Miniaturized Motion Design

The design for a new precision optical inspection device required a change of thinking. In the past, the solution included cams and other mechanical positioning devices. But with miniaturization and extended travel requirements, a cam application wouldn't work. Typical reciprocating ball screw solutions proved to be cost prohibitive and their physical characteristics were too large for the application.

"The customer toured our facility and noticed a precision high-speed, three axis positioning lead screw system that we designed and build for an electronics company," says John Pickering, president of CAR Engineering and Manufacturing (www.car-eng.com). "Once they recognized our precision lead screw expertise, they admitted that they had hit a glass ceiling with a current in-house design and wanted to look at other engineering options."

Miniaturization, Efficiency, Cost

Working closely with Kerk Motion Products (www.kerkmotion.com), the engineering team at CAR Engineering collaborated with the end-user in designing a multi-motion subassembly incorporating four lead screws, matching drive nuts, and two miniature servomotors. The compact assembly is only 1.5 inches high by 1 inch wide by 7 inches long.

The subassembly design uses forward and reverse motions on the four lead screws, working in conjunction with the two servo motors, to precisely position the unit's camera lens or mechanical probe. The result is 25 percent more articulation for the optical inspection device, and a reduction in the size of the motion control subassembly by about 50 percent from previous designs.

This computer-controlled optical inspection device utilizes distance-based articulation of a camera lens or mechanical probe, at distances from as little as 3 feet to as much as 40 feet from the operator console. The lens or probe can rotate 360 degrees, turn back on itself a full 90 degrees, and spin 90 degrees on its own axis for maximum flexibility.

Compact motion subassembly uses forward and reverse motions on the four lead screws, working in conjunction with servo motors on two of the lead screws, to precisely position the unit's camera lens or mechanical probe.

Ultimately, this peripheral vision allows the system to perform a variety of inspection tasks to detect subtle irregularities within castings or other complex assemblies. It can insure the structural integrity of a complex aircraft engine component or diagnose a simple cause of an automobile squeak or rattle.

Engineering Challenges

Once the technical requirements of the application had been fully defined, Pickering confirmed that the project didn't require a recirculating ball lead screw device. With miniaturization as a major design goal, he says that the Kerk lead screw alternative proved to be a tenth the cost of a typical design incorporating miniature ball screws. The collaboration on the design produced several different possible solutions that CAR Engineering prototyped and presented as "try before you buy" assemblies to the end user customer.

Pickering says that they could have miniaturized the design further if the articulation requirement had been less demanding, but as the travel requirements increased, the design encountered flex and distortion problems. In response, the design team increased the size of the lead screw slightly but he says the final system is still very compact and efficient.

A user-friendly engineering model developed by Kerk Motion simplified the formalization of the motion system design, engineering efficiencies, loads, speed recommendations, and potential heat build-up attributed to friction. Kerk Motion also assisted the engineering effort by helping with sizing for both the lead screw and its compatible drive nut geometry.

"Driving capacity was limited but we had a predefined amount of travel and load that we needed to manage, and we had to maximize efficiencies," says Pickering. "Load efficiency on the screw was critical to assure not only that the subassembly would operate properly, but also guarantee long-term durability."

According to Pickering, the project posed a two-fold engineering challenge: miniaturization of the subassembly and optimizing the coefficient of friction on the lead screw profile designed to meet the articulation requirements of the inspection device. Since the subassembly is packed as tightly as possible within the inspection device's handle, there is no room for error in the mechanical subassembly because of unusually tight space restrictions. The subassembly design also required dedicated areas for a circuit board and onboard optical sensors to be inserted during final product assembly.

With initial efficiencies at 10-15 percent, the system had to be optimized to achieve the efficiencies required

Mini screwrail is constructed of KerKite composite polymer material and compounded with lubricants, reinforcements, and unique thermoplastic characteristics formulated to provide optimum performance.

for the application. Kerk Motion built special tooling that modified the way its lead screw is typically rolled, increasing efficiencies beyond the typical 20 percent threshold, ultimately achieving 25-28 percent in the final product design.

"The miniature lead screws allowed us to create a mechanical subassembly with the high precision and superior efficiency we were looking for," says Pickering.

Built for hundreds of millions of inches of travel without requiring external lubrication, maintenance, or adjustment, Kerk Motion's Mini Series is constructed with proprietary composite polymer material. The products are compounded with lubricants, reinforcements, and unique thermoplastic characteristics formulated to provide optimum performance.

The polymer material was a key element to the success of this particular application because, unlike typical miniature subassembly applications, the optical inspection device required an especially high load capacity and an extremely smooth motion.

"A key advantage is design flexibility and material selection," says David Arguin, an applications specialist at Kerk Motion Products. "The Kerkite material allows for longer product wear life, and is capable of meeting the high speeds and loads that this application requires."

CAR Engineering is supplying the mechanical build and testing of the subassembly package including the four lead screws, four drive nuts, and servomotors. CAR Engineering worked with its customer to define assembly and performance criteria which allows them to provide plug-and-play sub-assemblies. Every subassembly is shipped from CAR serialized with a detailed test report providing a full history of each part. "Later, if units encounter problems in the field, we can use SAP evaluation techniques to find out why it failed through traceable manufacturing trending data," says Pickering. The end user is doing the final package assembly including the electrical and computer interface for the inspection device.

CAR Engineering is a multi-disciplined engineering operation offering services from tool design and engineering to high-speed metal stamping, multi-slides, punch presses, screw machines, hybrid CNC milling/turning centers, wire EDM, light welding and fabrication, plus a clean room assembly capability.

"Our customer has an internal engineering team which is a robust group of people but this was slightly outside their window of confidence," says Pickering. "By bringing us in, we were able to off-load engineering and build responsibility for the motion subsystem, freeing them to leverage their core competencies in respect to other challenging engineering aspects of the overall project."