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Nov 07, 2014, 11:21 PM
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contact lens agitator #3





It was finally time for a 3rd stab at a contact lens agitator. After years of levitation & home made linear motors, this one would be a tried & true laboratory shaker, using a stepper motor. The linear motor couldn't make enough thrust, was too noisy, & shed metal filings. The levitator had proven completely ineffective but looked neat.

There was hope a computer fan could do the job. Those turned out to use a dead simple half bridge. 1 pin is always 12V. The other 2 pins alternate going to ground. They spin in only 1 direction, no matter the polarity.

The next step was a traditional brushless gimbal motor. A 3 phase motor controller would have been ideal, but completely unaffordable. A pair of BJT's from a burned out lightbulb would do the job. They didn't need spudger diodes like MOSFETs. The same half bridge arrangement of a computer fan actually provided enough of a shaking motion when applied to a brushless gimbal motor.

After much effort, a C program for driving it with PWM wouldn't compile properly. The compiler choked on a counter equality comparison. After redoing it in assembly, PWM was once again a noisy failure. Even at 22khz, it was too noisy because the PIC at 8Mhz couldn't get the timing close enough. A linear regulator would lower the 20V input to control speed, the heat from which would heat the payload.



How to mount a regulator on a CPU heatsink to heat the payload.



A lock washer proved essential.

Rattling was a big problem. It turned out there was an optimum, high shaking frequency which wouldn't rattle. Using the lowest possible voltage damped it further. Keeping it from rattling at lower speeds would require a complicated mechanical damping mechanism.



The wood post design.

Motor heating was a big problem, since the wood post wouldn't dissipate heat. At 12V, it overheated. 6V was the minimum before the shaking would slip. 8V was the lowest compromise. It still hit 65C at that voltage. A metal post would be ideal.



The metal post design.

It consumed roughly 500mA for any voltage from 8V - 12V. The BJTs passed roughly 500mA, for which they needed 25mA of base current. They were the worst BJTs ever, but free. They pulled the microcontroller pins down to 3.5V. Most of the energy going into the system went to heat. At 8V, the metal post got the motor down to 50C. The linear regulator got up to 60C. The BJT's only got to 45C.



Contact lens agitator #3 (0 min 31 sec)


The slow motion cam revealed a sympathetic rattle still bounced it half way back across the shaking distance before the reverse coil was activated. It needed some kind of damping. Submerging a part in fluid or clever magnet pulsing might do the job, but for now, it was good enough.




Except for the use of BJTs instead of MOSFETS, this was exactly the same circuit which went in the 1st agitator, 10 years ago. The process had come full circle, except the stepper motor was far more efficient than the linear motor.
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