Motors: Stepper Vs Servo

Stepper motors:

A stepper motor's shaft has permanent magnets attached to it. Around the body of the motor is a series of coils that create a magnetic field that interacts with the permanent magnets. When the coils are turned on and off the magnetic field causes the rotor to move and it is this sequence of on and off that determines whether the motor rotates forwards in reverse. This sequence is known as the phase pattern and there are several types of patterns that will cause the motor to turn. Common types are full-double phase, full-single phase, and half step.

In order for a stepper motor to rotate, the coils must be constantly turned on and off. If only one coil is energized the motor will just jump to that position and stay there resisting change. This energized coil can pull the full current, without moving. This ability to stay rigidly in position is often considered to be one of the main advantages of stepper motors. The torque at standstill is called the holding torque. As stepper motors can be controlled by turning coils on and off they are easy to control with digital circuitry and microcontroller chips. The controller simply energizes the coils in a certain pattern and the motor will move accordingly. At any given time the computer will detect the position of the motor, as it is easy to track the number of steps that are moved. Most stepper motor control systems will have a home switch associated with each motor that will allow the software to determine the starting or reference "home" position.

Servo motors:


Although there are several different types of servo motors a normal DC motor has one coil with 2 wires. If a charge is connected to the wires the motor will spin, unlike a stepper, which will move a step, and then stay stationary until the coil is turned on and off again.

Servo motors have control circuits, and a potentiometer (a variable resistor, aka pot) that is connected to the output shaft, which allows the control circuitry to monitor the current angle of the servo motor. If the shaft is at the correct angle, then the motor will shut off. If the circuit finds that the angle is not correct, it will turn the motor the correct direction until the angle is correct. A normal servo is used to control an angular motion of between 0 and 180 degrees; the mechanical stop, built on the output gear, will prevent it from turning any further.

The amount of power applied to the motor is proportional to the distance it needs to travel. So, if the shaft needs to turn a large distance, the motor will run at full speed. If it needs to turn only a small amount, the motor will run at a slower speed, a process that is known as proportional control.

RC Servos:

The type of servo motor that I will be considering for this project is the type that is commonly found in hobby airplanes and cars, known as RC (remote controlled) servo motors. Although such motors are housed in small boxes, they contain a complete servo system, including; a motor, a gearbox, feedback device (pot), servo control circuitry, and a drive circuit.

Unlike DC motors, RC servos normally have 3 wires: +v, ground, control. The control wire is used to communicate the angle, which is determined by the duration of a pulse that is applied to the control wire. This is called Pulse Coded Modulation. The servo expects to receive a pulse every 20 milliseconds (.02 seconds) and it is the length of the pulse that will determine how far the motor turns. For example, a 1.5 millisecond pulse will make the motor turn to the 90 degree position (often called the neutral position). If the pulse is shorter than 1.5 ms, then the motor will turn the shaft to closer to 0 degrees and if the pulse is longer than 1.5ms, the shaft will turn closer to 180 degrees.