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What is a Servo Motor?

Several inventions in history have made a huge impact on our everyday lives and have been with us for quite some time. One of these inventions is the servo motors. This small yet powerful component exists around us, ranging from toys to printers to the airline industry.

A servo motor is a type of linear actuator or rotary actuator which precisely controls the angular or linear position, velocity, and acceleration. This motor often has a control circuit that provides feedback on the instantaneous position of the motor shaft so that it can rotate with high accuracy. It is mostly used in applications where there is a need to control angles and distances with high precision and accuracy.

It is a simple motor that is operated via a servo mechanism. This type of motor comes with a gear arrangement that helps maintain high torque with smaller packages. They are also known for their compact and higher powers, variable speeds, and electromagnetic brake types.

Mechanism of Servo Motor  

The servo motor operates on the closed-loop servomechanism and uses closed-loop feedback for controlling the angle and position. The input can be in the form of analog or digital. The feedback system in the servo motor controls the motion and position of the motor’s shaft by measuring the position /speed of the shaft and feeding it back into the controller. This feedback signal is then compared with a reference signal. The control signal for the accurate speed/position of the servo motor is generated by the controller based on this feedback signal and the reference signal. The servo maintains the output at a certain decided value even in the presence of noise or other interference.

Working Principle of Servo Motor

Allen Bradley Servo Motor TL-A2540P-HJ32AN

All types of servo motors have a potentiometer, a gear assembly, and a controlling circuit. The gear assembly is used for decreasing RPM and increasing the torque of the motor. For example, we can assume that initially at the zero position of the motor the position of the potentiometer is set such that no electric signal is present at the output of the potentiometer. The electrical signal is then presented to the input terminal of the error detect amplifier and the variation between these two signals will be processed in the closed loop and the output will be generated in the form of an error signal which will be responsible for the input for the motor, and it starts rotating. The motor shaft is attached to the potentiometer and with the rotation of the motor, the potentiometer will also start rotating and will be generating a signal. As the rotatory angle of the potentiometer varies, the output signal changes too till the time is reached when the position of the potentiometer is according to the signal provided and there is no variation between the external signal which is being provided to the motor and the signal at the potentiometer and so the motor stops rotating.

Servo Motor Control

The latest servo motors are often designed and supplied with a defined controller module. Integration of servo motors with microcontrollers is quite easy. A servo usually has three connection wires, as shown in Fig 1.

Here the brown wire is for the ground (-), the red is for Vcc (+), and the orange denotes the PWM (signal) for the microcontroller. Sometimes this color coding is different, and many vary in some models, but mostly it remains the same.

Servo is controlled by PWM wire, where we have to set a minimum pulse, a maximum pulse, and a repetition rate. It works on the PWM (Pulse Width Modulation) principle, so the rotatory angle is controlled by the duty cycle of the PWM signal. This PWM signal with a specified duty cycle can be generated via microcontrollers. The duty cycle determines the change in shaft angle of the servo motor.

Types of Servo Motors

Servo motors are distinguished into two types, AC and DC servo motors. Here is a brief description of each of these types:

DC Servo Motor

In these types of motor, there is a separate DC source for field and armature winding. This motor is controlled by changing the field current or armature current. Each of these controls has its own benefits and depends on the application. They are also known as brushless servo motors as they don’t contain brushes. These motors are usually preferred in high power applications.

Following are the types of DC servo motors:

Series Motors: These motors have high starting torque, and also draw a large amount of starting current. There is lesser velocity regulation and turnarounds are obtained by changing the polarity of the field voltage (Vf ) which is done by splitting the series field winding. That’s why this motor is less efficient and has poor regulation.  

Spilled Series Motors: They have fewer partial kilowatts and are rated via split field. They can act as individual energized field-controlled motors and the armature can supply ample current. The motor forms a common curve with a torque speed. So, there is more stall torque which decreases if we increase the speed. This also provides excellent damping.

Shunt Control Motor: These motors come with two types of windings: field winding and armature winding. The former is present on the stator of the motor, while the latter is present on the rotor. Each winding is connected with a separate DC source for the motor’s operation.

Permanent Magnet Shunt Motor: The field is present in the motor because of a permanent magnet and there is no input current consumption for excitation. They are utilized in vehicle starters, wipers, and ACs.

AC Servo Motor

There is an encoder in this type of motor which is utilized by the microcontrollers for giving feedback and the closed-loop control. They are highly accurate and can be controlled precisely according to the application we are using them for. For getting a better amount of torque, they have a superior design. They help in facilitating variations in voltages more comfortably than a DC motor. They are extensively used in the fields of robotics, automated fields, and CNC.

Following are the types of AC servo motors:

Positional Rotation Servo Motor: They are the most used AC servo motors and have many applications. The shaft rotates 180 degrees in these motors, and they have actual physical stops which are placed in the gear mechanism for the prevention of rotation. They are used in robots, airplanes, toys, and remote cars.

