Machine and system designers now have greater options in choosing a motor to meet motion control requirements. While basic step motors with open-loop control schemes abound in the marketplace, different motor and drive designs offer interesting solutions for smarter, faster, quieter operations with more torque. Some incorporate smart drives for higher-level streaming commands or onboard motion control, while others add encoders for greater torque and accuracy. These options can offer better solutions as applications become more sophisticated and demanding, while companies address initiatives for greener operations. This article outlines different step motor options available on the market and considerations for their use.

Open-Loop Step Motors with Basic Drives

Open-loop step motors with basic step-and-direction drives are popular due to their low cost and easy implementation. Able to accurately position loads without a feedback mechanism, open-loop step motors are inherently precise due to their toothed rotors and stators that create a fine-resolution, incremental motion. When the stator field rotates at a constant rate, as defined by a precise quartz crystal clock, the average open-loop velocity is nearly perfect, with near 0% speed variation.

Step motor with smart drive.

Ideal for OEM applications requiring low-cost solutions for loads that are constant and predictable, these basic drives easily interface with PLCs, motion controllers, indexers, and PCs that generate the digital step (pulse) and direction signals necessary to control the position and speed of the motor. Common applications include peristaltic pumps, positioning conveyors, belt-actuated positioning axes, linear actuators, and 3D printers.

Open-Loop Step Motors with Smart Drives

A step motor drive can be much more than the electronic amplifier that simply converts digital step-and-direction signals into phase currents to drive the step motor. Smart, or programmable step motor drives offer additional control options including streaming commands, stored program execution, and industrial Fieldbus and Ethernet connections.

An ultra-high-torque step motor offers higher holding torque within the same mechanical dimensions as a conventional step motor.

Smart step motor drives free users from writing code to generate step-and-direction signals by providing a higher-level machine language of commands interpreted by the drive as instructions to move, set I/O, and provide system status. Smart drives handle trajectory generation automatically, meaning the time and number of steps taken during acceleration, deceleration, and constant speed — as well as the distance to reach the target position accurately — are calculated by the drive. Users can focus more on machine sequencing tasks and allow the smart drive to handle the details of motion path creation and execution.

In streaming commands, smart drives receive commands sent by a machine controller or HMI. The machine controller “streams” structured text commands to the smart drive over a serial network connection, such as RS-485 or Ethernet, and the smart drive executes the commands in sequence. A typical sequence of higher-level commands streamed from a machine controller to a smart step motor drive is shown below. The sequence includes one command for each parameter of the motion profile: acceleration rate, deceleration rate, velocity, and distance traveled. The sequence ends with a single command that initiates motion.

Some smart drives take higher-level machine language a step further by providing non-volatile memory for the storage of command sequences, which can be executed on demand by the machine controller through the drive's discrete inputs or at power-up. This stored program execution includes command sets for motion parameters and I/O control, data register manipulation, math operations, and more. Challenging applications like label feeding and encoder following are managed with onboard motion profiles, greatly simplifying the user's effort to program complex motion.

Pairing streaming commands and stored program execution with industrial networking protocols such as Modbus or Ethernet/IP further expands the options for integrating smart drives and step motors into the machine architecture of the user's choosing.

Ultra-High-Torque Step Motors

For machine designers in need of more torque from an existing step motor but without the available space to increase the size of the motor, ultra-high-torque step motors provide an effective solution. Offering 25 to 45% greater torque in the same motor body as a conventional step motor, ultra-high-torque step motors can avoid the need for users to specify a larger motor to obtain enough torque for the application.

Exploded view of integrated motor shows motor and drive parts.

The additional torque is achieved with an enhanced magnetic design that adds rare earth magnets of opposite polarity between stator teeth. These additional magnets enhance the variation of magnetic permeability between teeth, allowing more of the magnetic flux between that stator and rotor to be focused on producing torque. For example, a NEMA 34 frame step motor with conventional magnet construction produces 5.9 N-m of holding torque. The ultra-high-torque version of the same motor produces 9 N-m. To achieve the same holding torque with a conventional step motor would require specifying a 31% longer motor.

