Driver Technology OverviewThe stepper motor driver receives step and direction signals from the indexer or control system and converts them into electrical signals to run the step motor. One pulse is required for every step of the motor shaft. In full step mode, with a standard 200-step motor, 200 step pulses are required to complete one revolution. The speed of rotation is directly proportional to the pulse frequency. Some drivers have an on-board oscillator which allows the use of an external analog signal or joystick to set the motor speed.
Speed and torque performance of the step motor is based on the flow of current from the driver to the motor winding. The factor that inhibits the flow, or limits the time it takes for the current to energize the winding, is known as inductance. The effects of inductance, most types of driver circuits are designed to supply a greater amount of voltage than the motor's rated voltage. The higher the output voltage from the driver, the higher the level of torque vs. speed. Generally, the driver output voltage (bus voltage) should be rated at 5 to 20 times higher than the motor voltage rating. In order to protect the motor from being damaged, the step motor drive should be current-limited to the step motor current rating.
Indexer OverviewThe indexer, or controller, provides step and direction outputs to the driver. Most applications require that the indexer manage other control functions as well, including acceleration, deceleration, steps per second and distance. The indexer can also interface to and control many other external signals.
Communication to the indexer is through an RS-232 serial port and in some cases an RS485 port. In either case, the indexer is capable of receiving high-level commands from a host computer and generating the necessary step and direction pulses to the driver.
The indexer includes auxiliary I/O for monitoring inputs from external sources such as a Go, Jog, Home or Limit switch. It can also initiate other machine functions through the I/O output pins.
Stand-Alone OperationIn a stand-alone mode the indexer can operate independent of the host computer. Once downloaded to the non-volatile memory, motion programs can be initiated from various types of operator interfaces, such as a keypad or touchscreen, or from a switch through the auxiliary I/O inputs. A stand-alone stepper motor control system is often packaged with a driver and power supply and optional encoder feedback for "closed loop" applications that require stall detection and exact motor position compensation.
Multi-Axis ControlMany motion applications have more than one stepper motor to control. In such cases a multi-axis control system is available. A HUB 444 networking hub, for example, may have up to four stepper drives connected to it, with each drive connected to a separate stepper motor. The networking hub provides coordinated movement for applications requiring a high degree of synchronization, such as circular or linear interpolation.
Choosing a Stepper Motor and DriveThe choice of a step motor depends on the application's torque and speed requirements. Use the motor's torque-speed curve (found in each drive's specifications, example in figure C) to select a motor that will do the job. Every stepper drive in the Omegamation line shows the torque-speed curves for that drive's recommended motors. If your torque and speed requirements can be met by multiple step motors, choose a drive based upon the needs of your motion system- step/direction, stand-alone programmable, analog inputs, microstepping- then choose one of the recommended motors for that drive. The recommended motor list is based on extensive testing by the manufacturer to ensure optimal performance of the step motor and drive combination.
|Types of Stepper Motor Drives|
|Step and Direction
These step motor drives accept step pulses and direction/enable signals from a controller, such as a PLC or PC. Each step pulse causes the motor to rotate a precise angle, with the frequency of the pulses determining the speed of rotation. The direction signal determines the direction of rotation (CW or CCW), while the enable signal turns the motor on or off.
Step motor drives with a built-in digital oscillator accept an analog input or joystick for speed control. These systems are generally used in applications requiring continuous motion rather than position control, such as mixers, blenders, and dispensers.
All of these stepper drives can be programmed for stand-alone operation; the motion control program is created with a simple drag-and-drop high-level software interface (included free), then downloaded and executed upon power-up. The motion control program typically waits for an input such as a switch closure or button press before executing the programmed motion.
These step motor drives offer advanced features such as self-test diagnostics, fault protection, auto-tuning, torque ripple smoothing, command signal smoothing, and anti-resonance algorithms. Some drives are stand-alone programmable, while others offer step/direction and analog inputs. High-performance drives will provide the best possible performance for your motion control system.
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