Ye Ol' G115 and G120PD Scanners

[Draco]

I have seen some audio amps that use back EMF, oddly i am driving these with a low power car amp to generate the Lissajous, the output is differential so i don't need a dual rail power supply, I was also wondering if these have a low frequency limit or can they go to near dc for short amounts of time? I was surprised by the specs for the mass they can move, defently well built

[lasermaster1977]

I have seen some audio amps that use back EMF, oddly i am driving these with a low power car amp to generate the Lissajous, the output is differential so i don't need a dual rail power supply, I was also wondering if these have a low frequency limit or can they go to near dc for short amounts of time? I was surprised by the specs for the mass they can move, defently well built

Load free, the resonant (natural) frequency of the G-310 was 230 Hz, the G-320 was 165 Hz and the G-330 was 130 Hz. You can drive them down to DC for short periods, yes. Having them deflect and stay in a single position for longer periods results in slight, and usually temporary magnetization of the armature causing a shift in the at-rest position. Of course, this is reversible and usually wears off in time and use in the AC driven mode. So with small, low-mass mirrors you should be able to drive them open-loop by a sine wave in the 200 Hz range before you start seeing a scan angle reduction. I know someone on this forum who used these with 1 inch mirrors, doing planetarium laser shows back in the late 70's which really lowered their upper frequency limit. Here are some other specs for you on the G-310:

Armature inertia, gm-cm2 =3.7
Torque---approx., gm-cm =950 (it could certainly swing large masses)
Coil inductance, mH = 30 (coils in parallel)
Sensitivity, mA/degree = 100
Back-emf, mv/degree/second = 2.9
Temperature Coefficient of
Sensitivity = +0.02%/deg. C
Linearity = 1% of peak-to-peak, or better
Repeatability = .05% of peak-to-peak, or better
Hysteresis, % of
peak-to-peak = 0.7


In comparison, the G-115 had a load free resonant frequency of 950 Hz.

Cheers

[laserist]

Who in hell would want to do that?

These open loop scanners had torsion bars. Think tight spring. When you drove the scanner to some position it would ring because of the spring tension. You could use back emf to dampen the ringing.

[lasermaster1977]

These open loop scanners had torsion bars. Think tight spring. When you drove the scanner to some position it would ring because of the spring tension. You could use back emf to dampen the ringing.

Great observation, laserist. Both the open and closed loop scanners from GS back then had torsion bar springs, one end connected to the bottom of the rotating armature and the other anchored to the mounting base of the scanner. I have a picture from GS on this I'll scan if anyone is interested.

The total deflection errors were derived from non-linearity in the torsion bar, the armature and torsion bar forming a mass-spring resonance system, and the no-current hysteresis due to magnetic remanence from previous deflections.

Positional error signals in the "PD" closed-loop scanners were derived from a pair of capacitive position transducers made up between the electrically isolated stator pole pieces, an air gap, and a short length of the moving armature. This provided a far better method of accurately sensing where the armature was than the open-loop scanner's quasi-positional error signals using back emf of the drive coils through a series current-sensing resistor. I've a diagram of the back-emf feedback schematic if anyone is interested.

[mixedgas]

I'll post my Back Emf circuit if you post yours!

Steve

[lasermaster1977]

I'll post my Back Emf circuit if you post yours!

Steve

Steve, fair is fair. I'd love to see someone else's solution. Here is one I derived back in the early 80's from GS's suggestion.

[Draco]

i would love to see a picture of the insides of one of these.

[lasermaster1977]

i would love to see a picture of the insides of one of these.

The smaller G100PD galvo on the right depicts the rotor or armature. The torsion spring is the very thin line near the bottom of the rotor between the two cylindrical shapes.



Credits are due here:

These photos are from a paper published by Pierre J. Brosens and Edward P. Grenda of General Scanning, Inc. titled "APPLICATIONS OF GALVANOMETERS TO LASER SCANNING" and presented at the 18th Annual Technical Meeting, Society of Photo-Optical Instrumentation Engineers, August 19-23, 1974 in San Diego, CA 92108

[mixedgas]

Oh, You got "RoC=L" working from the GSI apnote. I'll publish something when I get home.

Steve

[lasermaster1977]

Oh, You got "RoC=L" working from the GSI apnote. I'll publish something when I get home.

Steve

yessir, you nailed it Steve.