Functional diagram of closed-loop step motor system showing drive, motor, and encoder feedback in relation to input coming from machine controller and output rotor (shaft) position.

Ultra-high-torque step motors are useable with both basic and smart step motor drives and in the same applications as conventional step motors, though a tradeoff might exist between torque vs. speed performance when switching to an ultra-high-torque motor. Users should always consult published speed-torque curves when selecting the best motor for an application.

Integrated Step Motors

Integrated step motors, also referred to as integrated steppers or integrated drives, are the combination of the step motor and drive into a single package. The rear housing of the motor is redesigned to accommodate the drive electronics, with drive connections for DC power, inputs, and outputs (including step and direction inputs), and communications added to the side or rear of the motor.

Integrated motors save on space, wiring, and cost over conventional motor systems comprised of separate motor and drive components. Using integrated motors simplifies the product selection process by eliminating questions about compatibility between different motors and drives. Configuration of the integrated motor is simpler by eliminating wiring between the motor and drive. Design cycles are reduced as users no longer must specify or install external electronics to drive the motor. The elimination of external drives also reduces the size and number of needed control panels, which can save considerable money in multi-axis applications.

Integrated step motors are available with basic step-and-direction control and with smart drive options for streaming commands, stored program execution, and industrial networking. Integrated motors provide the same performance as their separate-components counterparts and are suitable for the same applications. Areas where integrated motors are particularly useful include high-axis-count applications such as smart conveyors and converting machines as well as compact machines without control panels such as industrial printers and packaging machines.

Closed-Loop Step Motor Systems

Although easy and inexpensive to use, open-loop step motors are not optimal for every application. Open-loop motor systems do not monitor actual position and speed vs. commanded position and speed as no feedback mechanism exists in the system. As a result, machine controllers are not always aware of the actual position of the step motor. What if the motor doesn't reach the final position or stops operating altogether? If the open-loop motor loses steps, it does not automatically correct its position. If the torque demand exceeds the motor's available torque, the motor will stall without warning. Many times, companies buy larger or longer motors to ensure that the motor has enough torque for the application. This wastes space, money, and energy.

Adding an encoder to a step motor system and incorporating servo control features into the drive electronics provide numerous benefits to the user. Closed-loop step motor control means that the actual position and velocity of the step motor are continuously monitored by the drive using encoder feedback, and current in the motor is controlled automatically to eliminate errors. As closed-loop step motor systems produce more torque to accelerate and decelerate faster, they better address today's high-throughput and high-speed processes. Drawing only enough current to meet the torque demand, closed-loop motors consume less energy and waste less power as heat than open-loop motors. Open-loop step motors draw full current regardless of load conditions. As a result, closed-loop step motor systems consume up to 2/3 less power than their open-loop counterparts, saving power and improving motor efficiency.

Due to the better use of current control in closed-loop systems, step motors also run quieter and more smoothly than open-loop step motors. This is advantageous in applications such as laboratory automation equipment and environments where audible noise from the motor is problematic. Closed-loop step motor systems are suitable for all step motor applications, especially those where increased torque, higher acceleration rates, quieter operation, smoother motion, higher efficiency, and greater system accuracy are required.

Conclusion

Choosing the right type of step motor and drive solution comes down to application requirements. Basic step-and-direction drives are best suited for high-volume, low-cost applications. Smart drives relieve the user from many programming tasks and offer numerous integration features, making them more useful for smart manufacturing and Industry 4.0 requirements. Ultra-high-torque step motors can save existing applications from trouble when torque demand increases. Integrated motors are particularly useful for high-axis-count applications where control panels are large or numerous. Closed-loop step motor systems are best where conventional step motors cannot provide the performance needed, whether it be torque, throughput, accuracy, or efficiency.

This article was contributed by Applied Motion Products, Watsonville, CA. For more information, visit here .


Motion Design Magazine

This article first appeared in the February, 2020 issue of Motion Design Magazine.

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