No, A slew rate limiter on a modern galvo amp is not a low pass filter. It looks at input DV/DT (rate of change) and makes a yes or no decision to remove the input signal until the rate is below what the Galvo can handle. It drives an analog switch or removes drive from the motor coil. And it is angle dependent, it will ignore small changes. Modern ones use DSP.
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Lars, you lived until now, in the audio world, a world of highly symmetrical signals, where the DC offset is removed. You have much to learn when you extend the bandwidth down to DC, which effectively is a signal of infinite bandwidth. Small changes in the phase angle with respect to frequency of a laser show control signal show up, and are unsightly to the viewer. This gets integrated across the whole small signal bandwidth, as James so elegantly expressed. It then goes on to excite resonances in the control system, which are well beyond the scope of this post.
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The transition region from DC to about 20 Hertz is very important in vector laser displays.
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The protection units in the past based on a low pass topology, get bypassed, in Laser Show use due to the distortion. Why?, Because the command to jump to a DC offset position, which is very important in laser graphics, has a infinite series of harmonics which pass thru the filter as a spike. The viewer then sees a distracting flash on the screen. The low and mid frequency portion of the image passes thru, while you watch the capacitor in the integrator charge up with the DC portion, and half of your image slowly slews to its new position while the rest catches up. It looks like hell to the customer, and even worse to a viewer. While this concept is just fine in protecting the galvos of a laser engraving system, it looks like garbage to a human viewer, especially in a beam or abstract show..
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Believe me, I have one set of amps that does that... For a good reason, because they drive large 25 mm mirrors that would torque themselves out of the mount if overdriven in Slew... I might shoot you a video on the weekend, and it will clearly show how you'd be wasting your time... It will even scan the ILDA test pattern at 24K with the large mirrors, which is unheard of in the show community. Yet if issue a command for the test pattern image to move across the screen with a DC offset, the image will tear apart for about one second until the new steady state is reached.
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I'll pull up a post of a galvo transfer function from the past tomorrow... You might be shocked with what your dealing with... Please get a pair of closed loop, Dragon Tiger or Cambridge or PT Scanners, and learn what your dealing with. This is a very complex system...
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And NO, as I have explained multiple times in the past on this forum and in others, there is NO solid relation between KPPS and frequency response. The divide by 12 portion for the "circle in the square" portion of the test pattern is an approximation for a fast one to two degree jump of the beam position in the image. Scanner manufacturers specify a small angle or step bandwidth and a large signal bandwidth. The divide by 12 is a guess at the small step bandwidth in one test condition, and has little to no relation to the large step bandwidth. I know the skilled engineer who developed that approximation, and he'd be the first to tell you that your dealing with a much more complex transfer function then that..
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Scanner Speed during an image is dependent on the instantaneous commanded angle in a way that requires a complex polynomial to describe.
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Steve
Last edited by mixedgas; 04-11-2016 at 10:59.
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When I still could have...