There are different types of motors available on the market, but servo motors are one of the most expensive motors with various applications.
So, why exactly are they expensive? What makes them stand out from the rest? And are they worth their price?
Servo motors are expensive because of their sophisticated closed-loop control mechanisms. This is implemented by using an encoder that tracks the position of the motor shaft and runs a feedback algorithm to precisely achieve the desired direction, speed, and angle of rotation.
This article discusses why servo motors are expensive compared to other motors. It also explains the applications of servo motors and the differences when compared to a DC motor.
Servos Motors: Why So Expensive?
Servo motors have proven to provide higher accuracy and better speed-torque characteristics that are best suited for CNC machining.
These motors provide various advantages over other motors, which increases the complexity of their construction, thereby increasing the cost.
Closed-Loop Control
A servo motor is simply a DC motor with a feedback mechanism that allows the user to control the position, speed, and rotation angle precisely.
Other motors used in CNC machines, like stepper motors, are open-loop devices that don’t have any mechanism to get feedback from the cutter.
As a result, if the load torque is very high, the stepper motor might miss steps, which would lead to inaccuracies in the operation.
Speed-Torque Characteristics
Servo motors offer speed-torque characteristics that are suited for CNC machining applications.
Unlike stepper motors, the output torque of servo motors doesn’t reduce upon increasing the speed. They are able to maintain almost constant output torque even at top speeds.
This feature allows servo motors to be used at different speeds depending on the particular operation.
In contrast, stepper motors are best suited for low-speed applications, as their torque increases with the decrease in speed.
Servo motors also have a faster acceleration from low to high RPM compared to other motors.
This factor would make a difference when the spindle has to change directions frequently during a job. If the acceleration is slow, the cycle time will increase.
Servo motors have very low holding torque hence they have to constantly send a feedback signal to the controller to adjust the position of the shaft to keep it stationary.
This results in constant oscillations and if the controller is not properly tuned, these oscillations can become high and cause undesirable movement of the spindle.
Hence, the controllers that are used in servo motors should be of good quality, which adds to the cost of the motor.
Need for Encoder
As mentioned earlier, servo motors use encoders to send feedback to the controller to maintain a precise output shaft position, angle of rotation, and speed.
Encoders add to the servo motor system and good quality encoders with high resolution will offer very precise control over the motor shaft position.
As a result, the cost of the servo motor increases and is directly proportional to the quality of the encoder used.
Other motors like the stepper motors don’t have a feedback mechanism, which reduces the cost of the motor but may cause inaccuracies in the operation of the motor.
Load Control
Servo motors draw current proportional to the output load. This requires a sensor that detects the load on the system and supplies the current accordingly, increasing the overall cost of stepper motors.
So using a servo motor for applications that involve low loads and volume of production, won’t give you any cost advantage. In such cases using a stepper motor is a better choice.
Maintenance
Servo motors consist of an output shaft coupled with a gearbox, along with an encoder and controller circuitry.
More parts make the construction complex and as a result, increase the probability of failure of one or more of the components.
The brushes of the servo motor generally need to be replaced after 2000-hours of use.
This increases the maintenance cost of servo motors compared to other motors.
Applications of Servo Motors
CNC Machines
Servo motors are an integral part of high-end CNC machines.
The spindle moves in the workspace according to the toolpath specified by the G-code. This is achieved by servo motors that control the movement of the spindle in the X, Y, and Z-axis.
CNC machines also use servo motors as spindle motors. This provides very high precision when working with costly workpieces that may get damaged if the motor misses steps or rotates beyond the desired angle.
Robotics
There are numerous robotic applications that require precision in the speed and angle of rotation. Stepper motors and servo motors are generally used for such applications.
A few of the robotic applications include robotic welding where the robotic arm needs to accurately actuate to the desired coordinates at every point of the operation for suitable welding.
Increasing the number of joints in a robotic arm increases the degree of freedom but also increases the complexity, which requires the high precision provided by servo motors.
