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What You Need to Know About Servo Motors in Industrial Automation

In most industrial applications, whether it is the food industry, textile, plastics, or even health care, servo motors play a major role in how we automate our surroundings. While it is easy to take them at face value, there is a lot of engineering that went into how these motors were designed, how they function, and how they are controlled. The ecosystem of servo motors may be small compared to the likes of PLCs, but there is quite a bit to know before stepping into the wonderful world of automation.

The term “Servo motor” dates back to the 1860s under the French name “Le servomoteur”, translating to slave motor, and was used to describe hydraulic and steam engines upon boats by a French gentleman named Joseph Farcot. He used it to describe “a power amplifying machine under precise control as to position by a human operator”. Fast forward to the 1800s with the inventions of the battery, electromagnet, and the discovery of a magnetic field, the first electric motor was built and introduced by Mortiz Jacobi in 1834. While it was only a single-phase, single-direction weak motor, it still showed its potential. The 1880s is when the three-phase motor began to emerge from different inventors and in the 1950s is when automation began to grow within the United States. While factories at the time were using pneumatic and hydraulic-controlled machines, they proved to be unreliable, imprecise, and prone to leaks. The servo motor then began to be adopted as a more precise and user-friendly alternative. Now they control some of the most advanced robotics in automation.

What Exactly Do Servo Motors Do?

Servo motors convert electricity into precise, bidirectional rotational movements. Controlled by either a PLC or a dedicated controller, they take input data and rotate based on the commands given by the controller. This rotation is then applied to the machine or system that the motor is attached to through the shaft, whether it is to roll a conveyer belt, control the joints of a robotic arm, or power a CNC mill.

What Makes a Servo Motor So Precise?

The secret ingredient for precise motor function is an encoder. These devices are connected to both the motor and the controller to observe the position of the actuator shaft, the change of position, and the rate of change. They also convert the mechanical input into electrical impulses to feed into the controller in the form of quadrature signals. There are two types of encoders:

Magnetic Encoders

Mechanical encoders work by using a sensor to identify the change in the magnetic field of a rotating magnetized wheel. The wheel has a series of magnetic poles around its circumference, and they output a predictable response to the sensing circuit to determine its position. Likewise, increasing the number of magnetic poles will increase the resolution and accuracy of the encoder. These types of encoders are more suitable for contaminated environments where excessive shock, moisture, and dust are prevalent.

Optical Encoders

Optical encoders use a similar method, but instead of a magnetized wheel, they use a light, a disk, and a sensor. While the magnetized encoder uses magnets, the optical encoder uses slits in a disk where light from its attached LED can pass through to the sensor on the other side. When the disk, or code wheel, is spun, the light from the LED will shine through one slit at a time to pass the light to the photo sensor to read its position. Instead of shining through, optical encoders can also reflect light from a LED to a sensor off of the disk. Both of these methods are reliable and accurate, and like the magnetic encoders, can increase in resolution based on the number of slits in the code wheel. They are also easier to improve accuracy and resolution compared to a magnetic encoder and can also be miniaturized to fit into smaller motors. These encoders are better suited for applications with a much higher magnetic field, such as in larger motors

Alternatively, encoders can be substituted for potentiometers, resolvers, or Hall effect transducers though may show a decrease in responsiveness and reliability.

What Types of Servo Motors are There?

The most common types of motors found in industrial applications are AC and DC motors. The main difference between the two is how they are powered. DC motors use DC power to operate and its speed is directly proportional to the voltage being supplied while AC motors use AC power and are controlled by the frequency of the applied voltage.

