Linear Motion Design Considerations for Medical Device OEMs

Linear actuators — in particular, electromechanical linear actuators — have become integral components of modern medical devices because of their high precision, accuracy, and ability to deliver repeatable motion control. Patient comfort, positioning and mobility, robotic surgery, imaging equipment, infusion, and pumping are just a few of the applications where the use of linear actuators has revolutionized the way medical devices are designed, improving patient outcomes and enhancing the overall quality of care.

Electric linear actuators offer significant and game-changing advantages in many medical device and equipment applications over conventional manual or fluid power driven solutions:

• They are more precise and provide greater accuracy and control.

• In many cases they can offer a smaller footprint to aid in creating more mobile, more elegant design solutions.

• Electromechanical solutions are also more reliable and less maintenance intensive, reducing downtime and costs.

• They are quieter to support improved patient experience.

• Finally, they can improve the efficiency and speed of medical procedures, reducing wait times and improving patient flow.

For medical device and equipment designs using electric linear actuators for motion control, detailing exact requirements for performance and reliability, as well as environmental and human factors, will help ensure successful outcomes.

Motion Design Considerations

In any new linear motion design exercise, the first requirement set to be evaluated typically, are the mass and forces to be managed. Linear actuators must meet the specific performance requirements for each aspect of device movement and often with multiple interactions, including target accuracy, motion precision, speed, force, and linear travel.

For example, in infusion pumps the speed and force of the linear motion is needed to ensure that system can provide the proper the flow rate to reliably ensure the precise and correct dosage is delivered to each patient. In the case of surgical robots, the accuracy and precision of the motion control system determine the accuracy of the surgical tool’s placement, which is critical for achieving surgical objectives while minimizing trauma to surrounding tissue.

For design engineers, the use of application worksheets and electromechanical sizing software assists in identifying, collecting, and calculating actuator specifications that will perform successfully in their application. It is critical that calculations be performed correctly to deliver the expected performance.

Rod-Style or Rodless Actuators

An application’s requirements will most often guide the selection of either a rodstyle or rodless actuator.

Rod style actuators are ideal for situations where linear thrust only is required, in applications such as fluid compression, dispensing, or injecting procedures. Rod style actuators offer limited stroke length and loads, but support higher forces designed for pushing, pulling, or pressing with force up to 50,000 lbf.

In contrast, rodless actuators can support and carry loads, eliminating the need, cost, and design complexity for additional load-bearing and guiding elements. A rodless actuator’s stroke lies completely within the length of its body, resulting in a smaller working footprint and a more efficient design that supports frequent cleaning, aesthetic requirements, or even decontamination processes.

The loads and forces carried by a rodless cylinder makes it necessary to calculate the various moments (torques) that are being placed on the actuator’s bearing system, based on the position, size, and weight of the load, including off-center or side loads. Moment loading should also be calculated for the Mx axis (roll) and the Mz axis (yaw). The farther a load is from the center of the load-carrying device, the larger the resulting moment.

In addition, dynamic bending moments are created by end-of-stroke acceleration or deceleration. Some applications contain compound moments that involve two or more of the moments described above. Each must be evaluated and calculated to determine whether the actuator can handle the combined moment forces.

Service Life and Reliability

Identifying the actuator’s duty cycle is a key component of sizing and selection. Duty cycle is defined as the ratio of operating time to resting time of an electric actuator, expressed as a percentage. For example, an actuator that is moving for two seconds and stopped for two seconds has a duty cycle of 50 percent. Underestimating the impact of duty cycle on an actuator can lead to overheating, faster wear, and premature component failure.

Overestimating the impact of duty cycle can lead to higher initial costs due to oversizing. Overly conservative duty cycle estimates often stem from an incomplete understanding of the application. This is another advantage of using sizing software programs as they can factor in duty cycles, move times, and velocities.

Operation and Usage

Medical environments require low-noise operation to ensure patient comfort and minimize interference with sensitive equipment. Noise and vibrations can also affect the accuracy and precision of the linear actuator technology. Actuators should be designed to operate quietly and produce minimal vibrations, particularly in devices used during patient examinations or procedures.

Prior to prototype, intangible aspects like noise, vibration, or other aspects including visual appearance can at times be the most challenging and hardest to quantify and manage throughout a design exercise.

Size and Weight

The size and weight of the linear motion and actuator technologies are also vital design considerations. In many cases, medical devices are designed to be compact and lightweight for portability, function, operational environments, and ease of use, while other applications may be fixed installations, quite large, heavy, and demand utmost levels of stability for accuracy and movement precision. Even in the largest, heavy-duty applications, there are often underlying objectives to produce desired performance outcomes while minimizing the physical footprint and weight to deliver a more practical and functional execution.

Operator and Patient Safety

In medical devices and equipment, the importance of human factors cannot be overstated. From functional considerations for operators to essential safety aspects for the patient, consistency and failsafe performance are of the utmost importance. Linear actuators and motion systems may be required to incorporate safety features such as position feedback sensors, limit switches, and emergency stop mechanisms to prevent accidents or injuries during operation. These safety features can help ensure proper device functionality while protecting patients and healthcare professionals.

Device Serviceability

Ease of maintenance and serviceability is crucial for medical devices, as they often require periodic inspections, repairs or component replacements. Electric linear actuators should be designed with accessibility in mind, allowing for straightforward maintenance procedures and minimizing device downtime. Specification of IP-rated actuators (such as IP55, IP67, or IP69K) not only protects the performance and life cycle of the component but helps ensure that the device itself is tolerant of real-world cleaning and maintenance practices in medical and healthcare environments.

Manufacturability

Once the proper performance specifications have been calculated, and operation, safety, service, and life cycle characteristics have been identified, the design process then turns to evaluate the viability of a solution. When considering a component provider, it’s best to identify a vendor with the expertise, agility, and willingness to make modifications. It’s important to remember that “build to order” does not immediately involve a complex and costly custom development cycle.

In cases where custom innovation is required, the development partner you select should be not only qualified but collaborative, with demonstrated capabilities, engineering capacity, and willingness to engage in a robust customization dialog and process. The right partner can also help identify the critical design requirements and work with you to evaluate iterations of the finished product with consideration given to budget impacts and lead times.

Conclusion

The continued development of emerging technologies ranging from surgical, procedural, and diagnostic devices to more advanced iterations of patient positioning, ergonomics, and mobility assistance, will propel the use of an enlarging scope of electric linear actuators to provide both simple and complex controlled movements. By considering the factors that reach beyond performance specifications to address the larger impacts of the operating environments, human factors, and longterm serviceability and maintenance, designers can ensure that their medical devices are safe, reliable, and effective in meeting the growing needs of patients and healthcare professionals.

This article was written by Ryan Klemetson, Business Development Manager, and John Fenske, Director of Marketing, at Tolomatic, Inc., Hamel, MN. For more information, visit www.tolomatic.com . A free white paper, “Linear Motion Design Considerations for Medical Device OEMs” offers deeper guidance on the steps to successful design and integration of linear motion in medical applications. Download the white paper here .

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