Problem
A manufacturer of ink jet cartridges was having unacceptably high failure rates in the production line. Additional off-line tests revealed that most of the failed cartridges were good. The problem was calibration of the production line test equipment. The asked Alpha Omega to produce a test device that would allow more precise calibration of the test equipment.
Solution
Working with the manufacturer's engineers we tried several approaches for achieving better calibration. Each method involved making precise measurements of energy being delivered to the devices under test.
The first attempt used a machined metal "calibration pen" body to carry a circuit board with high precision load resistors, and contact pads to mate with the test equipment probes. Wire loops extending through the top of the cal pen allowed current measurements to be made with a Tektronix hand-held current probe. An additional feature was a four foot long flex circuit that mated with the pen driver electronics in a printer or wide bed plotter to allow measurement of currents from actual printers. This unit was only moderately better than calibration methods already in use.
The second version was a very ambitious plan to incorporate the electronics of four Tektronix current probes inside a pen body. It was not clear at the beginning that this was possible. The hand-held current probe assemblies each were larger than the ink cartridge volume and we had to fit four of them plus switching circuitry inside one pen cartridge! Working with the customer's mechanical engineers we came up with an EDM machined metal pen body with very thin walls that contained sufficient volume to hold four of the current probe detector elements, eight mil-spec mercury wetted relays to switch the circuits, and circuit boards to mount all the parts. Machined delrin spacers held the parts in place inside the metal shell. A cable exited the top of the cal pen to connect to the Tektronix current probe amplifier and to our external switching control circuit. Amazingly, it all fit into the tiny cartridge! Furthermore, the unit worked spectacularly and reduced production line waste to a fraction of what it had been.
Not satisfied with the original cal pen, which was designed for a single type ink pen, our customer wanted to rework the product to make it adaptable for multiple variations of the same basic pen design. This involved making the front contact PC board unpluggable to allow different variations to be installed. While we were at it we added an internal heat sink to remove heat from the relays and transfer it to the external metal shell, and a thermistor to measure temperature inside the assembly. There wasn't enough empty space inside the unit for a small ant to move around!
Results
The cal pen worked extremely well, reducing production line waste by a reported $125 million per year.
