When it comes to solving test and measurement applications, many engineers may already have a background in electrical or computer engineering. However, motion control is a field that often involves mechanical or controls engineering, which is a rather different discipline.
To better understand motion control in test and measurement applications, let's first explore the functions of a motion controller and its role in a motion control system.
The Goal of Motion Control
The goal of motion control is to move a load from one location to another while controlling the system's overall kinematics. Kinematics involves controlling the position, velocity, and acceleration of the system to achieve the desired outcome.
Motion is more than just on/off control
To illustrate the concept of kinematics, let's use a simple analogy. Imagine two drivers in different cars. The first driver is a novice and does not know how to control the gas pedal very well. He can only step on the gas pedal fully, or let go completely. The second driver, being more experienced, knows how to gradually increase the car's speed using the gas pedal.
Both drivers can reach their destination eventually, but riding with the first driver would be incredibly jerky and uncomfortable. On the other hand, the ride with the second driver would be much smoother, since he has better control over the car’s speed.
With this in mind, controlling velocity and acceleration is crucial in many test and measurement scenarios with motion control. Here are a few examples:
- When using certain cameras, jerky motion can lead to blurred pictures. Therefore, it's important to control the velocity and acceleration of the movement.
- Some chemical specimens can be ruined by jerky motion, so it's necessary to control the velocity and acceleration of the movement.
- To simulate real aircraft movements when rotating a wing specimen in a wind tunnel, it's important to control the velocity and acceleration of the movement.
- And many more…
In short, if you are concerned not only with whether something moves, but also with how it moves, it is essential to have control over the velocity and acceleration of the movement.
Components of a Motion Control System
Now that we’ve established the importance of motion control, let’s look at the typical components in a motion control system.
- PC and application software
- The PC and application software (LabVIEW) send high-level commands to the motion controller. During application development, the programmer can use the high-level function calls to define motion movements.
- Motion controller
- The motion controller is the core of the motion control system. It receives high-level commands from the PC and translates into low-level signal commands that are sent to the motor drive or amplifier. There is an onboard trajectory generator that plans the motion path according to the specified parameters of the motion move. The motion controller also monitors feedback signals from the motor and sends status information back to the PC.
- Motor drive or amplifier
- The motor drive or amplifier is responsible for providing the power for the motors to turn. The specification for the motor drive will be in Amps or Watts, to represent the amount of electrical power it can provide.
- Motor
- The motor is the physical device that converts electrical energy from the motor drive into mechanical energy.
- Motor stage or assembly
- The motor stage, or assembly, is the mechanical component responsible for physically moving the load. It usually consists of gears, couplings, and pulleys that provide translational and rotational movement.
The Role of a Motion Controller
Now that we have gone through the components of a motion control system, let’s turn our focus to the motion controller. The motion controller is responsible for the following tasks:
- Plan the move: Before the move be can executed, a mathematical trajectory must be derived from the desired velocity and acceleration parameters. This is done through an onboard trajectory generator.
- Execute the move: the motion controller sends the low-level signal commands to the motor drive. These commands may be in the form of I/O signals such as pulse commands, or in the form of communication, such as Modbus or EtherCAT.
- Monitor feedback from drive and I/O: During execution of the move, the motion controller checks for any fault signals from the motor drive. Additionally, if any limit switches and emergency switches are triggered, the motion controller can interrupt the current movement by to prevent collisions and damage.
Benefits of a dedicated motion controller
While it is possible to use general I/O such as data acquisition devices for motion control, it is important to keep in mind the advantages of using a dedicated motion controller. These benefits include:
- Precise timing: Hardware-based controllers have onboard clocks that provide precise timing, which is essential for achieving accurate and precise motion trajectories. In contrast, software-based controllers with general I/O rely on the timing of the computer's operating system, which can be inaccurate and susceptible to jitter.
- Real-time processing: Hardware-based controllers can process motion commands in real-time, which is critical for applications that require precise motion control. This is not possible with software-based controllers, which can introduce delays due to the time needed to process commands.
- Dedicated computing resources: Hardware-based controllers have dedicated computing resources that are used exclusively for motion control. This means that they do not have to compete with other applications running on the same computer for computing resources.
- Advanced features: Hardware-based controllers often offer advanced features that are not available with software-based controllers, such as breakpoint triggering and high-speed position capture.
Conclusion
Motion control is a critical component in many test and measurement applications, and understanding the functions of a motion controller is essential to achieve the desired outcomes. With full control of motion velocities and accelerations, hardware-based motion controllers offer advanced features that improve performance and reliability, making them the better choice for test and measurement engineers.
For more information on integrating motion control with LabVIEW, download our free 33-page presentation here:
Integrating Motion Control with LabVIEW - TENET.pdf