Wednesday, February 14, 2001
Petersfield Hampshire, UK--Although ballscrew accuracy
may be adequate for the next few years, it leaves little margin for testing the
increasingly dense circuit assemblies of the future. At least that's the
contention of engineers at electronics test specialist DiagnoSYS (www.diagnosys.com). The company recently
made the switch to linear motor technology on its next generation AutoPoint II
"flying probe" tester.
The upgrade from a lead screw and motor-based design to a new type
of linear motor called the ThrustTube (www.thrusttube.com), from Linear Drives Limited (LDL), helped
the company meet future design goals for speed, accuracy, and maintenance
ThrustTube exploits a simple patented design based around a
tubular thrust rod and a moving thrust block. The block carries a set of
thrust-generating, circular armature coils that, unlike common flat and c-frame
linear motors, fully encircle its stator magnets. So all coils cut the
electromagnetic flux lines from the entire magnet surface to produce pure linear
thrust and ensure efficient use of stored energy.
"With no precision air gap or precision alignment," explains
DiagnoSYS' Technical Director Stephen Marshall, "it's very quick and easy both
to use and install." Since the motor is electrically identical to rotary
servomotors, industry-standard servo drives power and control it.
In addition, the motor block is designed to carry a load directly,
without need of extra brackets or bearings normally demanded by conventional
direct linear drives. In fact, the ThrustTube module uses wide series HRW21
bearings from THK (www.thk.com) that have a
basic load rating of 6.18 kN and are rated up to speeds of 5 m/sec.
The ThrustTube's speed ranges from a few microns/sec to greater
than 10 m/sec. It also has a high motor force-to-weight ratio and can deliver up
to 20g accelerations. "It's a 'plug-in and go' module," explains Marshall.
"There's no set up required. You take it out of the box and plug it into
standard mains. Then apply a demand signal and you have linear motion. It not
only makes it extremely easy to install and configure into our machines, but
simplifies field maintenance."
As a result of these engineering benefits and the ease of fitting,
DiagnoSYS was able to introduce its next-generation AutoPoint II machine
equipped with the new ThrustTube motor in just eight weeks. "The LDL ThrustTube
is essentially a performance boosting, drop-in replacement for a lead screw
design," summarizes Marshall. "And although it has worked perfectly in our
specific PCB test application, its performance benefits could be transferred to
almost any industrial machine application."