An innovative robotic XYZ lead screw mechanism, slotting the motion control subsystem into a smaller footprint design, is a critical element in systems that speed analysis of scientific samples. Using the combination of a lead screw and two spline shaft rails, the motion subsystem of a new liquid chromatography system saves space by using the spline rail as both a linear sliding device and as a driver in the Z direction for efficient handling of liquid samples.
Advances in Liquid Chromatography
New ACQUITY Ultra Performance Liquid Chromatography™ (UPLC) systems from Waters Corp. (http://rbi.ims.ca/4928-556) have created a new category of separation science. The systems use 1.7-micron particles to give laboratories as much as nine times higher throughput, three times higher routine sensitivity, and two times greater resolution or peak capacity than today's HPLC instruments achieve with larger particles.
The UPLC process reduces time to analyze each sample from one minute to a minimum analysis time of 15 seconds. By offering increased resolution, sensitivity and speed, the company says that UPLC greatly improves productivity and provides scientists with considerably more information in a single run.
Design Challenge: Smaller Footprint
While performance of the ACQUITY system proved to be the top priority for engineers developing the product, the team also worked toward a system with a much smaller footprint. Smaller footprint design consumes less of a laboratory's valuable bench space and guarantees that users can more easily stack and arrange various modules.
Accomplishing this goal requires use of smaller internal components in both the main unit and the optional module, which holds an additional 21 sample plates. Initial efforts to create the required parts did not yield the desired results, according to Ken Plant, a principal engineer on the Waters UPLC project.
Customized Motion Solution
To design the new mechanism, Plant worked with Kerk Motion Products (http://rbi.ims.ca/4928-557) to produce a sample manager that would meet the reduced size requirements and produce the increased speed and performance vital to the UPLC concept.
"While other vendors were selling off-the-shelf commodity items, Kerk's products allowed for customization and flexibility," says Plant. "Kerk was willing to customize their components and ensure that they would mesh into our system."
Within the sample manager, the motion subsystem drives a robotic XYZ mechanism holding the needle that draws liquid samples from each sample plate. To achieve the "X" and "Y" movements, the ACQUITY arm incorporates one Kerk /-inch lead screw and two specially adapted Kerk nuts. The "Z" motion is accomplished through two spline shafts and splined bushings.
In an optional sample organizer, a ˝-inch lead screw and anti-backlash nut is employed to produce the Z-axis motion, allowing the robotic machine to shuttle as many as 21 sample plates back and forth to the sample manager.
The system's design uses the lead screw and two spline shaft rails that run parallel to the lead screw, with the lead screw being centrally located between the two spline shafts. One axis advances along the screw, but there is also a need for two Z-axis motions in that same space. A key aspect of the design is leveraging the spline rail as both a linear, sliding device and as a driver in the Z direction. By having the two linear spline shafts, the design produces the Z motion by driving either one or the other spline shafts.
"The rails serve a dual purpose for the sliding motion, as well as the driver for the Z-axis carriage assembly," says Bob Hawkins, an applications engineer for Kerk Motion who worked on the project. "The system positions over a tray of samples and, once they position over a bottle, penetrate the top of the bottle with a hollow needle." The second Z axis puts a tube down through that puncture for extracting the liquid from the sample. For puncturing, a relatively high force is required, but the second Z axis requires almost no force at all.
Custom Pulley Design
Another design challenge proved to be the pulley arrangement on the spline shafts. On the bearing area of the pulley/slider combination, wall thicknesses ended up down to as small as 10 thousands of an inch. The team tried different approaches because the pulleys are so small in size and there were concerns about concentricity and the interface with the spline. On the ends of the pulleys, outside of the belt, a bearing on each side of the pulley supports the carriage when it slides back and forth.
"We originally started with aluminum pulley stock, bored out the center and pressed in a composite spline bushing," says Hawkins. "It turned out that the wall thicknesses were too thin to use traditional keys or other methods of affixing the two components. The internal bushings would spin because the torque was too great. We couldn't come up with a reliable method for attaching those two components. More than that, the bushing on the aluminum pulley, even with its TFE anodized surface treatment, was still wearing unacceptably."
As a result, the team specified 300-series stainless material and, along with the molded pulley, 303 stainless sleeves are manufactured that press on the ends that ride on the bushings. The pulleys are molded, machined, broached and the sleeves are pressed on. The size of the whole pulley is less than 1 half-inch long.
Focus on Reliability
Coming up with a design that would work, given the tight space needs and features required, also demanded a design focus on reliability.
"Especially in lab and medical type applications, everything needs to be smaller, more reliable and lower cost," says Hawkins. "We used the Kerkote® TFE coating on the spline shafts to reduce friction and wear which is key in this application because there is not much material to wear. Any wear on the spline shaft and/or bushing would result in lost rotational motion."