Monitoring solenoid health 4

[CGSmith]

We have several electro-mechanical actuators with fail safe brakes which release using the solenoid. If the solenoid doesn’t release the brake, the actuator won’t work and hatches/doors won’t open. I’d like to find a way to monitor the solenoid coil when the actuators are not in use....they sit for a long time. It is important that they work when commanded. Anyone have experience with techniques or products that monitor solenoid coils? Thanks!

 

[IRstuff]

Are the solenoids powered continuously and then released, or unpowered, then actuated?

But, what's the failure mechanism? You can't monitor for a mechanical jamming, unless you actually actuate it to find out. Ditto for possible electrical failures, although you can measure the solenoid current or resistance, without actuating it.

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[itsmoked]

I'm currently working on a job using a Productivity 2000 PLC (Automation Direct) that has output modules that throw a fault code if there is no continuity thru the connected load. This tests for a broken wire issue without the output having to be turned on.

This is done with a milli-ammeter approach. A couple of mA are allowed thru the load and an optoisolator monitors that current. If it goes missing the circuit is broken. Essentially a resistor and an opto. How they're plumbed in depends on whether your system is sourcing or sinking to the solenoids.

As IR sez though, it doesn't prove the hatch will open.

If you want to know if the hatch has opened you can add microswitches to the hatches to detect physical opening.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[VEBill]

My car can tell me that an external lamp is burned out, specific right down to individual bulbs (hilariously, even Left or Right license plate illumination). If I understand how it works, every single load has its own power control MOSFET and wiring, and the CAM bus modules monitor the bulk current. So the computer software modulates the voltage to each load and looks for a corresponding signal in the bulk current. If a given circuit is open (due to a burned-out lamp), then the software knows that that particular bulb is open circuit. Speculation alert, but general concept of monitoring the current applies.

More specifically to solenoids. The car's Cam Position Adjusters are solenoid controlled. The computer monitors the resulting Cam Position and throws an error if it's not working properly. This is more of an end-to-end test, looking for the closed loop feedback. This is a bigger loop than just looking for the current to match the command. It can detect if the mechanical components are functioning (or not).

Just two concepts I've seen from the diver's seat.

 

[CGSmith]

Thanks for the great replies. The biggest difference between this application and something like a light is that we really need to detect if the solenoid is bad before we try to use it. That’s what seems to make this more difficult because the solenoid will be off. Also, the solenoids we’re using only have a few ohms of resistance when just measured at the cable leads. When you think about it, they are just coils of wire when not energized so tough to measure without any current.

I’m interested in that productivity 2000 PLC....great suggestion. Running current through it would work. We’ve been looking at a product found online yesterday by a company called Pretech Solutions also (can’t remember the product name though). It checks the solenoid and wiring to the solenoid. Sounds similar to the productivity 2000 based on your description. Thanks for all of the great replies. I’ll check out more info on both and let you all know how it turns out.

 

[VEBill]

Short pulses are the trick. Long enough to measure the resultant pulse of current, but too short to have any mechanical effect. On the order of milliseconds. If the wiring or solenoid are open circuit, then there would be no current signal.

Short circuits would be slightly trickier to distinguish. Calibration may be required. Software could self-calibrate with each normal operation, if the system is in regular use. This is called 'health monitoring' in general. The system pays attention to the details of the voltages and currents, and advises of any changes. If the linkage needs lubrication, ideally it should notice.

If this is a very critical system and worth the expense, then map out a complete list of all imaginable failures. Then see if they can be distinguished in advance. Probably need to add sensors, and redundant or interlocked actuators.

Interlocked would be that it requires both A and B. Each can be actuated individually and confirmed to work (with a sensor), but the system is only triggered with both A and B actuated at the same time. Just leaves software bugs as probably the only remaining failure mode. Of course, software never goes wrong, LOL.

This type of safety critical design is not uncommon. History of Apollo spacecraft has many examples of this sort of failure resistant or fail-safe design concepts. Same thing in nuclear reactors, based on books I've read. Aircraft too.

 

[itsmoked]

That would be expensive VBill but sure would completely prove things nicely. I like it. Easy with a PLC too. Actually tests the solenoid's function!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

Conceptually, isn't this the same as trip circuit supervision on larger circuit breaker trip coils?

EDMS Australia

 

[davidbeach]

Freddy - Yes it is.

But that doesn't sound fail safe to me. If I've read the thread correctly, the fail safe approach would be that the solenoid holds the brakes on and springs release the brakes if the solenoid fails or the power fails. All that fancy discussion about monitoring the solenoid matters not a darned bit if there's no power. Sounds more like a fail secure design than a fail safe design.

