Servo motors are motors combined with a sensor, so that the position/velocity/torque of the motor at a given instance can be measured and corrected using the fundamentals of control systems. In servomotors, a desired value of position/velocity/torque is provided as an input and the actual value of the same is taken as the output. The voltage/current is varied in such a way that the difference between the output and input is minimized. The device which varies the voltage/current of the motor is called as the controller of the motor. One of the most famous and widely used controllers are pid controllers. Now, that we are clear about what servomotors are we can see why they are preferred in robots. Robots are designed in such a way that the are able to perform a wide variety of tasks. Move from one point to another at different speeds. Pick objects from any random point and place it at any other point. So, while doing this the speed and position of the must be precisely controlled. This can be achieved using servo motors. There are other popular motors types , that are used in robots, stepper motors is one example. However, due to torque and speed considerations servo motors are preferred.
Answers
Servo implies an error sensing feedback control which is utilized to correct the performance of a system. It also requires a generally sophisticated controller, often a dedicated module designed particularly for use with servomotors. Servo motors are DC motors that allows for precise control of angular position. They are actually DC motors whose speed is slowly lowered by the gears. The servo motors usually have a revolution cutoff from 90° to 180°. A few servo motors also have revolution cutoff of 360° or more. But servo motors do not rotate constantly. Their rotation is limited in between the fixed angles.
The servo motor is actually an assembly of four things: a normal DC motor, a gear reduction unit, a position-sensing device and a control circuit. The DC motor is connected with a gear mechanism which provides feedback to a position sensor which is mostly a potentiometer. From the gear box, the output of the motor is delivered via servo spline to the servo arm. For standard servo motors, the gear is normally made up of plastic whereas for high power servos, the gear is made up of metal.
A servo motor consists of three wires- a black wire connected to ground, a white/yellow wire connected to control unit and a red wire connected to power supply.
The function of the servo motor is to receive a control signal that represents a desired output position of the servo shaft and apply power to its DC motor until its shaft turns to that position.
It uses the position sensing device to figure out the rotational position of the shaft, so it knows which way the motor must turn to move the shaft to the instructed position. The shaft commonly does not rotate freely around similar to a DC motor, however rather can just turn 200 degrees.
Servo Motor
Servo Motor
From the position of the rotor, a rotating magnetic field is created to efficiently generate toque. Current flows in the winding to create a rotating magnetic field. The shaft transmits the motor output power. The load is driven through the transfer mechanism. A high-function rare earth or other permanent magnet is positioned externally to the shaft. The optical encoder always watches the number of rotations and the position of the shaft.
Working of a Servo Motor
The Servo Motor basically consists of a DC Motor, a Gear system, a position sensor and a control circuit. The DC motors get powered from a battery and run at high speed and low torque. The Gear and shaft assembly connected to the DC motors lower this speed into sufficient speed and higher torque. The position sensor senses the position of the shaft from its definite position and feeds the information to the control circuit. The control circuit accordingly decodes the signals from the position sensor and compares the actual position of the motors with the desired position and accordingly controls the direction of rotation of the DC motor to get the required position. The Servo Motor generally requires DC supply of 4.8V to 6 V.
Controlling a Servo Motor
A servo motor is controlled by controlling its position using Pulse Width Modulation Technique. The width of the pulse applied to the motor is varied and send for a fixed amount of time.
The pulse width determines the angular position of the servo motor. For example a pulse width of 1 ms causes a angular position of 0 degrees, whereas a pulse width of 2 ms causes a angular width of 180 degrees.
Advantages:
If a heavy load is placed on the motor, the driver will increase the current to the motor coil as it attempts to rotate the motor. Basically, there is no out-of-step condition.
High-speed operation is possible.
Disadvantages:
Since the servomotor tries to rotate according to the command pulses, but lags behind, it is not suitable for precision control of rotation.
Higher cost.
When stopped, the motor’s rotor continues to move back and forth one pulse, so that it is not suitable if you need to prevent vibration
7 Applications of Servo Motors
Servomotors are used in applications requiring rapid variations in speed without the motor getting overheated.
In Industries they are used in machine tools, packaging, factory automation, material handling, printing converting, assembly lines, and many other demanding applications robotics, CNC machinery or automated manufacturing.
They are also used in radio controlled airplanes to control the positioning and movement of elevators.
They are used in robots because of their smooth switching on and off and accurate positioning.
They are also used by aerospace industry to maintain hydraulic fluid in their hydraulic systems.
They are used in many radio controlled toys.
They are used in electronic devices such as DVDs or Blue ray Disc players to extend or replay the disc trays.
They are also being used in automobiles to maintain the speed of vehicles.