A Nipkow disc with no holes.
Posted: Tue Jun 12, 2007 3:57 pmI’ve had this idea rolling around in my empty skull for some time. A Nipkow disc that doesn’t have holes. Well, not for scanning anyway.
The idea is to use a linear sensor array to scan the image. The TSL201R seems being ideal (pdf attached). However, if its active position is at 3 o’clock it scans across the lines. But if positioned either at 12 o’clock or 6 o’clock as shown in the sketch in red, this will scan an NBTV line of 64 pixels.
The smaller ellipse is a representation of a lens, a 50mm focal length lens from an old SLR camera would be ideal.
The output is analogue and needs to be ‘stretched’ as one complete frame would be scanned in 2.5mS.
There is also the issue of integration time. With a single sensor on a disk rotating at 750RPM there is very little time for the device to capture the ambient light upon it. It’s within spec. but I suspect that a great deal of light would be needed. Here that’s not a problem; we receive three times the solar energy that Europe does.
So to combat this I considered increasing the quantity of sensors to three which would reduce the speed of the disc to 250RPM and increase the integration time by a factor of three. Using three sensors spaced at 120 degrees would allow the use of AC synchronous motors that run at 250RPM (RS & Farnell). The problem is that the analogue signal still needs to be stretched.
To get a continuous output waveform that doesn’t need stretching would require a lot more sensors and doing some simple maths they would simply not fit next to each other, there’s just not enough space.
There is also the problem of getting the signal in and out of the devices as well. They need two inputs, the output plus power and ground. Slip rings are a possibility (yuk), the other is opto-coupling with power provided by batteries on the disc.
This is all very conceptual, and partly why I posted the ‘PCB discs’ thread some time ago. The electronics would be quite complex, but I think possible.
Steve A.
The idea is to use a linear sensor array to scan the image. The TSL201R seems being ideal (pdf attached). However, if its active position is at 3 o’clock it scans across the lines. But if positioned either at 12 o’clock or 6 o’clock as shown in the sketch in red, this will scan an NBTV line of 64 pixels.
The smaller ellipse is a representation of a lens, a 50mm focal length lens from an old SLR camera would be ideal.
The output is analogue and needs to be ‘stretched’ as one complete frame would be scanned in 2.5mS.
There is also the issue of integration time. With a single sensor on a disk rotating at 750RPM there is very little time for the device to capture the ambient light upon it. It’s within spec. but I suspect that a great deal of light would be needed. Here that’s not a problem; we receive three times the solar energy that Europe does.
So to combat this I considered increasing the quantity of sensors to three which would reduce the speed of the disc to 250RPM and increase the integration time by a factor of three. Using three sensors spaced at 120 degrees would allow the use of AC synchronous motors that run at 250RPM (RS & Farnell). The problem is that the analogue signal still needs to be stretched.
To get a continuous output waveform that doesn’t need stretching would require a lot more sensors and doing some simple maths they would simply not fit next to each other, there’s just not enough space.
There is also the problem of getting the signal in and out of the devices as well. They need two inputs, the output plus power and ground. Slip rings are a possibility (yuk), the other is opto-coupling with power provided by batteries on the disc.
This is all very conceptual, and partly why I posted the ‘PCB discs’ thread some time ago. The electronics would be quite complex, but I think possible.
Steve A.