Other applications include robotic vehicles used for remote navigation, bomb disarmament, etc.
Solar tracking
A solar panel is a device used to convert solar energy to electrical energy.
The amount of electrical energy generated is directly proportional to the intensity of sunlight that is incident on the solar panel.
The intensity of sunlight is maximum when the sunlight falls perpendicular to the solar panel.
Since the sun doesn’t stay in the same position throughout the day, the solar panel should track the position of the sun and follow it to generate maximum electrical energy output.
This solar tracking is achieved with the help of servo motors. The angle of the solar panel is adjusted according to the position of the sun and servo motors tilt the solar panel in the direction of the sun as it moves from east to west.
Industrial applications
Other than CNC machines, servo motors are used in various industrial machines.
Such machines include conveyor belts. Conveyor belts may need to stop at different points along the assembly line for specific operations. Hence the movement of the conveyor belt needs to be precise.
Metal forming and bending machines also need precise control to achieve accuracy in their operations. Hence these machines use high-torque servo motors to achieve this precision.
Printing presses need motors that can start and stop the printing heads at a particular point accurately in every cycle and move the paper along for printing multiple rows of text and graphics.
Difference between Servo and DC Motor
| Parameter | Servo Motor | DC Motor |
|---|---|---|
| Type of supply | AC or DC | DC |
| Number of terminals | 3 | 2 |
| Control | Closed loop | Open loop |
| Components | DC motor, gear system, encoder, control circuit, etc. | DC motor is a single component with no external parts |
Servo motors and DC motors can be differentiated on the basis of various factors.
DC motors are open-loop systems, meaning that they don’t have any feedback mechanism for error correction.
You power up a DC motor and it will start running but you won’t be able to control the angle of rotation or detect the position of the shaft.
Servo motors on the other hand employ complex electronics along with sensors and encoders to accurately measure the position of the shaft and control the angle of rotation.
The sensors send a feedback signal to the controller for error measurement and adjust the position or the rotation angle of the output shaft accordingly.
DC motors as the name suggest work on DC, hence they have two wires coming out to be connected to the supply.
Servo motors are available in AC and DC versions. AC servo motors are used in industrial applications whereas DC servo motors are more suited for small DIY projects.
Servo motors have three wires setup, two for supply and one for control. The control wire is the feedback of output shaft position and speed that is fed into the controller.
Final Thoughts
Servo motors are used when the user needs to precisely control the angle of rotation and speed of the motor’s shaft.
However, these added advantages come with an increase in the price.
As a result, the servo motor can not be implemented everywhere. You have to take into account the initial cost of the motor and the running costs to see if it going to suit your requirements.
However, if you are working with costly workpieces, you might want to consider systems that have servo motor drives, so that you don’t end up damaging the workpiece in case of any fault.
Frequently Asked Questions (FAQ)
Is a servo motor a DC motor?
A servo motor is not technically a DC motor, however, a DC servo motor’s prime mover is a simple DC motor. A servo motor has some additional components such as a gear system, encoder, and controller. These components make the servo motor capable of precise control of speed and angle of rotation of the output shaft, which is not possible in a DC motor.
What is the difference between an open-loop system and a closed-loop system?
An open-loop system is a system where the output is only dependent on the input. There is no feedback and hence no error measurement to correct the output if it becomes undesirable. Closed-loop systems, on the other hand, have feedback from the output to detect the error and adjust the output of the system accordingly.
How is a servo motor different from a stepper motor?
A servo motor operates on a closed-loop system that takes feedback from the output to control the speed and angle of rotation precisely. A stepper motor is an open-loop system that also has precise control over the speed and angle of rotation, but by stepping or micro-stepping the rotation of the shaft. The higher the number of steps in a stepper motor, the higher the resolution of the rotation angle. A servo motor is capable of providing better control over the output parameters; however, they tend to be costlier than a stepper motor of similar ratings due to the additional components used for feedback control.
