Feedback signal points exist/possible with CB6580 scanner amps?

[taggalucci]

Can anyone help me find an appropriate way to connect my Cambridge Technology CB6580 scanner amps to a MediaLas SafeGuard Lite? I can't see any obvious feedback signal points on the PCB.

Thanks in advance for your help

Tony

[mixedgas]

TP1 is feedback out, TP2 is ground

see:

http://www.skywise711.com/lasers/scanner/DSC_2073.gif

Steve

[taggalucci]

Thanks Steve

I've taken a photo below with what I've identified as TP1 and TP2 circled. Perhaps this will be of help to others too in the future...

[LaserGuy216]

MediaLas SafeGuard Lite, im unable to find this on the website, can you point me to it please.... thanks

[taggalucci]

MediaLas SafeGuard Lite, im unable to find this on the website, can you point me to it please.... thanks

I originally discovered the existance of such a board from the Medialas catalogue PDF that they mail out. I'm on their distribution list. I think it also used to be called ScanGuard, and this is a newer reduced form factor replacement. I'm not sure exactly, but I believe it achieves the same thing.

I can't see it on their site. In fact there are many products on their PDF catalogue which do not appear on their site.

Here are two links to what I belive may be the predecessor:

http://www.medialas.de/data/Safe_Guard_II_eng.pdf
http://www.medialas.de/data/Safe_Guard_II_eng_ilda.pdf

I found these PDFs at http://www.medialas.de/documentation.html

[taggalucci]

Here's the info I received from MediaLas...

Connectors:
+24V GND -24V - Power supply +/- 18V ...+/- 30V
X+ - Input signal X+
Y+ - Input signal Y+
FBX - Feedback X-signal
FBY - Feedback Y-signal

Test points:
TP...

General
The electronic board is supplied with the +-24V Power supply used for scanning. In operation the feedback signals FBX and FBY are analysed. The signals x+ and y+ are used for correct adjustment. The red LED ́s indicate the stop of one axis. If both axes stop moving the laser will be turned off in 0.25s.

Connection
The power supply (+-18V ... +-30V) has to be connected to the supply connector. The signals x+,y+, FBX and FBY have to be connected to the marked places. A single laser can be connected at blanking out and the modulation source at blanking in. For the use in multicolour systems there are three channels in which the modulation signals are interrupted while fault condition.
The signals R,G,B are connected to the marked places. At the connectors r out, g out, b out the modulation signals for the lasers are available including the safe function.

Tuning
Using a suitable source an object will be outputted. Due to simplicity the geometry of this object should not be too complex. Using a two channel oscilloscope the amplitude of FBX will be adapted to x+. Therefore both signals are measured at the test points TP FBX and TP X. By changing the setting of the potentiometer ADJ FB X the amplitude of the signal FBX is changed. The signals FBX and X+ should have the same amplitude.
In the same way the y channel will be adjusted.

Tuning of hysteresis
By turning the potentiometers Hysteresis X and Y the switching threshold of both axes is set separately. Make your laser projector draw a short horizontal line. This length is the shortest distance without triggering a fault condition. Turn the potentiometer Hysteresis X until the laser switches off.
The hysteresis of the y axis is adjusted in the same way.

The Safe Guard Lite is set up by default to the factory settings.

So... I think I've got it mostly figured out. The thing I can't work out relates the the GND connection shown alongside the signal inputs (X+, Y+, FBX, FBY). Thanks to Steve, I assume I know where to connect FBX and FBY (TP 1 of each scanner amp), but what should I connect to this GND?

My guesses are:

1. Bridge TP 2 on both boards and use this
2. Use the common DC single point ground
3. Do not connect anything to it, common DC single point ground is already provided with +/-24v

[buffo]

Answer # 3 is correct.

I already replied to your other thread, but for details on *WHY* this is the right thing to do, have a look at this article concerning projector grounds that was written by Bill Benner of Pangolin.

Adam

[Phredy1]

I already replied to your other thread, but for details on *WHY* this is the right thing to do, have a look at this article concerning projector grounds that was written by Bill Benner of Pangolin.
Adam

I was going to email Bill on that article, but maybe here is just as good to ask a question. On Figure 2 it states that during hard acceleration the power ground will reach 4 volts.

How is that possible? Wouldn't 4 volts to ground be a direct short?

Gene

[mixedgas]

I was going to email Bill on that article, but maybe here is just as good to ask a question. On Figure 2 it states that during hard acceleration the power ground will reach 4 volts.

How is that possible? Wouldn't 4 volts to ground be a direct short?

Gene

Its called parasitic resistance. And its a real parasite/pest if not handled carefully.

Its real possible. Say your scanner amp ground wire and connections has a resistance of a tenth of a ohm between the scanner and the power supply . Ohms law still applies, by definition electrons do not flow unless there is a complete circuit. The scanner amp , driven hard on a long jump, for a few milliseconds draws 40 amps peak from the power supply caps when trying to stop the scanner on a dime.
Ohm's Law
E = I *R V = amps * resistance 4volts = 40 amps * one tenth ohm.

Remember a good Cambridge amp might have +/- 30 volt 4 amp psus, per axis, so this kind of surge is possible from the storage caps

So someplace in the system 4 volts is there. Obviously other parts of the system are designed to see 0 volts, ie ground. SO if they are grounded at the point that has .1 ohm drop to ground, their ground is now 4 volts above ground, and a 5 volt signal going into a opamp whose ground is 4 volts above ground is NOW a 1 volt signal or in some cases a 9 volt signal... So you have a massive spike passing through sensitive electronics and you get glitches.

This is why Bill is preaching "HOME RUN" wiring, where each power signal has its own path back to a central ground point, without sharing paths with other currents.

There are other rules to this, but Bill explains them clearly in his paper..

BTW, far worse cases of this exist.. Think of car audio for example...

My buddy has a light aircraft... At night you can hear the strobes popping in the radio and intercom headsets... Followed by the chirp of the inverter as it recharges the capacitors in the strobes.
He is a electrical engineer, knows the solution is home run wiring, but can't legally change it.
The return current uses the metal in the wings for the ground wiring... The dash panel uses the metal in the airframe back to the battery ground for the radios.. Airplanes are made of aluminum, and rivets through oxidized aluminum are not known to be superconductors.... Far from it... The strobe is say 160 amps peak for a few tens of microseconds.. click.. whirrr click whirrr click whirrr.. click whirrr...

Steve

[Phredy1]

How is that possible? Wouldn't 4 volts to ground be a direct short?

I found the problem with my post(s) on the scanner signal ground voltage issue. It was a one - D - one - zero - T error by me.

I didn't read far enough down the article to see you don't connect the signal grounds to anything.

It all makes sense now.

Gene