Ways to control servo motor speed: IC, MCU, or even Arduino

According to Zhang Peng’s article “Industrial Control Digital Controller”, servo control refers to the designer’s ability to adjust the speed and position of the motor based on feedback signals. But as designers know, there’s more than one way to control servo motor speed and position. In this article, we review new chips (and methods) that enhance speed control of such motors.

But before we delve into these methods, it might be useful to first distinguish the differences between stepper motors and servo motors.

Stepper motors and servo motors

Stepper motors move in predefined “steps”. Stepper motors themselves do not “know” where they start or where they stop.

On the other hand, servo motors have complex control mechanisms that can adjust the speed and position of the motor based on feedback signals. This usually involves a speed loop that defines how fast the servo motor is turning, and a position loop that determines the correct position.

Schematic diagram of different servo control types

Diagram of different servo control types: As described by the author, “(a) SCR control; (b) Pulse width determines the average voltage; (c) Pulse frequency modulation determines the average voltage.” Image provided by Zhang Peng

It is important to control the speed to maintain stability and ensure that the servo does not overshoot its designated position.

New speed and torque control IC

Performance Motion Devices (PMD) has added three new members to its proven Juno family of speed and torque control ICs. The Juno MC71113 and MC73113 are designed to work with brushed DC motors and brushless DC motors respectively, while the MC78113 can be programmed by the user to handle either motor.

Juno IC

Juno IC.Image courtesy of PMD

These units are designed for applications such as centrifuges, spindle control, peristaltic pumps, packaging automation and laboratory automation. They are available in a 64-pin TQFP package measuring 12 mm x 12 mm.

Block Diagram of MC71113, MC73113 and MC78113

Block diagram of the MC71113, MC73113, and MC78113 servo motor controllers.Image courtesy of PMD

On power-up or reset, the three new Juno chips check the configuration commands stored in NVRAM. If there is no message, the default value is used and the chip receives the travel command via SPI or CANbus 2.0 from the controlling MCU.

The new Juno chip provides a PI loop for speed control functionality. The measured speed may come from sources such as Hall sensors, quadrature encoders, or tachometer feedback.

Current control is performed by first accessing the current flowing through the motor windings. This measurement, along with the required current for each phase, is used to generate the necessary PWM signal.

Other ways to control motors: MCU and Arduino

While Performance Motion Devices introduced ICs to enhance control of servo motor speed, several other companies use a different approach to controlling motors, namely MCUs.

For example, Cypress Semiconductor uses a field-oriented control (FOC) algorithm to control servo motor speed. This method is based on Cypress’s Cortex-M4 MCU, which enables fast response to speed changes and high-speed control accuracy.

Block diagram of Cypress's FOC control method.

Block diagram of Cypress’s FOC control method.Image source Cypress Semiconductor

The outer loop is used for speed control and is compared to the input statement for the desired speed of the unit. The inner loop is used for current control.

Texas Instruments (TI) also uses MCUs to affect servo control. Its MSP430 series, including the MSP430FR2000 and MSP430FR21xx, are ultra-low-power MCUs designed to save power in battery-powered portable devices. This control is aided by the family’s low-power FRAM non-volatile rewritable memory. These devices are available in a 3 mm × 3 mm VQFN package.

Of course, for early prototyping or maker projects, one can always use a joystick module connected to an Arduino to control a servo motor, as detailed step-by-step on our sister site Maker Pro .

on the factory floor

In which areas do servo motors and servo ICs play a leading role? We don’t need to go any further than a factory facility.

Robots play an important role in modern manufacturing. Rather than being replaced, humans are complemented by collaborative robots or collaborative industrial robots. Collaborative robotics technology is designed to work with people. They work in hazardous environments, keeping humans safe from danger and doing most of the heavy lifting. Fortunately, they also free humans from mindless repetition.

Some of today’s super robot stars include Scara, Delta, Cartesian and articulated robots.

Engineers, how many months (or years!) does this type of servo control technology shave off the typical time to market for your electric products? Management, does this technology eliminate the need to hire highly paid internal control systems experts? Share your experiences in the comments below.

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