servo motor

                             SERVO MOTOR

This is nothing but a simple electrical motor, controlled with the help of servomechanism. If the motor as controlled device, associated with servomechanism is DC motor, then it is commonly known DC Servo Motor. If the controlled motor is operated by AC, it is called AC Servo Motor.

It consists of three parts:

1. Controlled device
2. Output sensor
3. Feedback system

OPERATION PRINCIPLE OF SERVO MOTOR:

• Servo motors are geared DC motors with the closed-loop circuitry incorporated within them. The basic configuration of a servo motor composed of a DC motor, gearbox, potentiometer and control circuit.
• DC motor is used to move a gearbox with a large reduction ratio. The final shaft imposes a force on the external load and simultaneously acts on the axis of the feedback potentiometer. So, the potentiometer senses the position of the axis and sends a corresponding voltage to an operational amplifier. This voltage compared to the input voltage, that determines the desired position of the shaft, producing a voltage in the output of the comparator. 
• This voltage powers the motor such that the shaft moves in the necessary direction to align with the angle that corresponds to the voltage applied to the input.

                

                                      

                            simplified block diagram

How is the servo controlled?

• Servos are controlled by sending an electrical pulse of variable width, or pulse width modulation (PWM), through the control wire. There is a minimum pulse, a maximum pulse, and a repetition rate .A servo motor can usually only turn 90 degrees in either direction for a total of 180 degree movement. The motor's neutral position is defined as the position where the servo has the same amount of potential rotation in the both the clockwise or counter-clockwise direction. The PWM sent to the motor determines position of the shaft, and based on the duration of the pulse sent via the control wire; the rotor will turn to the desired position. The servo motor expects to see a pulse every 20 milliseconds (ms) and the length of the pulse will determine how far the motor turns. For example, a 1.5 ms pulse will make the motor turn to the 90-degree position .Shorter than 1.5 ms moves it to 0 degrees, and any longer than 1.5ms will turn the servo to 180 degrees. 

Types of Servo Motors:

• Basically, servo motors are classified into AC and DC servo motors depending upon the nature of supply used for its operation. Brushed permanent magnet DC servo motors are used for simple applications owing to their cost, efficiency and simplicity.
• These are best suited for smaller applications. With the advancement of microprocessor and power transistor, AC servo motors are used more often due to their high accuracy control.

CHARACTERISTICS:

Power Supply Voltage and Current :

• The power supply voltage and current values are specified for each type of servo motors and depend on the application. The common RC servo motors powered from supplies in the range of 4 – 6 V and 100 mA - 2 A . 

Operating  Speed :

• Operating speed of a servo motor is defined as the time required for the shaft to reach a specified position. Common servos have operating speeds in the range of 0,05 to 0,2 s/60 degree.

Torque :

• Typical values of torques of servo motors are in the range of 0,5 to 10 kg/cm. 

Control Pulse :

• Control pulse is referred to the type of pulse used to position the shaft. Two main types of control pulses used in RC applications: center position in 1-2 ms and 1,25-1,75 ms. 

Resolution :

• It defines the precision with which the shaft is positioned when it receives an external command signal. Typical servo motors have resolutions in the range from 1 degree to 10 degree. 

Size and Weight :

• This characteristic is important in the mechanic design of projects. Typical RC servo motors have a weight range between 15 and 200 g.