 

[aussiejohn2]

David,
I suppose anything is possible but the ones I have seen the brake is applied by the springs and the solenoid releases it so it is fail-safe against the likely failure modes, and "TCS" would be effective for finding open circuit wiring or solenoid.
J.

 

[CGSmith]

Yes, the brake is fail safe. The springs set the brake and the solenoid releases it. So, you need power to release the brake. However, the issue comes when you "must" release the brake in order to open the door/hatch. If the solenoid or wire to the solenoid is burned out, the brake won't release. The system is "safe" but not functional which causes a whole host of other issues.

 

[CGSmith]

VEIBLL, thanks for the great reply! I'm looking for a PLC output card that can do this. I assume you are saying to program a PLC output card to pulse constantly and measure current. The PLC output card would also need to be able to actuate the solenoid as well?

 

[CGSmith]

Itsmoked, I've been reviewing the Productivity 2000 modules and haven't been able to pinpoint the exact module yet but still reviewing. I did complete my review of this DIN rail mounted device made by Pretech. This looks similar to the solution that VEIBILL was referring to as well. The website makes it sound like a turnkey solution...no programming. I'm not a software guy so this sounds good to me so far but we'll see.

https://pretechsolutionsinc.com/solenoid-coil-monitor

I've emailed them to see what they cost and find out more.

 

[BrianE22]

You might want to "dither" the solenoid every once in awhile. Dithering is done by commanding a high frequency low amplitude current to the coil. Just enough current and movement to break away from any impending lockup due to increased friction.

 

[VEBill]

If you can find off the shelf PLC hardware that offers the required hardware capabilities, that'd be great.

I was silently assuming that somebody would design and build the required circuitry from scratch, because the required 'health monitoring' software might still dominate the total project cost. The hardware is "just" the power supply, high side drivers, power MOSFETs with protection, current sensor, and any other sensor inputs. The software will require the other 90%+ of the budget.

 

[electricpete]

On the pulse, it deserves to be discussed further what you will monitor. Estimate of resistance? Change in waveform which might indicate a single shorted turn or ground path?

Either way all, you get is some indication of electrical condition of the coil. There are of course a lot of mechanical failure modes. If you have processing power built into the monitoring system, it might be helpful to combine the pulse checks with trending of pulllup times (which can probably be estimated from current: for example ac coil changes reluctance on pull-in) and dropout times when the solenoid does operate so you might have a chance to see mechanical problems coming. (of course you’re not going to see problems that developed suddenly since last operation).

I’m not sure I understand what does dithering actually accomplish.


=====================================
(2B)+(2B)' ?

 

[IRstuff]

Dithering verifies mechanical freedom, to the degree the solenoid can be moved; still doesn't ensure that solenoid has full travel capability.

It seems to me that much depends on the OP's reliability requirement and consequences of failure; high levels for both would seem to indicate needing full hardware redundancy and/or redundant coils. One can easily imagine having two coils on the same actuator, but even then, two solenoids will lessen the probability of a stuck plunger.

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[VEBill]

One thing I've found amusing is the following:

More prominently some decades ago, there was a philosophy that reliability is enhanced by leaving out parts. I've seen examples where the design was stripped down to the bare minimum, with the goal of enhancing overall reliability. The late Bob Pease did a column on this exact topic, but I can't find it on-line.

The more modern 'Health Monitoring' approach is the complete opposite, where the complexity and number of parts skyrockets. But the goal is to provide enhanced overall reliability.

Both are perfectly correct, but the latter is more powerful.

 

[IRstuff]

At the end of the day, what really counts is what does your spec say, and what does your analysis say? After all the analysis, if this is a one-off, then the analysis is moot anything, since the analysis refers to some distribution of statistical things, and not the performance of a single systems

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[itsmoked]

Here's the 8 output module I happen to be using. They have others. They're a little hard to find because the up-front description only notes "Overload and Short Circuit" monitoring but also included is the open circuit monitoring.

It works as advertised. The Productivity line has output displays on all modules that typically show the status, for instance if an output is ON/OFF. On these 'protected' modules they include a button, "SEL"ect, when pushed the then displayed numbers show which are having a fault - overload/short-circuit OR open-circuit.

Of course, the module has set I/O points available to the ladder logic. These modules have the normal output addresses to turn on the outputs and an additional set of inputs showing alarm status. In your code you can monitor them to see if any outputs have faulted.

Self Monitoring Output Module

Keith Cress
kcress -

http://www.flaminsystems.com