Designer’s guide: Industrial motor control

As an essential component of many different industrial processes, motor control has a wide variety of applications in fields ranging from manufacturing to autonomous robots. Controlling the motor in an effective manner not only improves performance but also guarantees both safety and precision. In this article, we will delve into the complexities of motor control, focusing on industrial applications.

Types of motors and control strategies

Industrial applications comprise a wide variety of tasks, each with its own motor specifications. Selecting the proper motor type and how to control its operation is crucial for maximizing performance and efficiency. The most prevalent types of industrial motors can be categorized as follows:

• Induction motors: Due to their reliability and simplicity, they are widely used in industrial applications. They operate on the principle of electromagnetic induction, where a rotating magnetic field induces a current in the rotor, causing it to rotate. They are suitable for applications requiring continuous operation, such as pumps, fans and conveyor belts.
• Synchronous motors: Operating in synchronicity with the supply frequency, these motors offer accurate control over speed and position. They are the best choice when constant speed under varying loads is required, such as in precision machinery and robotics.
• DC motors, available in the brushed and brushless DC (BLDC) variants: Unlike brushed DC motors, BLDC motors don’t have electrical contact with the rotor, replacing brushes with electronic commutation. Despite a higher cost and design complexity, BLDC motors offer higher efficiency, longer lifespan and reduced maintenance needs, making them suitable for applications demanding high performance and durability, such as computer numerical control (CNC) machines and robotic arms. Even though BLDC motors can have one to three phases, the single-phase and three-phase types are the most used.
• Stepper motors: Because they can move only in discrete steps, they are ideal for high-position–accuracy applications. They are commonly used in 3D printers, CNC routers and automated assembly lines.
• Servo motors: These motors provide accurate positioning/angle and speed control, combining the characteristics of DC and stepper motors with a closed-loop control method. Often incorporating feedback devices like encoders or resolvers, they are a high-performance alternative to stepper motors and find applications in industrial robots, CNC machines and automated manufacturing.
• Permanent magnet motors: These are electric motors that also contain permanent magnets. In certain high-efficiency applications, such as electric vehicles and compact electric motors, they are more efficient than induction motors or motors with field windings. Both AC and DC motors benefit from this technology.

It should be noted that motors for industrial applications often operate in challenging environments, including extreme temperatures, dust, vibration and humidity. Designers must carefully select motors that can withstand these conditions and ensure long-term reliability and safety.

A disassembled stepper motor, showing the rotor and the stator, is shown in Figure 1. This kind of motor achieves high precision with excellent speed and torque performance.

Figure 1: Stepper motor disassembled, with stator on the left and rotor on the right (Source: Shutterstock)

After selecting the motor that best satisfies the application’s requirements, the next step is to choose the right control strategy. In industrial motor control, two main control strategies are employed: open-loop and closed-loop control.

The open-loop control approach is simple: The motor is controlled without receiving any feedback from the system that it drives. The parameters, such as voltage or current levels, are predefined, and commands are sent based on those parameters. Open-loop control may be straightforward and economical, but it lacks precision and the ability to adjust to a wide variety of circumstances. It is appropriate for use in situations in which precision is not an absolute necessity, such as fans and simple conveyor systems.

With a closed-loop control approach, the performance of the motor is continuously monitored and adjusted through the use of feedback devices. Real-time data about the motor shaft’s speed, position and any other relevant parameters can be obtained using sensors like encoders and resolvers. Because of this feedback, the control system can make precise modifications so that it can continue to produce the desired result.

Greater accuracy, stability and adaptability to changing conditions are all ensured by using a closed-loop control system. Robotics, CNC machining and automated packing lines are some examples of applications in which precision, consistency and dynamic reflexes are of the utmost importance. Closed-loop motor controls often rely on proportional-integral-derivative (PID) control algorithms. PID parameters need to be carefully tuned to achieve the desired response, stability and minimal overshoot.

Besides encoders and resolvers, Hall-based sensors are widely used in motor control to determine the current rotor position. To do that, they need magnets appropriately positioned. Inductive position sensors are emerging as an alternative to magnetic sensors.

One example is Microchip Technology’s LX34070 high-speed inductive position sensor with differential output (see Figure 2) that is suitable for industrial and automotive (especially EV) applications. The two output signals representing the absolute position can be configured as single-ended to reduce the number of pins or as differential to maximize noise immunity in remote applications. PCB traces are used to generate a magnetic excitation signal and detect the presence of metal targets within the generated magnetic field. The benefit is a magnetic field sensor that does not require magnets, provides greater accuracy and is highly resistant to extraneous magnetic fields.

Figure 2: Microchip’s LX34070 inductive position sensor (Source: Microchip Technology)

Motor control techniques

A widely used technique for controlling motor speed and torque by adjusting the duty cycle of the applied voltage is pulse-width modulation (PWM). The main benefit of this technique is a smooth and step-less speed control, allowing motors to operate at different speeds without abrupt transitions, thus enhancing the overall system performance.

Moreover, by varying the duty cycle of the input voltage, PWM control reduces energy wastage by providing the necessary power only when needed, especially during lower-load conditions (this is the typical profile of an EV moving through the city traffic). PWM-controlled motors also reduce heat generation compared with traditional voltage-control methods, resulting in prolonged motor life and enhanced reliability.

Field-oriented control (FOC) is a more sophisticated control technique, mainly used with AC motors. FOC enables the independent control of torque and flux components, allowing motors to deliver precision torque levels while maintaining optimal flux levels.

FOC minimizes losses and maximizes efficiency, especially in applications with variable loads, by altering the current vectors in the stator to align with the rotor’s magnetic field. In addition, FOC ensures smooth and consistent motor operation even during abrupt load changes, making it suitable for applications requiring high levels of dynamic response.

When adding physical sensors is impractical or prohibitively expensive, sensorless control techniques, such as the ones listed below, are implemented:

• The motor parameters are estimated by sensorless control algorithms by analyzing the back electromotive force generated in the motor windings during operation. This estimation provides details regarding rotor position and speed.
• The sliding-mode control method predicts motor behavior and position based on input signals using mathematical models, allowing for precise control without the need for additional sensors. This technique adapts well to highly nonlinear systems, achieving excellent tracking performance and disturbance tolerance.
• Adaptive motor control algorithms continuously adjust parameters based on real-time motor behavior, compensating for fluctuations in load conditions and unpredictability.

Motor control design can be greatly simplified by using integrated motor controllers. An example is Infineon Technologies AG’s iMOTION IC series, integrating all the control and analog interface functions necessary for sensorless FOC of permanent magnet motors utilizing DC-link or leg shunt-current measurements.

In addition, they incorporate Infineon’s patent-protected and field-proven motion control engine (MCE), which eliminates the need for software coding during the motor control algorithm development process. Implementing a variable-speed drive is as simple as configuring the MCE for each motor. With the assistance of the iMOTION Solution Designer (iSD) tool, the motor can be operational with little effort.

The MCE employs FOC and space-vector PWM with sinusoidal signals for maximum energy efficiency. The MCE can control a sensorless motor or one with sensor support, such as analog or digital Hall sensors, and is equipped with algorithms for boost PFC implementations. The block diagram of the Infineon iMOTION integrated motor control (IMC) device is shown in Figure 3.

Figure 3: Block diagram of the iMOTION IMC device (Source: Infineon Technologies AG)