AL15/AL20K + Adapting galvos to work with another scanner amplifier?

[tribble]

Turning a vane on a shaft and measuring the amount of light blocked by the vane is not linear. The amount of decrease in the vane's shadow per degree of rotation varies by the cosine (or sine, depending on your perspective) of the current position. For very small angles subtended from a perpendicular starting position w.r.t. the emitter and detector I can see how you could approximate linear, but at 20-30 degrees you are way out of the "almost linear" section of the curve. So, it's interesting that there is a way to compensate for that.

[logsquared]

[QUOTE=tribble;304745]Turning a vane on a shaft and measuring the amount of light blocked by the vane is not linear. The amount of decrease in the vane's shadow per degree of rotation varies by the cosine (or sine, depending on your perspective) of the current position. For very small angles subtended from a perpendicular starting position w.r.t. the emitter and detector I can see how you could approximate linear, but at 20-30 degrees you are way out of the "almost linear" section of the curve. So, it's interesting that there is a way to compensate for that.[/QUOTE

They use two photo diodes. As one side gets less light the other gets more. Both signals get feed to a diff amp. so the difference is the position signal. The photo diodes are also "sampled" by the agc circuit where they are mixed together. The linearity pot helps balance the response differences in the photo diodes. The cambridge uses a more sophisticated light blocking vane than shown on the scanners above. It is shaped like a bow tie placed perpendicular to the shaft. The individual photo diode response will be more linear than the sin curve these scanners will have.