So this is a patch. It needs to go between the DAC board and the PID loop input stage.
Some systems have an internal DAC. In other cases the user/renter plugs their analog signal in. Hence needing to intercept.
When the galvo drivers were cloned in China, the. slew rate limiter, coil temperature. calculator, and notch filters were removed. For a long time the pads were even retained on the clone boards. The CTC was based on the RC4200 Analog Multipler which went obsolete. Eventually the notch filter plug in for damping the principal resonance returned.
There are input gain, servo, LF damping, HF damping, and differentiator 20 turn pots that need to be tuned to match the galvo to the amp, plus the notch filter. The notch filter went away at first as the cloners didn't understand it, and probably saw no reason to invest in the test gear. Eventually the cost savings was overcome by the return rate of units returned, damaged by shaft resonance problems.
No two units are the same due to very tiny machining, position detector, and mirror inertia differences.
The PID loop is tuned using a visual test pattern. Industrial users tune for low ringing and best performance on a square wave, then match the response of the two mirrors on a scope, by re tuning the faster small x mirror to match the slower, larger y mirror. We tune to a transfer function defined by the test pattern at a given scan angle. Tuning is thus a bit of an art form.
I have a very sopsticated clipper limiter working, based on the active one in the Nonlinear Circuits Handbook. Once plugged into the standard harness , that will stop the user's from driving the optical position sensor flag into the housing. This is a limited rotation moving magnet motor. Once the flag is bent or knocked off [ 0.040" thick piece of steel about 1.5 mm by 3 mm in size, the galvo pair is toast from a user stand point.
That leaves problem "B". The cloners, catering to uneducated
customer demands, have opened up the mechanical scan angle to the point that the position sensor can become saturated or non linear. The loop can recover from a DC overdrive, however, HF ac transients in the control signal plus users running at maximum angle can exceed the loop bandwidth , take the position sensor well out of its agc and linearity corrector operating range, resulting in chaotic loop oscillations , and cook the drive coil. the 60 or so turn 3.2 Ohm coil expands, pushes itself out of the stator, strikes the rotor, and shorts out.
Hence the concept of a gently sloping low pass, which will pass the design signal, but quash getting into the "Ballistic" region of the control law. The position sensor on a cheap clone is two photodiodes and an LED. The LED illuminates the flag, etc. The active current source drives the LED, with a resistor network feeding part of the A and B photodiode signal into the current source for linearity correction, known as "AGC" in the business.
Newer designs have four photodiodes, to allow wider angles,but
"cheap" rules and everything low cost is shipping with two.
So part "C" of my board turns off the Laser Diodes on a "scan fail" condition. I have access to the raw position signal in most cases on a three pin header. But no two sensors have the same
level or offset, so I look for ac components within a window.
Simple slew rate limiters cause problems on long jumps in the graphics. Hence the desire to limit the control signals hf content. All it takes is the user cranking up the refresh rate slider and image size sliders in the control software, and instant 400$ mistake.
Recent work by a very brilliant team
first resulted in a doubling of scan speed, then a 30 % increase.
So high end systems exist, running at the physical limits of what a galvo can do. Hence the software folks , have no intention of rolling back the controls.
Comparing the demand signal to the position feedback out doesn't help much, due to the fact that longer jumps are slow and short jumps are very fast, the mechanicl system is of course a delay.
So I want to get rid of that HF content.
It used to be the case that trained operators were sent out on tour. Now the artistic directors want control from their consoles. Thus "Don't do that!" does not work.
I'll post some FFTs and scope shots as in a few days.
Thank you all for re-educating me. My last position went from taking care of a few graduate labs to catering for the tech needs of four hundred or so students, both graduate and undergrad. So I was not doing much design for the past six years. I'd describe the job as 110% crisis managent, as Chemical Engineers are mainly trained in modeling systems.
My old friends are asking for help as marketers have taken the industry over. It is no longer the domain of artists and engineers, as commodity systems have flooded the market.
I need to run some FFTs and post a few schematics.
Thanks..More to come.
Steve
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