple coils that are in groups (phases) which rotate through a precise angle. One full rotation is divided into equal steps. When one phase is energized, it rotates one step (a well-defined motor angle). This is a great explainer. Unlike an ordinary DC motor, a stepper has rotating permanent magnets on the inside and static coils of wire on the outside. It is ideal for short quick moves and is used in motion control applications due to its precise positioning and speed control ability. You will find stepper motors in applications such as 3D printers, Camera Platforms and X, Y Plotters.
Servo Motors Explained
Other than the usual parts of an electric motor, the servo has a positional shaft that is fitted with a gear. The shaft is placed in a specific position by sending coded signals through electrical pulses. When the signal changes, the position of the shaft also changes. It has control wheels that regulates how much movement and in which direction. Servo are used in applications such elevators, rudders, grippers, and robots. Read more.
What are the major differences between a servo and a stepper motor?
A stepper motor has more poles with one rotation requiring many current exchanges. The poles allow the motor to move accurately and precisely as each pole stops the shaft. A Stepper does not require encoders as it moves precisely between poles using an open loop. It is much easier to drive a stepper motor to a precise position as a pulse moves the shaft one step and one only need to send the right number of pulses.
On the other hand, a servo requires position encoder to keep an accurate position of the shaft. Servo reads between the motors encoders and commanded position to adjust the current required to move. Servo is also mechanically complex and requires increased maintenance due to the presence of a gearbox and a position encoder. Learn more.
Torque (turning force) characteristics
A stepper has a constant holding torque that does not require the motor to be moved. It has maximum torque at low speeds making it excellent for applications that require high precision with low speed. The torque of a stepper at low speed is greater than that of a servo motor of the same size. Stepper does not have any reserve power and also tends to lose torque as it approaches its maximum drive speed.
A servo is able to maintain its torque rating at high speed and thus is great for high speed and high torque. The servo is more efficient compared to a stepper as it is able to supply twice the rated torque over a short period. More on the torque differences.
- A stepper motor may experience vibration and resonance issues while servo does not vibrate nor experience resonance issues.
- Stepper is cheaper and readily available compared to a servo that is expensive and rare to find.
Choosing between the two motors depends on the application, cost, torque requirement and positional accuracy.