Accomplishing motion control with digitally commanded electric motors is a responsive, precise, and energy-efficient approach suitable for a wide range of applications. But although the technology has been available for many years, the cost and complexity of integrating the required multiple devices has made this approach impractical in many cases.
A combination of two technologies — micro controllers and stepper motors — which have received recent improvements, has changed the circumstances, and now many types of hobbyist, commercial, and industrial applications can benefit from intelligent and practical microcontroller- and electrical motor-based motion control.
The first development is industrial-grade programmable logic controllers (PLCs) with built-in motion-specific instructions to perform most of the heavy lifting with regards to programming, and high-speed pulse train discrete outputs suitable for driving stepper motors. Traditional solutions have required separate dedicated motion controllers, or premium PLCs.
It is now possible to specify smaller PLCs, which can capably control a wide range of equipment, with the native ability to command a motion “axis.” Not only can a single axis be controlled, but two or three can be operated in a coordinated manner, which is extremely useful for applications like 3D printers or CNC machines with X/Y/Z positioning.
A compressed and simplified instruction set is included with these types of PLCs and includes these three essential commands:
Homing: The ability to drive the hardware to a known position (typically with a limit switch) to verify or “home” the axis calibration. There are multiple common homing methods.
Velocity move: Motion operations usually need to operate at a target velocity, and the motor command must be able to accelerate/decelerate either linearly or following an “S-curve” to achieve desired velocity. Acceleration must be carefully managed to smoothly handle payloads.
Position move: For many applications, it is necessary to define a target position that the equipment must reach, while following a defined accel/decel and velocity profile to do so. The ability to coordinate up to three axes for a position move is also important (Figure 1).
Standard AC induction or DC motors are economical and fine for running at approximate speeds, but not accurate enough for motion applications. Servo motors represent a high-performance closed-loop motion technology with equally expensive and complex integration requirements.
Stepper motors are a third option, available in standard DC and newer high-bus-voltage AC versions. They also provide some of the best characteristics of servo motors, but with lower costs and simpler implementation. Stepper motors follow rapid pulse commands to perform small discrete step movements. They operate open-loop, but provide a good combination of speed, torque, and accuracy, making them suitable for many types of projects.
The combination of modern PLCs with stepper motors makes it easy and cost effective for users to implement coordinated one-, two-, or three-axis combined motion control ( Figure 2). These products are straightforward to use, although developers will still need to make informed decisions about motor sizing and homing/movement commands.
Micro-PLCs with motion capability can also perform other associated control, input/output, data acquisition, and industrial internet of things (IIoT) functions — making them a good fit for many types of machine automation and other applications.
This article was written by Conor Perry, Technical Product Manager for the HMI and CLICK product groups at AutomationDirect (Cumming, GA). For more information, visit here .