When Is Precision Motion Control the Right Choice?

For many engineers working in the automation space, programmable logic controllers, industrial ethernet communication networks, and sensors are common tools of the trade. Those tools are used in motor control applications such as controlling fans, pumps, and blowers using AC motors; controlling process lines via torque and velocity control with high-powered brushed DC motors; microstepping control for precise applications using stepper motors; and controlling position, velocity and/or torque with servo motors. Each technology has its own benefits, and for the most part the world’s largest automation suppliers have an adequate solution no matter what technology is required. However, there is also a set of solutions aimed at a completely different level of motor control: precision motion control.

Precision motion control solutions supply an even higher level of motor control, often synchronized over several axes of motion or motors. A common representation of precision motion control is the control of a CNC milling machine, which requires multiple axes of motion to move in complete synchronization in order to machine parts out of raw materials. However, precision motion control has a much broader appeal than this and can be advantageous in a variety of applications.

Unlike the more general motion control associated with factory automation, precision motion control uses higher-resolution feedback and control devices with faster data rates for servo and current-loop control to position with high resolution, speed and/or accuracy. In a broad sense, precision motion control solutions take special care to synchronize commands to multiple motor drives or axes of motion, either via high-performance communication buses or multi-axis drive hardware.

Precision motion controllers also include complex mathematical engines for calculating precise motor positions and special programming constructs for integrating process-level control with motion control. The technology and performance improvements of these solutions have traditionally been reserved for highest-end applications; however, that no longer needs to be the case.

Many AC induction motors and brushed DC motors are being offset by AC brushless and permanent magnet synchronous motors (PMSM) due to their improved performance, increased efficiency, and growing volume production. Also, new microcontrollers now have the computational power to perform the required calculations to enable precision motion control.

Changes in the workforce and the core technology required to deploy solutions support increased adoption rates of precision motion control. For example, more graduates enter the workforce with the types of programming skills required to succeed using more precise controllers. Figure 1 shows the modern programming integrated development environment (IDE) of the Aerotech Automation1 Studio application, which offers a look and user experience that recent graduates will likely find familiar.

Identifying When You Are Ready

One indicator that an organization should consider a precision motion control solution is if controlling their process with higher levels of precision can provide a benefit to their customer or give them a competitive advantage in the market. During customer conversations, the word “precision” is one signal that precision motion control may be a viable solution, but often discussions about precision center on system accuracy or repeatability.

If customers describe a solution as “close but not where it needs to be,” they mean the solution in question isn’t accurate enough. Whether controlling a motor for position, velocity, torque or some other process variable, accuracy refers to how closely the actual value matches the desired reference value. When customers say “nobody in the market makes a system that works every time,” they are referring to a repeatable process. Repeatability refers to how well a system can duplicate results time after time.

The need for precision motion control can also be gauged by making internal observations. Some signs that an organization may be ready for a precision motion control solution include:

• Part tolerances, in general, are pushing the state-of-the-art on what is currently being achieved on the market.

• Higher throughput is necessary while maintaining the same, or better, part tolerances.

• Significant time is spent developing robust part fixturing.

• The mechanical structure of the machine is viewed as a critical item for performance.

• Environmental effects such as temperature or floor vibrations will adversely affect part tolerances that are produced on the machine.

In a basic sense, a precision motion control system is simply a control system that operates at higher data rates, using processes with a higher level of precision, with the purpose of increasing the bandwidth of control to systems to make them more accurate and perform with higher throughput. Often, precision motion control systems also have advanced features that allow developers to work around some common issues with mechanical systems. For example, all mechanical systems are affected by vibrations and temperature, but not all controllers are designed to minimize those environmental factors from affecting the system’s accuracy or repeatability.

It should be noted that there are plenty of other reasons to consider precision motion control for a project. For instance, most precision motion controllers enable specifying a motion path or trajectory using either the G-code language or a custom scripting language that may be easier to program than traditional IEC ladder logic. There can also be other process improvements — like faster I/O processing — that result from using a motion control solution. These considerations are all worth investigating and can lead to discovering even more benefits.

Pivoting Your Organization

Once an organization determines it needs to develop a precision motion control skillset, the next step is to start evaluating the requirements across all systems. It’s important to do this before a system build begins because the most pressing requirements, which can easily become the focus during the build, are often not the most influential. Laying the groundwork ahead of time includes documenting short- and long-term requirements. The most successful teams identify these requirements early and build a path to adopting new technologies. Taking a long-term view allows time to consider the impact a new control platform will have on the team and how influential each requirement is to the organization as a whole.

A good starting point for identifying requirements is determining the level of performance improvement needed for the system. The chosen solution should benefit the full spectrum of these needs. Note that every servo motor is configured, tuned and optimized for the task at hand. This process can be time consuming and increase build costs, but it cannot be avoided. Thus, a preferable solution will minimize the time it takes to set up a machine as well as provide the necessary features needed to accomplish the process.

Depending on the internal and external needs, organizations should also evaluate feature sets including:

• advanced control features

• supported feedback devices

• operating system compatibility

• programming APIs

• communication with other controllers

• support for different motor drive types and power ratings

• support for additional I/O, tools for custom developments.

Figure 2 shows a sampling of the software and hardware available in the Aerotech Automation1 motion control platform, which is an example of a platform that delivers the above features.

It is important to start the motion control supplier evaluation process early and give this process some time. Motion controllers tend to be complex systems that offer a variety of ways to accomplish different goals. A clear understanding of system requirements and available solutions allows organizations to confidently pick the motion control approach that is most likely to increase their system performance and customer satisfaction.

This article was written by Patrick Wheeler, Product Manager, Aerotech (Pittsburgh, PA). For more information visit, here  .

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