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.
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.
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.