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Thread: Analog scan rotator

  1. #1
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    Default Analog scan rotator

    Hi PhotonLexicon,

    I want to build an analog scan rotator (for any angle, not just flip X/Y).

    Mathematically:
    Xrotated = X * COS(angle) - Y * SIN(angle)
    Yrotated = X * SIN(angle) + Y * COS(angle)

    Can I use an op-amp to compute the COS and SIN functions?

    Kind regards,

    weartronics


  2. #2
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    This is a classic question for Dean C. Hammonds!

    James.

    PS I'm guessing there might be a way, but not through ordinary linear analog circuits. Sine and Cosine are not simple functions to calculate under any circumstances. Historically speaking, they were found in large books of lookup tables with varying precision; actually measured with drafting tools and real, drawn circles! There are infinite series notations for these things, but much like the relationship between a circle's radius and its diameter, it cannot be fully quantified to any rational value. Good Luck!

    James.
    Last edited by James Lehman; 08-10-2008 at 10:36.

  3. #3
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    I've built rotators before using an analog 4-quadrant multilplier chip.

    http://en.wikipedia.org/wiki/Analog_multiplier

    also: Google "Analog Devices" and "multiplier".

    Tim

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    Can you give us some more detales, please?

    James.

  5. #5
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    Here is a rotator I made. Signal path is analog, but the sin and cos tables are digital on eprom so the rotation can be stopped without drifting. Comes with documentation.
    http://cgi.ebay.com/ws/eBayISAPI.dll...m=150281723379

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    WOW! You made that? That's really neat! Nice job.

    James.

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    Default 4qmm

    You need 4 - 4 quadrant multiplier chips - we used to use the AD532 from analog devices. You can certainly use opamps to generate the sin/cos signals - it's called a quadrature osc. As an alternative a joystick moving in a circle does the same thing and allows you to "perform"...

  8. #8
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    Hello all and thank you for your suggestions,

    I am still not sure how it is possible to compute sin and cos using analog multipliers without LUT. I need to compute sin and cos very fast with better than 8-bit resolution, based on the rotation of my spinning projector. This is why I prefer to use the analog circuit, because the delay of ordinary ADC-LUT-DAC is too long.

    I was looking at sine encoders, which output sine wave signal proportional to rotation, but they are many cycles per revolution and I need one cycle per revolution. I thought of an even simpler solution, because the transmission of crossed polarisers is the sin of the angle. I can use a spinning disc of polarising film, geared down 2:1 from the projector using timing pulleys. Then with photodiodes and stationary polarisers on opposite sides of the disc, the output of the photodiodes is the sin and cos of the rotation. This is cheap and simple to make with no digital delay.

    Best regards,

    weartronics

  9. #9
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    You use the 4 quadrant analog multipliers to do the multiplication in the rotation formula. The sin/cos signale are classically (i.e. in my laserium days) provided to the circuit from a joystick moving in a circle. It could also be generated by a quadrature osc. or a look up table - I used to use a military surplus sin/cos potentiometer at one point - of course I haven't seen any of those for 25 years...

  10. #10
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    Quote Originally Posted by imobscure View Post
    You use the 4 quadrant analog multipliers to do the multiplication in the rotation formula. The sin/cos signale are classically (i.e. in my laserium days) provided to the circuit from a joystick moving in a circle. It could also be generated by a quadrature osc. or a look up table - I used to use a military surplus sin/cos potentiometer at one point - of course I haven't seen any of those for 25 years...
    PM me, I have a schematic for a 12 bit digital rotator. It uses two multipling dacs that are ~$7-$12 in quanity one from AD. I can scan it in and send it as a pdf. I only have Inet access once a week so it may take me time to get back to you (unemployed, no T3 to my desktop net at the office anymore :-( )

    Steve Roberts

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