'Flying probe' PCB tester

Multi-axis controller revolutionizes 'flying probe' test: Two 8-axis NextMove motion controllers are at the heart of a new machine which is speeding 'flying probe' test of bare printed-circuit boards by up to an order of magnitude. The new PC-controlled machine uses an ingenious system of 'flying grids' with 216 probes each to decrease the acceleration, speed and complexity of probe positioning, providing an economic test solution for high-density surface-mount PCBs.

The flying grid idea means that moves of less than 1mm are needed to access any circuit net, allowing the probe system to use economic and reliable stepper technology instead of more expensive and complex servos or linear motors.

In all, 13 axes of control are needed, 8 of which control the X-Y motion of four flying grids. These eight axes use a novel encoder-based system to maintain accuracy. As with any motion system, minute amounts of drift creep in through factors such as backlash. Each test pad is typically 75 microns wide, and although the error for an individual move might be just a few microns, a complete test sequence could entail many thousands of moves and the total error would quickly become significant if uncorrected. So, after each move is made in its optimum-speed 'open loop' fashion, the actual position is read back and used on-the-fly to adjust the following move. This simple approach allows the machine to make moves in around 20msec, compared with 100-200+ for a typical 'flying probe'.

The other axes on the system control PCB loading and positioning, a Z axis to adjust for different thickness PCBs, and a camera which provides feedback to correct for factors such as 'artwork stretch'.
Two problems were encountered when seeking a solution to the machine's motion control requirements. The first was the high level of integration: for economy it wanted a controller that offered a large number of axes; only a couple of suppliers offered solutions with as many as eight. The second issue was compatibility with Windows NT, the platform that had been chosen to control the machine and provide an operator interface. This narrowed the choice down to just one product: the NextMove PC from Baldor UK.

Baldor's Windows interface library provided a number of development approaches, and the Mint motion control language was chosen, providing a stream of high-level axis positioning commands from the host PC into each NextMove board's buffer. These commands are then executed using NextMove's on-board Mint interpreter.

This approach helped to build a working machine very quickly, one which was capable of performing positioning moves in just 60msec. But ideally, even faster perfromance was wanted and Baldor was asked for advice. A decision to switch to C was made, and the library of Mint-compatible C routines was used to convert the code.

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