Oscilloscope for troubleshooting encoders, drives etc.


Thread Starter

Helge Slettvoll

We sometimes have to bring to light our oscilloscope for troubleshooting. It is a big bulky CRT scope and I reluctantly drag this heavy instrument around. It says 'digital', but it's really not that sophisticated. 20 years old. 20 kg heavy.

I need some preferences for a good light weight scope and/or PC-adapter for troubleshooting industrial equipment, mostly encoder systems, DC drives/thyristor bridges and something of the sort. This is not an instrument that we will use very often, so it has to be fairly simple to use, but with the functionality enough to let's say detect and freeze on missing pulses from an encoder with varying speed.

I also would like to bring up a second issue: We sometimes look at the output from (DC) drives. It's always a challenge about grounding. The scope's chassis ground is the same potential as the protective ground. This doesn't work when measuring output from a drive. We solve this by temporarily disconnecting the ground wire from the scope's mains plug. I don't like this at all, but it's the only working solution found. How do you solve this?
We use HP digital scopes and like them a lot. The ones we use have a 16 channel digital pod in addition to the two analog channels which is great for doing timing work on automation equipment since you can tell exactly when your IO is transitioning. I dare say Tektronics has equivalent equipment. The scopes I've seen in the catalogs seem to be not as deep these days, which is handy when you are on a machine and the scope is propped on a chair or something of that sort.

Have a look at the Fluke Scopemeter. I have used it for a number of purposes and it is quite a useful device. You would normally use it on battery power, so that also solves the grounding problem.

Gerald Beaudoin

I used to do the same thing...drag my dual-channel dual-time base crt scope from home for the few occasions when I needed it.

Later we bought a Fluke 123 which took care of most of the our plant needs. Nice and portable to go anywhere. Was able to look at encoder pulses etc, but no dual time base or ext triggers. Unless your folks are really at ease with all those neat extra features, they will just be be superfluous and confusing. The 123 is a simple to use auto trigger scope that does most of what you need in day to day operations.

Channel ground isolation it does NOT have. I know that some of the other models/makes do have complete channel isolation which has become more essential these days.

In any case, whatever you decide seems to be a good fit, get the rep to lend you a demo for a week or two and test drive it. That's the only way to really find out.


Gerald Beaudoin

curt wuollet

I'm all over this one :^)

Very seldom over a long career have I found a tool so useful that I would call it out by name and risk actually endorsing it. At my last job, I bought a Rigol DSO specifically for one shot events, intermittent failures and the like. It was absolutely excellent for the purpose and arguably paid for itself at first use. The Rigol specifically, over all the reasonable cost competition for it's deep memory. For most automation work you don't need extreme bandwidth, in fact, the problem is much more often that things happen too slowly or infrequently for an analog scope to be of much use. And the jitter and triggering issues often confuse more than they illuminate. The solution is a long one shot real time record. This is what I call time machine mode. Let's say something is causing a fault once a week and you suspect a flakey signal or power issues. You can set up to watch power and the signal, and trigger (one shot) on the fault. The DSO will be recording, and _stop_ recording on the fault. With the long record (deep memory), you have a permanent record or what happened before the fault and after. If you have chosen your time frame correctly, you usually have your answer. This is extremely powerful and will impress management no end. Looking at an encoder with an analog scope usually gives an "envelope", an overlay of many cycles, and if one is missing, for example, you are unlikely to catch it. But with the DSO, you can get a long record of many cycles, not overlaid or mixed together but as they happened. You can scan back and forth and look for the bum counts that occur with worn bearings or a scratched disk. Start up problems, power issues, the list is long. Learning how to use this and especially the trigger options can save huge amounts of time in troubleshooting. They even asked me to teach a class in how to use the DSO for the entire department. For problems with many variables, there is a logic analyzer option which would allow triggering on a given word or up to 16 variables. I didn't get that, but I would sure consider it next time around if I'm still in the business. And the Rigol is cheap, around $400 for the DS1052E base model.

Oh, and the official way to solve the grounding problem is to use two probes and set up the vertical section for differential measurement. One probe on the low side and one probe on the high side. Take care not to exceed the common mode voltage limits, and it's generally safer to use 100X probes.

I have sucessfully used the Fluke 123 and other Fluke ScopeMeters on DC and AC drives for about 14 yrs... I'll agree with the other posters recommending a Fluke ScopeMeter