LDL's ThrustTube delivers future-proofed performance and accuracy gains for PCB test application

Citation

(2001), "LDL's ThrustTube delivers future-proofed performance and accuracy gains for PCB test application", Assembly Automation, Vol. 21 No. 4. https://doi.org/10.1108/aa.2001.03321daf.009

LDL's ThrustTube delivers future-proofed performance and accuracy gains for PCB test application

LDL's ThrustTube delivers future-proofed performance and accuracy gains for PCB test application

To enhance the speed, accuracy and capability of its latest PCB testers, and create a robust motion platform for future developments, electronics test specialist DiagnoSYS has employed ThrustTube linear motor technology.

Leading multinational test company DiagnoSYS has significantly improved the performance of its latest AutoPoint II range of "flying probe" printed circuit board (PCB) testers by replacing lead screw actuators with advanced linear motor modules from Linear Drives Limited (LDL) (see Plate 10).

The linear motor modules have replaced the lead screw actuators that formerly drove the machine's X-Y axes. Modern PCB assemblies require the test machine to probe several thousand test points on increasingly reducing pitches down to 0.4mm. The motion platform needs to be fast to maintain output, yet highly accurate to suit the density of modern electronics. Lead screw technology had provided a suitable solution, but was reaching the limits of its capability in terms of speed and accuracy. The linear motor solution provides higher speeds and greater accuracy for both DiagnoSYS' existing and future products.

Plate 10 The Autopoint II flying probe testers from DiagnoSYS employ advanced linear motor modules from Linear Drives Ltd

"One of the most powerful advantages of ThrustTube is that it has given us the technological potential to extend the capabilities of our AutoPoint II machine to a next generation design in the future", says Stephen Marshall, technical director with DiagnoSYS.

"The latest products will exploit a greater number of flying test probes so that they can perform a greater range of tests, and do them more rapidly. All of this would have been impossible to achieve with a lead-screw arrangement which, although perfectly adequate for a single probe based tester, could not cope with the greatly increased demands of multi-probe testing."

LDL's ThrustTube motor technology utilises a patented and remarkably simple design that comprises a tubular thrust rod and a moving thrust block. Unlike other linear motors designs, The block carries a set of thrust-generating, armature coils which completely encircle the thrust rod and the stator magnets within so that during operation every coil cuts the electromagnetic flux line of the magnets. This produces a direct linear thrust only in the direction required and ensures optimum use of stored energy, while allowing the motor to operate without need of forced cooling.

LDL specialises in providing a comprehensive, easy-to-install solution, each ThrustTube module is a complete, ready-to-use payload carrying axis including an industry-standard controller interface. The motors are electrically identical to rotary servo motors and the motor block is designed to carry a load directly without need of extra brackets or bearings normally demanded by other linear motor types. This makes it an ideal solution for machine manufacturers who were initially searching for a bespoke linear motor solution.

ThrustTube is free from the constrictively tight operating tolerances traditionally associated with other linear motor types. Its ingenious design means that it requires no precision air gaps or alignment, which makes it very quick and easy both to use and install. Apart from the usable linear force, it produces no other forces, resulting in a zero cogging force and zero attractive forces between its build components.

"We immediately recognised the advantages of the novel ThrustTube linear motor", says Marshall. "Unlike the other linear motors we assessed, it was a very simple, cost-effective and easy-to-use package that offered an enhanced level of performance over our existing lead screw motors.

Flying probe testers automatically test PCBs by using a single or multiple needle-like test probe that is programmed mechanically to lift up, move across, and re-position down onto a board to make electrical contact with specific components, solder connections and test points to check their functionality.

"Although flying probe testing is fast and precise, its speed and precision are governed by the accuracy, speed and positional repeatability of the motor used to control and drive its probing system", continues Marshall.

"Therefore, given that a more complex board can require thousands of points to be probed, even small improvements in probe speed and accuracy can have a dramatic impact upon overall test time and reliability."

The ThrustTube has a very wide speed range, ranging from microns to over ten metres per second. It also has a high motor force-to-weight ratio and can deliver accelerations of up to 20G. The ThrustTube offers a linear accuracy of ±2µm compared with ±100µm typical of a lead screw design.

The need for in-house maintenance is virtually eliminated (in contrast to the regular greasing requirements of a lead screw) because the ThrustTube has no internal moving parts and there is no contact between the thrust rod and motor housing. In addition, because of its "plug-in and go" design, ThrustTube is extremely easy to install.

For further information contact Denis Devlin, Linear Drives Ltd, Luckyn Lane, Basildon, Essex SS14 3BW, UK. Tel: +44 (0)1268 287070; Fax: +44 (0)1268 293344; E-mail: denisd@linear.co.uk