AL15/AL20K + Adapting galvos to work with another scanner amplifier?

[masterpj]

Mother always said I could use some "refinement" HAHAHA

Seriously though.... I think he said the coil was around 8ohms. 1.5V /8 =187mA. Is that really too much for the china motors if just "tapped" on the cell?

My older 6800's had the fuse on the motor. Probably why the amp doesn't have a fuse. Put one in series!

Steve's right these things are pretty delicate. If it were me I would use the lowest possible supply rails and fit a 100ohm resistor in series with the fuse and coil. This will help limit the current available to the motor.

The coil is 7.84.. I measured it multiple times incase oxidation on the outer pins would have some play on that.. so yes close to 8ohms.
I used one of those really small cell batteries which caused the coil to flop to one direction according to the polarity and did cause any damage after repeating this process.

I could add an fuse externally to the drive but I' m worried that breaking the connection to one of the galvos drive with a fuse could cause issues and damage to the amplifiers? (latter not sure)

[mixedgas]

The coil is 7.84.. I measured it multiple times incase oxidation on the outer pins would have some play on that.. so yes close to 8ohms.
I used one of those really small cell batteries which caused the coil to flop to one direction according to the polarity and did cause any damage after repeating this process.

I could add an fuse externally to the drive but I' m worried that breaking the connection to one of the galvos drive with a fuse could cause issues and damage to the amplifiers? (latter not sure)

One PLer's tap is another one's Short Circuit! Hence the caution. Good Cambridges with good bearings and good coils are not cheap.

Fuses and Polyfuses are a good thing. Redboards ship with a self reseting Polyfuse installed for good reason. A really sophisticated amp might have internal current limiting, partially negating the need for a fuse.

8 Ohms static, but remember its inductive when moving. A 6800 is 4.2 Ohms static per attached older data sheet.

The D battery causes end of travel impacts to be violent.

When I made the kit for Sunk!ng to test all the surplus 6810Ps, test currents were limited to 50 and 100 mA, switch selectable.

I'm working on 506 Amps tonight, I'll slip an old 6800 on a 6850 and see what the numbers are. All the 6800s I have are worn.
I keep a few around for the obvious reasons.

The Volts Per Degree calibration of the sensor is critical for good amp function. Its essentially the volts/per degree per unit time or slew rate of the feedback signal. That matters.


Steve

[mixedgas]

Peter, I'm adjusting amps right now. I promised I would make this measurement for you the other day. So I stuck a Cambridge 6810P and a 6850 Amp on the test rig. Iagc = 18 mA , Which is 18 mA thru the LED in the position sensor.

Steve

[masterpj]

Peter, I'm adjusting amps right now. I promised I would make this measurement for you the other day. So I stuck a Cambridge 6810P and a 6850 Amp on the test rig. Iagc = 18 mA , Which is 18 mA thru the LED in the position sensor.

Steve

Thanks! so my scanner led needs aprox half. Will do some tests once I know what the pinout is of this mentioned adapter cable so I can make my own as right now its a connector for G120,cambridge hybrid.

[mixedgas]

PJ, This is the schematic of a Older Cambridge Amplifier's photodetector and AGC circuits. AGC is an old radio term, "Automatic Gain Control".

I've crudely painted in a few things, which I will explain tomorrow.

I added the orange and green wires, the Clockwise and Counterclockwise photodiodes, the position vane, and the LED.

You can see how the linearity correction is done by feeding of the position signals to the AGC current source via the Va and Vb inputs.


R19 and R13 are what concerns you with reducing current. I'll discuss them tomorrow. I'm sure your amp will have something VERY similar, as this is the circuit Cambridge recommended to those who built their own amps.

Keep in mind if you change them, you need to adjust every tuning control on the amp. Making it hard to go back to the G120s.

Slight changes to the AGC adjustment have a serious effect on Amplifier Performance.

"Schematic posted under a stretched educational use clause."

Steve

[masterpj]

Making it hard to go back to the G120s.

Steve

Hey Steve that's te cool thing about this amp.. it has an toggle switch on it. It changes to a completely different tuning pot range and discards the others when flipped over. This has to be done to switch over from G120 to cambridge.
My schematic reading skills are "ok" but its one of those skills I need to practice more on.. so it'll take me a bit to properly read the schematic.

