Motion Control Strategies for Medical Device Apps

From servo and stepper motors to linear and ac induction actuators, the motors used in medical device applications come in a range of types and technologies. Behind the scenes, all of these motors--despite their differences--are regulated by software programs that control a variety of functions, including position, velocity, torque, and safety. However, while all motors operate in tandem with control software, manufacturers have adopted different control strategies. Some, for example, offer dedicated software platforms for controlling different motors, whereas others have developed a single, integrated solution that can work across a spectrum of varying motor types. Both approaches, according to industry experts, have advantages and disadvantages.

Customized Motion Control
"We choose a controller for its performance capabilities," comments Sacha Marcroft, vice president of Ibex Engineering (Newbury Park, CA). "Typically, with most medical device applications, you can get away with using Ford Taurus-type controllers that employ enough high-level command capability that one can hit 90% of the applications." Such a control technology, he adds, provides easy entry and intuitive software. Additionally it does not require intensive training or handholding. Single-axis pumps, for example, use very simple controls and do not require any high-level processing power.

"Motion control is application dependent," Marcroft emphasizes. "For example, scanning applications used in microscopy require very smooth velocity control. Smooth velocity improves imaging and reduces fringing. The better the imaging, the easier it is to perform image postprocessing, and the quicker the processing, the faster the slide throughput." Such scanning applications require high-resolution feedback, controls with high sampling rates (bandwidth), and some level of notch filtering. Off-the-shelf controllers with this type of technology are readily available, so that OEMs do not have to reinvent the wheel.

However, even simple motion control systems may eventually incorporate greater complexity and funcationality than they possess at present. "There are always going to be breakthroughs in computational algorithms and package size," Marcroft remarks. "But one trend we are starting to see is the employment of servo control with simple stepper motors. Using vector control with a simple 50-pole stepper and a rotary or linear encoder gives you the best of both worlds when it comes to closed-loop applications that require high torque and low speeds. "The benefits of integrating servo control with a simple stepper motor include zero-speed holding torque, zero dither when the motor is stopped, no stalling, improved resonance control, and low-cost motors, Marcroft adds. While this concept has been around for many years, the availability of this type of drive and controlller is on the increase.

Integrating and Modularizing
In contrast to Ibex Automation's customized control technologies, B&R Industrial Automation (Roswell, GA) takes an integrated approach to handling motion control. "What this means is that the application layer that a designer uses for any motion application is consistent and the same, regardless of whether it is for a robotics application, CNC equipment, or just general motion control," notes Corey Morton, the company's solutions architect. "The actuator type really does not make any difference because the control application layer is identical for all of them. From a software design standpoint, I don't care whether I'm driving a stepper, ac induction, or servo motor."

The greatest advantage of an integrated approach to motion control, according to Morton, is that the same software is used to control different actuator types. This approach results in simplicity of design and also reduces the learning curve for end users. "When customers wish to modify or perform maintenance on systems that have been delivered and commissioned, they only have to learn one thing and then apply it to all actuator varieties."

This integrated approach also enables designers to structure their software irrespective of the type of motor it will eventually control. "If I look at a machine of any type, I'm probably going to break it down into several different mechatronic components," Morton explains. "Because I don't care what actuator type I have, I can use the same software to control a stepper or a variable-frequency drive, and I can make it completely modular from the standpoint that I can plug and play both actuator types on my machine." This functionality provides a high degree of freedom. First, it's hardware independent, and second, it allows designers to organize the software in the same fashion as the mechanical components. For example, the software design for a lower-cost stepper-driven machine can be virtually identical to the software design for a higher-performance servo-based machine.

Because motion control, discrete logic, visualization, and safety have been integrated into a single software platform, the user has the ability to organize his software to match the different mechatronic components that make up the machine.  Although previously challenging, this modularization effort is easy in an integrated environment, offering a significant advantage, according to Morton. "From a mechanical perspective, designers have been dividing a machine into different sections for years. What's been lacking is the software equivalent of that." Previously, when engineers removed a machine module or replaced it with a new one, this required significant changes to the overall software design, Morton adds. Now, when they replace a mechanical module, they can deploy the associated software module, which includes all required components.

Simplified Programming
Another take on integrated motion control software is provided by Haydon Kerk Motion Solutions (Waterbury, CT), which integrates intelligence--or software--directly into a family of drives that can be used in such medical devices as MRI equipment, medical scanners, and x-ray machines. "When we speak of an integrated linear actuator, what we mean is that the drive, the controller, and the stepper motor linear actuator are all developed as one unit," explains Dan Montone, the company's marketing director. "Based on this integration, our IDEA line of stepper motor drive technologies is good for distributed control. In other words, it offers very localized control and the electromechanical device in a single package."

Consisting of a microcontroller and a graphical user interface (GUI), the controller portion of the integrated linear actuator enables engineers to program the company's drives without having to write lines of code. "While engineers typically program stepper motor controls by entering lines of code and then downloading them into the controller, our GUI makes it easy for customers to program the moves for their linear motion applications without using programming language," Montone says. "Thus, the user can push a button and enter a value in a field to instruct the motor to move a certain amount of distance."

Part of the challenge associated with programming stepper motor linear actuators is that those tasked with doing so typically do not have programming backgrounds. "The knowledge that's required for a person to be a programmer is quite different from that required for a person to be an engineer," Montone notes. "And because it's often mechanical or electrical engineers rather than programmers that are called upon to integrate motion control systems, our software interface platform makes it easy for them to program the drive. The GUI makes our controller different from other available controllers."

Contributing to the motion control flexibility, each of the company's controller/drive models relies on the same GUI for programming any desired parameters. "You download the GUI to your computer once," Montone comments, "and then you can use this same interface to program any of our units."