  • AC Synchronous Motors: An AC Synchronous motor is a motor whose rotation speed is synchronized with the alternating current. They are powered by a three-phase alternating current to their stator coils to produce a rotating magnetic field that is proportional to its power frequency. This is paired with the coils on the motor’s rotor that are powered by direct current to produce alternating magnetic fields. The opposite poles of the stator and rotor attract each other, thus, synchronizing their speed continuously and proportionally to the electrical frequency produced by the alternating current. The number of magnetic poles the motor has also affected its speed, whereas the number of coils affects the speed and the torque. The more coils there are, the slower the speed but with more torque. Synchronous motors do come with their shortcomings as a force applied to the shaft that is opposite to the rotor’s direction can cause the motor to get out of sync, thus making it static. The stators cannot spin the static rotor as the frequency changes too fast to overcome the magnetic force keeping it still. This is solved by what is called a ‘squirrel cage’ around the rotor. This allows the synchronous motor to act like an induction motor for a moment in regards that the stator windings rotating magnetic field inducing power into the squirrel cage to spin the rotor. Once the rotor has gained speed, it is then energized and begins to work as a synchronous motor again. These motors are also constructed with permanent magnets to improve reliability.
  • AC Asynchronous / Induction Motors: An induction motor works by using electromagnetism from the stator windings to induce an electric current to the motor’s rotor to produce the torque needed to spin. The stator on an induction motor is made of a three-coil winding that allows AC power to power it. The windings pass through steel slots on the stator within its cast iron structure. When three-phase AC power is applied to these coils, it produces a rotating magnetic field that spins the rotor. This allows the motor to be self-starting and able to have its speed increased or decreased with the frequency of its input power supply. What makes these motors asynchronous is that the rotor always spins slightly slower than the stator’s magnetic field. The difference between these speeds is called the ‘slip speed’. These motors are very simple as they don’t have permanent magnets, brushes, position sensors, or commutator rings.
  • Brushed DC Motors: These types of motors are the most common due to their simplicity. They are constructed with a set of permanent magnets on their stator and an armature, or rotor, with insulated wire coiled tightly around an iron core. Those wires are then attached to a mechanical commutator. To pass voltage to a constantly spinning rotor, power is delivered through either carbon or graphite brushes to the spinning commutator to energize its coils at the right time. As this current is passed through the commutator and to its armature windings, it creates an electrical field to either attract or repel the permanent magnets on the stator to produce the torque needed to spin. While these motors are simple in design and are often cheap, they do require periodic maintenance on the brushes as they deteriorate and create dust. These motors are also not precise, making them unsuitable for robotic applications. The entire motor must be replaced if the commutators wear down as well.
  • Brushless DC Motors: Instead of using physical brushes and a commutator, brushless motors are driven by a drive circuit. A semiconductor switch attached to each of its three coils actuates at a certain sequence to alternate the current flow to produce the rotating magnetic field. These semiconductors are actuated by the orientation of the permanent magnets on the rotor using a magnetic sensor or a hall sensor. This eliminates the need to replace physical wearing parts and vastly increases the longevity of the motor. Brushless motors are also much quieter when compared to brushed motors.

How Are Servo Motors Controlled?

To set up a servo motor or multiple motors in any application, it is important to understand how this operation works. To have a successful setup, three main items are mandatory. While further research will be needed to match the correct drives, controllers, and motors, here is how they operate:

  • Servo Motor Controllers: These are the brains of the entire setup. The controller’s job is to close the loop on the system by reading the encoder signal to calculate a path of trajectory. The controller then sends this low-power signal to the drive.
  • Servo Motor Drive / Power Amplifier: The servo drive will take the command signal for position, torque, and velocity from its controller to amplify the low power signal to then feed that power, typically in amps, to the motor to make it rotate. The motor then communicates with the drive through a feedback signal and the drive will then adjust the frequency of the voltage to match the signal from the control panel.
  • The Motor: It is important to pair a motor to a drive that matches its power type. AC motors are generally paired with variable frequency drives while DC motors require a drive specifically for DC power or a DC power source. As stated before, research is needed to pair your motor with the most effective drive and controller setup.

Do Servo Motors Require Maintenance?

Like everything with moving parts, servo motors do require maintenance. A faulty motor can lead to unexpected downtime in an industrial setting while it is getting replaced when it could have been prevented with regularly scheduled maintenance. Not only will it cost money to replace that motor, but it will also render the part it is attached to inoperable, which also costs money due to downtime.

Thankfully, preventative maintenance isn’t difficult with intermediate mechanical knowledge. This requires separating the motor’s face plate from its main body to reach the internals. This allows the technician to perform a wear check on the internal parts, such as bearings, seals, and the brake. Bearings are one of the parts that require extra maintenance as they are worn quite often, leading to servo breakdowns with high repair costs. Seals prevent unwanted dust, water, oil, and debris from working their way into the motor to cause internal damage. The breaks on the motor can fail if oil is present on its mechanical parts. Lack of maintenance can also cause the brake to block and produce metallic dust in its system.

How Are Servo Motors Used?

Servo motors play a role in a variety of applications. Due to their precision and reliability, they can tackle tasks that demand repetitive yet precise movements. Some real-world applications are as follows:

  • CNC and Woodworking: CNC machines use software to mill or drill very precise holes and valleys in the wood or metal. Servo motors are also present in lathes and augers.
  • Textile: Servo motors are responsible for the fast movements of the looms and weaving machines that produce the clothes we wear or the carpet we step on.
  • Robotics: Every joint of an automated robotic utilizes a servo motor for precision control. These are prevalent in car manufacturing plants, recycling plants, and many more.
  • Automatic Door Openers: Most modern hospitals and stores use automatic sliding doors. Those are also controlled by servo motors to make sure they open and close at a set distance, speed, and torque.
  • Conveyor Belts: Within package sorting plants and warehouses, conveyor belts use servo motors to start, halt, and move products along at a set pace.

Conclusion

Servo motors provide an efficient and reliable way to control our modern industrial plants. These motors save an exponential amount of manpower and have significantly improved production and economical growth. While understanding how these motors function may seem like a daunting task to some people, this guide has been written to help everyone understand just how simple these machines truly are and how they can impact the daily lives of everyone around them.

This entry was posted on January 23rd, 2023 and is filed under Uncategorized. Both comments and pings are currently closed.

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