Continuous Rotation Servo Motor: This and the positional rotation servo motor are the same, except the advantage of this type of motor is that it can go in every direction without mentioning a limit. The control signal is responsible for locating the static point of the motor for understanding the velocity of the rotation. They are mostly used in radar dishes.

Linear Servo Motor: It is also similar to a positional rotation servo motor, but it contains an extra mechanism to change the output and make it vibrate back and forth. They are not found extensively, and their applications are also limited although we may find them inside a hobbyist store where they can be used as actuators.

AC Servo Motor DC Servo Motor 
Speed is calculated from supply voltage’s frequency and number of magnetic poles Speed has direct relation with supply voltage  
Less efficiency More efficient 
More torque per weight  
High precision Less precise 
Reduced RF noise Noisy operation 
Less maintenance needed (no commutators) More maintenance needed (have commutators0 
Complex controller Simple Control 

The Efficiency of Servo Motors

Servo motors can be used in a variety of applications, whether it is for simple hobbies or industrial uses and the main thing to note is the energy efficiency. Servo motors are very efficient because of a variety of reasons.

First is their design and production which is different from the stepper motors. Another thing is their strong permanent magnets. These help in decreasing the length of the servo by at least 20 percent which resultantly increases the torque by up to 50 percent. They also have a higher resistance to demagnetization. An increase in torque helps in increasing efficiency and also regulating speed and when it comes to speed, the servo is very effective with over 85 percent efficiency. Also, if we compare them to cheaper models, they are produced by winding more wire in laminations so the space is filled and as a rule of thumb, more slots filled results in delivering more torque with greater efficiency, and it is estimated that manufacturing motors with this technique can increase the efficiency up to 85 percent.

Advantages of Servo Motors

There is no out-of-step condition in the servo motor. This means that if a heavy weight is on the motor the driver will itself raise the current towards the coil in an attempt of rotating it. Higher speed operation is also possible in these types of motor. Other features may include controlling the torque, smaller sizes, smoother running, more efficient, more power output, feedback control, very quiet operation, highly reliable, accuracy, and high acceleration.

Disadvantages of a Servo Motor

It is not much recommended to use in high precision applications as the motor tries to rotate according to the commanding pulse, but it lags. It has higher costs and also when it is stopped, the rotor still continues to vibrate so it is not very useful.

Applications of Servo Motor 

These types of motors are used where we need quick changes in speeds without the motor getting too heated. So, it can be used in: 

Allen Bradley Servo Motor TL-A2540P-HJ32AN
  • There are various industrial uses of servo motors and is used in machinery, tools, packaging, industrial automation applications, materials handling, printing, assembling, and other applications of robots, CNC, and automated production.  
  • They are used extensively in radio-controlled airplanes for controlling the position and movements of elevators. 
  • They are used in robotic applications for smoother on and off switching and precise positioning.  
  • They help in maintaining hydraulic fluid in the airline industry in the hydraulic systems.  
  • They are extensively found in radio-controlled toys.  
  • They are found in players for DVDs. 
  • They are used in vehicles for maintaining speeds.   
  • They are used in glue dispensers in industries and their manufacturing line is dependent on servo motors.  
  • Various super marts and buildings use servo motors for managing auto-control doors. 
  • Servo motors also find their applications in antenna positioning like NRAO.  
  • Angles of the solar systems can be adjusted via these sorts of motors so that the panels would be facing the sun all day long. It helps in absorbing maximum energy from the sun.  

The Circuitry Inside the Servo Motor

There are three inputs in a circuit which are the positive supply, ground, and a PWM input signal. The pulse width of the square is responsible for deciding the speeds and direction of the servo motor. Here is a circuit for the servo motor. The potentiometer is responsible for the clockwise and anticlockwise direction of the motor.

How to Choose a Servo Motor For Your Application?  

Carefully consider the following specifications of the motor while selecting one for your application.

Continuous and Peak Torque: Continuous torque is a time-weighted average of the torque during the complete cycle. To maintain the appropriate speed, this must fall in the continuous region of the torque curve. Peak torque is the maximum amount of torque needed at any one time during the cycle.

Speed: Knowing the needed speed (RPMs) is just as crucial as the load’s weight when choosing a servo motor. Generally speaking, the potential torque decreases with increasing motor speed.

Gear ratio: A servo motor in a geared system drives a driver gear that drives a second gear, closely linked to the payload. A servo motor may contain more than one gear combination. The gear ratio is determined by the ratio of the number of teeth between the gear pairings. The real torque of the motor is then determined using the standardized torque equation.

Inertia Ratio: The ratio between the inertia of the load and the motor is known as the inertia ratio. As the inertia decrease, the motor’s performance increases. As the inertia ratio drops, control loop optimization and machine performance increase.

Environment factors: The choice of the motor can be influenced by environmental factors in a variety of ways. For instance, if a motor is operated with a temperature higher than its mentioned ambient conditions, then the motor may produce less continuous torque than specified.

Efficiency: The amount of current required to produce a constant torque value is the simplest way to measure a servo motor’s efficiency. Usually, this constant torque is mentioned in the datasheet of the motor.

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