RE: [CAD_CAM_EDM_DRO] Need help with O-scope

One bit I wrote below is confusing.

I wrote 'checking that the non inverted and inverted are the
exact opposite to each other, including any noise'

There are two types of noise, the normal one being harmless in a
differential system.

If A is positive, the inv A should be negative, or opposite at 0V
(Differential signals can be centred around 0V but for encoders in these
systems are typically centred around 2.5V). Noise that is OK would push both
traces in the same direction, 5V and 0V with 0.1V of common mode noise,
become 5.1V and 0.1V. Since both carry the noise, the difference between is
the same (5V) and noise exists no more.

There is also differential mode noise, if you get noise in opposite
directions (i.e. 0.1V noise becomes 4.9V and 0.1V), this noise is a problem
since 5V signal has now dropped to 4.8volts. This is normally caused by
another wire like the motor servo wire coming between the twisted pairs and
will be a problem. It can also be a short in one side of a differential pair
to ground, or damaged line driver in the encoder, but you will see that the
traces are no longer opposite when this happens.



This is a little difficult to explain without picture but I hope it covers
it.



Regs Mark





Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Mark Vaughan
Sent: 14 April 2007 20:56
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: RE: [CAD_CAM_EDM_DRO] Need help with O-scope



We're all getting into complications here of the more peculiar scope
measurements that are very rarey needed.

Lets try to attack this another way.

Don't worry about isolation transformers, we are 99.9% sure you won't need
one.

Connect scope to same supply as mill, or lathe. The ouside of the BNC
connector, the scope croc clip, and it's metal case should all be at the
same potential.

Since the probe croc clip is earth, if you attach it to something with a
voltage on it, you may earth out that voltage, first thing to fry will
probably be the little lead on the croc clip.

Assuming machine 0V, computer 0V etc are all grounded then it is OK to hook
the croc clip onto one of these. Then the probe tip can be used to look at
step direction signals, and encoder signals.

If you use differential encoders and want to study the signals you really
need a dual channel scope, hook one channel to the A, and one to the inverse
of A and look at both signals on the screen, then you can move the probes
between the signals checking that the non inverted and inverted are the
exact opposite to each other, including any noise, and that A and B are 90
degrees to each other.

For step and direction, you can look at both with two probes, check that the
direction changes before the edge of the step of pulses may be lost when you
reverse direction.

For the motor output, these are hi voltage so be careful, but again one
probe on each wire should let you see the pwm signals going to the motor.
Remember the motor is inductive so you are going to see loads of spikes on
these.

You can also look along your DC power supply and earth lines, power supplies
should be smooth DC, 0V should be smooth at or close to 0V, you should be
able to follow any noise back to it's source, when you find the source check
earth connections are tight, perhaps add a ferrite core of capacitor to
reduce the noise. If two earth / 0V lines are daisy chained so the noise of
one is appearing on the other, change the circuit so as to break the chain.

I hope this makes it a little easier. Sorry we all get a bit carried away.

Dr. Mark Vaughan Ph'D. B.Eng. M0VAU

Managing Director

Vaughan Industries Ltd, reg in UK no 2561068

Water Care Technology Ltd, reg in UK no 4129351

Addr Unit3, Sydney House, Blackwater, Truro, Cornwall, TR4 8HH, UK.
Phone/Fax 44 1872 561288

RSGB DRM111(Cornwall)

_____

From: CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com] On Behalf Of Peter Reilley
Sent: 14 April 2007 20:07
To: CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Need help with O-scope

There is another technique that can get effective isolation
from ground. That is to use the A - B setting that some
dual channel scopes have. This subtracts the B input channel
from the A channel. Thus the trace produced is the voltage
difference between the two inputs.

To use it put the A and B channel probes on the two points
that you want to measure. Connect the B probe to where
you would normally connect the ground. Do not connect
the ground leads to anything.

You are still limited by the max voltage limits for the scope
inputs and probes however.

Pete.

----- Original Message -----
From: "Jon Elson" <elson@pico-systems. <mailto:elson%40pico-systems.com>
com>
To: <CAD_CAM_EDM_ <mailto:CAD_CAM_EDM_DRO%40yahoogroups.com>
DRO@yahoogroups. <mailto:DRO%40yahoogroups.com> com>
Sent: Saturday, April 14, 2007 2:58 PM
Subject: Re: [CAD_CAM_EDM_DRO] Need help with O-scope

> David G. LeVine wrote:
>> At 10:06 AM 4/14/2007, you wrote:
>>
>>>I am having a problem with a mill set up with Gecko drives. There seems
>>>to be a noise problem that has cropped up on the Z axis and I would like
>>>to look at it with a scope. I have the use of a scope but have not used
>>>one for several years and have forgotten a few things. The part I worry
>>>about is; do I have to use an isolation transformer to keep from popping
>>>an IC do to different ground potentials? The scope is a B K Precision
>>>model 1570A with API 1X and 10X probes. Any help appreciated. Ed.
>>
>>
>> Most 'scopes had isolation transformers built in. BK Precision
>> SHOULD, but may not. It couldn't hurt.
>>
> No, this is not true. No oscilloscope is powered directly off
> the mains, but that is not the same as an isolation transformer.
> The frame of the scope, and the barrel of the BNC probe
> connectors will be connected to the power plug ground pin.
> Only on battery powered scopes will this not be the case.
> If in doubt, use an Ohmmeter to measure from the BNC barrels to
> the power plug ground pin, and you will have your answer.
> Making a mistake with this could turn the entire scope into
> scrap or get you seriously burned or shocked.
>
> There are cases where you need to isolate the scope from ground,
> such as to measure across the motor output terminals of a full
> bridge driver. There are usually much better ways to make such
> a measurement, for instance by making a differential measurement.
>
> Jon
>

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