Unless its the pots that are covered with a warranty seal.. in that case yeah that issue will be present and I would be hesitant with the adjustement.. I want to use these amps because they provide a nice voltage/gain for scanners which could yield a performance boost on some galvos.

[mixedgas]

R13 is the position scale pot. On Cambridge it is CAREFULLY set to something close to five tenths of a volt of input per mechanical degree. Optical scanned degrees are TWICE the mechanical degrees because of the double path on the mirror..

So 10V of control signal divided by 0.5 volts per degree gives you 20 degrees mechanical angle. That turns into the 40' maximum optical angle of the scanner.

R19, is the current source sensing and limiting resistor. It is only changed with some trepidation.

So to set the Position Scale, one sets up a known target distance on the wall and sets the scan pair a very precise distance from the wall, at the center of the line. One can then do some mild trig on the calculator and calculate the angle.

There is a very precise, easy to follow, procedure for this in some manuals I have.

There will be a similar procedure for setting up your amp.

After the "Volts Per Degree" is set, you tune your amp.

Then you adjust the Linearity potentiometer for the least AC voltage at the Linearity test point while scanning a grid or similar symmetric test pattern. This takes care of errors in the sensor vane and the slight electrical and mechanical differences in the photodiodes. I find it best to monitor all this on a oscilloscope. However it can be done extremely carefully with a really good voltmeter.

Every time I describe this procedure to people who want to calibrate their scanners from scratch, they get scared and leave it be. Its not that difficult if you have the manufacturer's parameters for the amp input and the galvo signal. The specification usually calls for a 20 Hz square wave test signal for the initial setup. This gets ran with enough servo gain to be saturated and enough LF damping to stop any ringing.

You may have enough dynamic range in your existing scanner amp to go down to 8 mA with no modifications. If that is the case, you will probably want to limit your maximum control signal to one half normal. One can do this by running single ended on the ILDA connector, or by changing the scale factor resistors on the position detector circuit on the amps. Or by adding a image size potentiometer if you do not have one. Since the manufacturer of your amp is being helpful, this is probably easily taken care of.

On the 506 Amplifiers, Per Mr. Benner's specification, I change two resistors to handle this issue. Those are R2 and R4 on the attached drawing.

VA is the output voltage of one photodiode on the schematic***. VB is the other voltage. The Op-Amp U4-B computes the difference of the photodiode signals in real time. Hence the term "differential amplifier" for the circuit of U4-A, U4-B, and U4-C. This circuit is common and the mathematics of it are well understood. Wiki has a decent page on its math.

*** In reality a current from each photodiode is converted to a voltage by U4-A and U4-C respectively. The minute current returns to ground via the negative input of the opamp.

KEYWORDs: Amplifier Tuning , Position Sensor , Linearity , Photodiode , Position Demodulator .
Steve

[masterpj]

it makes more sense now Thanks Steve!

Also a little off topic but I tuned and scoped the scanner amp with the G120
Had bought a allen key at size recommended 1/16th of an inch and it didn't fit in the set screw of the G120.. apparently my set screw is slightly smaller and I ordered a smaller allen key.
I had scoped the amp and the results were showing nothing was wrong with the scanner amplifier.

What was noticeable was a wobble on the x-axis only (or y axis if the scanner signal cables were switched over). Depending how far you'd project off center from either side the wobble would increase and with size increases eventually caused that squeeking sound for only that one faulty g120 galvo.

My allen key comes in tuesday so I can open it up and see if indeed the torsion bar was broken as was assumed.
I talked to DZ and I paid him shipping as he wanted to send me a G120.

He was also surprised that the amplifiers were able to let the G120's do 30kpps.
I will let the pictures do the talking as soon as I have the G120 in and a mirror epoxied *if none is attached*

Having listened to the noise of the scanners I think I could have even pushed further and im not very scared of tuning.. in fact I kinda enjoy it.
I hope to soon post results soon.

[tribble]

I find it interesting that you can get anything approaching a linear position determination out of such a decidedly non-linear sensor arrangement! That is a pretty nice trick.

[masterpj]

Its quite linear but for very small work or very high distance projection the linearity might visually decrease but overall its more then enough for most applications.
There are different sensor arrangement.. for one the new scannermax scanners has a completely new sensor design/arrangement and can be found somewhere in a list of patents online and maybe some engraving scanners, etc use non optical feedback sensor arrangements