Wednesday, January 22, 2020

Motion Controller and Driver Selection Tips You Should Know

Motion Controller
The first thing that should be checked when selecting a motion controller is compatibility with other system components such as the PLC/PC, stepper driver and voltage sources.Motor speed is controlled by the pulse rate (pulses per second) supplied by the driver via the controller.Depending on the speed required, one must ensure that the motion controller can output the corresponding pulse rate to reach that speed. The motion controller must be able to output this pulse rate, and the stepper drive must be able to receive this pulse rate and send over to the SMLA. The equation for determining required pulse rate is shown in the following page.

Motion Controller and Driver Selection Tips You Should Know

• Linear speed: desired linear travel speed (in/s)
• Lead: lead screw for stepper motor travel per one full revolution of the screw (in/rev)
• Motor steps per rev: how many steps per one full revolution of the motor (steps/rev). All SMLAs are 200 steps per revolution.
• Pulses per step: pulses per step (pulse/step). SMLAs are 1 pulse per 1 step.
• Microstep: Micro-stepping resolution (micro-step/step)
After calculating the required pulse rate, it may come to light that your PLC, PC or microcontroller is capable of sending out this frequency to the stepper drive without the need of a separate motion controller.

Stepper Drive
Like a controller, the user should select a drive based on its ability to interface with all other components in the system – specifically the stepper motor and controller. For Thomson standard SMLA stepper motors, a drive must be able to allow the connection of a four-wire, bipolar motor. Electrical properties of the system must also be taken into consideration. Items such as desired output current to the motor, max input voltage from the power supply, and motor inductance will need to be reviewed.Just like a controller, required pulse rate will also need to be considered to ensure the driver is capable of driving the motor to the required speed.

There are many unique stepper motor drives out there that offer various micro-stepping resolutions. Depending on your requirement, micro-stepping may be worth considering, specifically if a smoothmotion is required. Micro-stepping essentially takes the standard 200 steps per revolution of the SMLA motor and breaks down each step to smaller increments such as from ½ step, ¼ step and even all the way to 1/256 step. Figure 7 illustrates the difference between full-stepping and micro-stepping.

Figure 7: Simple illustration comparing the motion between full stepping and micro-stepping.An important thing to note about micro-stepping is that it does not improve positional accuracy by goingto finer resolutions. A typical rotational accuracy for a stepper motor is approximately +/- 0.09 degreesregardless of micro-stepping resolution.

Final Considerations
Ultimately, experience is the best tool at one’s disposal for building a stepper motor-based system.The guidance mentioned above should only be utilized as a way of getting in the ballpark for a system build.Some experimentation with trial and error may need to be conducted to get a completely functional system. Always utilize the help of an experienced system designer and add a decent margin to system calculations when possible. When it comes to SMLA selection, Thomson can help recommend a product to get the performance you need. PLC, motion controller and drive manufacturers will also have dedicated engineers to help assist you in selecting one of their products.

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