As a controls tech working in the water and waste water industry with mainly the Micrologix series PLC's and a variety of different brand drives, I come across situations where I have to solve noise problems with analog circuits. Bad vfd's. There are also the occasional communications problems like dedicated leased land line, rs232, rs485, ethernet/ip and other varietals protocols.

I'm sure an oscilloscope would help with diagnostics and I'm wondering how much of an asset a scope is for a troubleshooter? Does anyone use one for diagnostics? From what I gather, Tektronix and HP/Agilent seem to be good names for scopes. Can anyone give any insight as to whether or not it would be worth my while to have one for trouble shooting? If so, what kind of specs for a scope would be good for all around trouble shooting when it comes to features like bandwidth, sample rate, max voltage input etc.? Can anyone recommend one in the used or new market like what I might find on eBay or elsewhere in the $300 to $400 range?

 

I would get a digital storage scope with a lot of capture memory. We have older HP units that have two channels and a digital breakout for 16 channel digital. We use these to debug our PLCs sometimes when they misbehave since you can have a lot of signals on the screen.

100MHz is probably a good all around scope for bandwidth but is probably dated if you are going to be debugging ethernet (this doesn't tend to be an issue for me).

There are some laptop based scopes, not sure how good they are. You probably won't go wrong sticking with HP or Tek. I don't think I could be in the automation business without access to a scope.

KEJR

 

This came up a few months back...check out thread #1285276375

Cheers

Gerald Beaudoin

 

We had a recent thread on this which you might research. My recommendation for the low end would be the Rigol Digital Storage Scopes. You have all the capability of an old used analog scope and the tremendous power of the digital storage technology which is particularly suited to automation work with non-repetitive and low repetition rate signals which are extremely hard to capture with an analog scope. I've quoted my part in the earlier thread for your convenience. If you think I'm sold on the DSO approach, you're correct. As a troubleshooter, this has been extremely useful.

> 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.

Regards
cww Happily in my depth.

 

Oddly, we don't actually end up pulling out the scope all that often. Of course, when you DO need a scope, there is absolutely no replacement. Like several other commenters, I think your situation calls for a portable scope, like something out of the Fluke ScopeMeter line or the Agilent U1600 series.

Agilent, Fluke, and Tektronix all make excellent scopes. HP no longer makes scopes; they spun off Agilent to handle that business. I can't comment on Rigol specifically, but I'll take cww's word for it.

$300 to $400 range? Hah! Not even close. Try triple that as a minimum. There is one thing you can do to cut costs. Use a PC-based DAQ system, such as www.bitscope.com, www.softdsp.com, or www.picotech.com. Agilent also has USB-based scopes, but they're as expensive as their portable scopes, so why bother? National Instruments has some USB DAQ options, but the software is on the complex side.

Hope that helps.

-James Ingraham
Sage Automation, Inc.

 

One thing I forgot to mention that speaks to quality. Rigol makes or has made the low end Agilent scopes. If you look at the pictures, there's more than a resemblance. I did a lot of research and it's pretty hard to beat the Rigol 1052 even at twice the price. If I was working, I would own one before the prices go up. And I already have an analog scope. Most of the competition has much less storage which is the most valuable parameter for time machine mode.

Regards
cww