In modern usage the term servo or servo-mechanism is restricted to a feedback control system in which the controlled variable is mechanical position or time derivatives of position such as velocity and acceleration.
A servo is a device, electrical, mechanical or electro mechanical, that upon receipt of a stimulus or input, will employ feedback for velocity and/or position control, creating a closed loop.
A feedback system is a control system which tends to maintain a prescribed relationship between a controlled quantity and a reference quantity by comparing their functions and using the difference as a means of control
There are mainly two types of servo-motors
1)AC Servo motor
2)DC Servo-motor
AC servo-motors are generally preferred for low-power use. And for high-power use DC servo motors are preferred because they operate more efficiently than comparable to AC servo motors.
Unlike large industrial motors, dc servomotors are not used for continuous energy conversion. The basic operating principle is same as other electromagnetic motors.
Construction:
1.It has construction as same as dc motor. It is consist of stator and rotor and controlling parts.2.It has feedback generator for generating feedback for controlling the speed & torque.
3.It has two ports one for dc supply and other for controlled dc supply.
Field Controlled DC Servo Motor Theory:
The figure below illustrates the schematic diagram for a field controlled DC servo motor. In this arrangement the field of DC motor is excited be the amplified error signal and armature winding is energized by a constant current source.
The field is controlled below the knee point of magnetizing saturation curve. At that portion of the curve the mmf linearly varies with excitation current. That means torque developed in the DC motor is directly proportional to the field current below the knee point of magnetizing saturation curve.
Armature Controlled DC Servo Motor Theory:
The figure below shows the schematic diagram for an armature controlled DC servo motor. Here the armature is energized by amplified error signal and field is excited by a constant current source.
The field is operated at well beyond the knee point of magnetizing saturation curve. In this portion of the curve, for huge change in magnetizing current, there is very small change in mmf in the motor field. This makes the servo motor is less sensitive to change in field current. Actually for armature controlled DC servo motor, we do not want that, the motor should response to any change ofPerformance Specifications:
DC servomotors share many performance specifications that are applicable to all types of?DC motors. To properly size a motor, these specifications must be matched according to the load requirements of the application.
Shaft speed (RPM) defines the speed at which the shaft rotates, expressed in rotations per minute (RPM). Typically, the speed provided by the manufacturer is the no-load speed of the output shaft, or the speed at which the motor's output torque is zero.
Terminal voltage refers to the design voltage of the DC motor. Essentially the voltage determines the speed of a DC motor,and speed is controlled by raising or lowering the voltage supplied to the motor.
Torque is the rotational force generated by the motor shaft.The torque required for the motor is determined by the speed-torquecharacteristics of thevarious loads experienced in the targetapplication.
Starting torque - The torque required when starting the motor,which istypically higher than the continuous torque.
Continuous torque - The output torque capability of the motor under constant running conditions.
Some ratings of dc servo-motor available:
Shaft Speed:
Less than 1,610 rpm
1,610 to 3,187 rpm
3,187 to 4,700 rpm
4,700 to 7,090 rpm
7,090 rpm and up
Terminal Voltage:
Less than 20 VDC
20 to 50 VDC
50 to 100 VDC
100 to 180 VDC
180 VDC and up
Continuous Current:
Less than 1 amps
1 to 4 amps 4 to 8 amps
8 to 17 amps
17 amps and up
Continuous Torque:
Less than 0.45 Nm
0.45 Nm to 1.70 Nm
1.70 Nm to 5 Nm
5 Nm to 17 Nm
17 Nm and up
Continuous Output Power:
Less than 0.4 HP
0.4 to 1 HP 1 to 2 HP
2 to 6 HP
6 HP and up
Advantages:
- High output power relative to motor size and weight.Encoder determines accuracy and resolution.
- High efficiency. It can approach 90% at light loads.High torque to inertia ratio.It can rapidly accelerate loads.
- Has "reserve" power. 2-3 times continuous power for short periods.
- Has "reserve" torque. 5-10 times rated torque for short periods.
- Motor stays cool. Current draw proportional to load.
- Usable high speed torque.
- Maintains rated torque to 90% of NL RPM
- Audibly quiet at high speeds.Resonance and vibration free operation.
Disadvantages:
- Requires "tuning" to stabilize feedback loop.Motor "runs away" when something breaks. Safety circuits are required.
- Complex. Requires encoder.Brush wear out limits life to 2,000 hrs. Service is then required.
- Peak torque is limited to a 1% duty cycle.Motor can be damaged by sustained overload.
- Bewildering choice of motors, encoders, and servo-drives.
- Power supply current 10 times average to use peak torque.
- Motor develops peak power at higher speeds. Gearing often required.
- Poor motor cooling. Ventilated motors are easily contaminated.
Applications:
DC servomotors finds its applications in various domain. Some of them are given below:
- For very high voltage power systems, dc motors are preferred because they operate more efficiently than comparable ac servomotor.
- It has also find its application in inkjet printers and RC helicopters.
- To drive conveyors used in Industrial manufacturing and assembling units to pass an object from one assembly station to another.
- It is also used in solar tracking system.DC servomotors are widely used in robots, toy cars and other position controlled devices.
- Widely used in radars, computers, robots, machine tools tracking system, process controllers etc.
Fasttobuy co.,ltd is a professional and experienced company which specializes in producing and researching,located in changzhou city,jiangsu province, china. Our main products are stepper motors and drivers,servo motor and drivers,gear motor and other automatic device, applied widely in printing equipment, engraving machine textile machine, computer external application equipment, medical instruments, stage light equipment, robot, CNC machine and other automatic controlling system. Exported to United States, Canada, Germany, United Kingdom, France, Switzerland, Italy, Russia, Korea and so on more than ten countries and regions in the world.
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