Stepper motors are widely used in applications requiring precise control of position and speed, such as robotics, CNC machines, and camera positioning systems. In this article, we'll explore how to control a stepper motor using National Instruments' Single-Board RIO (sbRIO) platform and LabVIEW, specifically utilizing LabVIEW RT and LabVIEW FPGA for real-time control and precise signal generation.
What Is a Stepper Motor?
A stepper motor is an electromechanical device that moves in discrete steps rather than continuous motion like a DC Motor. Each step corresponds to a fraction of a revolution, enabling precise control of angular movement.
Stepper motors are typically driven by a series of digital pulses that modify a waveform sent to electromagnets in the stepper. The common wiring setup of the electromagnets includes two main windings: A/A' (A and A not) and B/B' (B and B not). These windings represent the coils that, when energized in sequence, cause the motor to rotate and go to a designated rotary position. The diagram below illustrates a typical stepper motor wiring setup, which helps to convey this concept.
Role of the RIO Platform and LabVIEW and CircaFlex™ in Stepper Control
National Instruments' Single-Board RIO (sbRIO) and CircaFlex™ boards offer a flexible, powerful platform for controlling stepper motors. However, they cannot directly connect to the stepper motor's windings, as they do not supply the necessary current and voltage in the correct waveform. This is where a stepper motor driver comes into play.
The sbRIO, programmed using LabVIEW RT and LabVIEW FPGA, handles the logic of motor control—deciding how far to move, how fast, and in which direction. These decisions are based on various inputs, such as user commands, sensor data, or image processing algorithms.
Example of Stepper Motor Movement Calculation
To control a stepper motor effectively, we need to calculate the steps required to achieve a desired movement. Suppose you want to move an object a certain distance based on an image captured by a camera. The sbRIO processes the image and converts the necessary movement from pixels to millimeters. Then, using system specifications like the ball screw's pitch and the stepper motor's resolution, you can calculate the exact number of steps required.
Let’s assume:
The ball screw pitch is 5 turns per millimeter.
The stepper motor has 2,000 steps per revolution.
If the software determines that the motor needs to move 3 mm, the necessary number of steps can be calculated as follows:
Steps required = Distance in mm × Turns per mm × Steps per revolution
Steps required = 3 mm × 5 turns/mm × 2000 steps/turn =30,000 steps
This example demonstrates how easily sbRIO and CircaFlex™ can convert real-world units into precise motor movements.
Outputting the Steps: Velocity and Frequency Calculation
Once the required number of steps is calculated, the system needs to output the steps at the correct frequency to control the motor's speed. If we want the object to move at 10 mm per second, we can calculate the required step frequency:
Frequency (steps per second) = (Steps required / Distance to travel ) × Speed
Frequency (steps per second) = (30,000 / 3 mm) × 10 mm/sec = 100,000 steps/sec
The software in sbRIO and CircaFlex™ can then output these steps to the motor driver at the correct frequency.
Controlling Duty Cycle and Direction
Each stepper motor requires a pulse signal with a duty cycle, which is typically 50%. This means that for every pulse, the signal is high for 1 microsecond and low for 1 microsecond. The sbRIO outputs these pulses through its FPGA using LabVIEW FPGA, which gives precise control over timing and signal generation.
Additionally, the software determines the direction of movement, either forward or reverse, based on system logic or user inputs. The correct signal is sent to the stepper motor driver, which then outputs the appropriate waveforms to the motor windings.
Acceleration and Deceleration Profiles
It’s crucial not to start or stop the stepper motor abruptly. Rapid acceleration or deceleration can cause missed steps or mechanical issues. Instead, an acceleration profile should be calculated to gradually increase the motor's speed. For example, if we want to reach 3 mm/sec, the software can apply a 30 mm/sec² acceleration, which brings the motor to full speed in 100 milliseconds.
Note, in more sophisticated systems, the acceleration does not have to be strictly constant, even the acceleration can speed up and slow down. This more advanced calculation takes into consideration a desired value of "jerk", which is the rate of change of acceleration, or the second derivative of velocity. Therefore the amount of change in acceleration (m/s²) per second, jerk is measured in mm/s³!
Integrating sbRIO and CircaFlex™ with the Stepper Driver
Once all the necessary parameters—steps, frequency, duty cycle, and direction—are calculated, the sbRIO or CircaFlex™ outputs the corresponding signals to the stepper motor driver. The driver amplifies these signals and converts them into the correct waveforms for the motor windings, enabling precise motor control.
The stepper motor driver has its own variety of settings, depending on the manufacturer.
Conclusion
Controlling a stepper motor using the sbRIO platform and LabVIEW provides a robust, flexible solution for embedded applications requiring high-precision motor control. By leveraging LabVIEW RT and LabVIEW FPGA, engineers can create complex control algorithms that are executed in real time, ensuring smooth, efficient operation.
The combination of sbRIO and CircaFlex™ is ideal for embedded control projects, offering powerful integration capabilities with stepper motors and other systems. Whether you're moving an object based on sensor data or controlling motion in a CNC machine, this platform offers a comprehensive, scalable solution.
For more information on how to implement stepper motor control using sbRIO, CircaFlex™, and LabVIEW, or to inquire about our integration services, feel free to contact our engineering team. We're happy to assist with any embedded control project. We have the experience and skills to help you choose the correct products to operate a stepper motor, and we can also help demonstrate equipment or provide startup assistance with any equipment you purchase.
Comments