Forums

Solenoids

Started by Don Y March 1, 2015
Hi,

What (and how!) can I deduce about the operation of a
particular (e.g., previously empirically characterized)
solenoid by observing it's electrical characteristics
UNDER OPERATION?

E.g., I can tell if the coil is *open*, obviously enough.
Likewise, I can tell if it has been shorted.

[Of course, these assume you can resolve the differences
with whatever detection means you employ]

But, can I determine if, for example, the actuator has
traveled it's normal length (without encountering some
mechanical interference)?

On a related note, how much control (in the hammer driver)
can I exert over the actual *motion* of the actuator?
E.g., "slow" the motion vs. speed it up?  What factors
are likely to complicate this, over time?

Thx!
On Sat, 28 Feb 2015 21:57:58 -0700, Don Y <this@is.not.me.com> wrote:

>Hi, > >What (and how!) can I deduce about the operation of a >particular (e.g., previously empirically characterized) >solenoid by observing it's electrical characteristics >UNDER OPERATION? > >E.g., I can tell if the coil is *open*, obviously enough. >Likewise, I can tell if it has been shorted. > >[Of course, these assume you can resolve the differences >with whatever detection means you employ] > >But, can I determine if, for example, the actuator has >traveled it's normal length (without encountering some >mechanical interference)?
Sure. Measure its inductance. -- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
On Sunday, March 1, 2015 at 4:57:57 AM UTC, Don Y wrote:

> Hi, > > What (and how!) can I deduce about the operation of a > particular (e.g., previously empirically characterized) > solenoid by observing it's electrical characteristics > UNDER OPERATION? > > E.g., I can tell if the coil is *open*, obviously enough. > Likewise, I can tell if it has been shorted. > > [Of course, these assume you can resolve the differences > with whatever detection means you employ] > > But, can I determine if, for example, the actuator has > traveled it's normal length (without encountering some > mechanical interference)? > > On a related note, how much control (in the hammer driver) > can I exert over the actual *motion* of the actuator? > E.g., "slow" the motion vs. speed it up? What factors > are likely to complicate this, over time? > > Thx!
Inductance increases as it closes, so with ac drive or a small ac component you can measure v,i and deduce L. You can measure its cold R & present R & work out how hot it is. If you drive it with high enough f you can deduce where it is in its travel from L. A suitable controller can then give you complete control over its position - within its mechanical and control loop limitations at least. You can speed it up by driving it with a whacking great spike or overvoltage - ratings are for continuous operation and there's masses of scope to exceed them briefly. By monitoring spikes you could also tell if the sometimes included diode is faulty, fwiw. You could monitor switch contact waveforms to detect deterioration & predict failure. I don't know whether you could also initiate a switch cleaning cycle with the load otherwise disconnected. And finally you could simultaneously use it as a radio receiver for a suitably low frequency and a moving iron speaker driver coil. NT
On Sat, 28 Feb 2015 21:57:58 -0700, Don Y <this@is.not.me.com> wrote:

> Hi, > > What (and how!) can I deduce about the operation of a > particular (e.g., previously empirically characterized) > solenoid by observing it's electrical characteristics > UNDER OPERATION? > > E.g., I can tell if the coil is *open*, obviously enough. > Likewise, I can tell if it has been shorted. > > [Of course, these assume you can resolve the differences > with whatever detection means you employ] > > But, can I determine if, for example, the actuator has > traveled it's normal length (without encountering some > mechanical interference)? > > On a related note, how much control (in the hammer driver) > can I exert over the actual *motion* of the actuator? > E.g., "slow" the motion vs. speed it up? What factors > are likely to complicate this, over time? > > Thx!
Haven't done this with solenoid, but with stepper motor control to get much of what you asked for. Don't know how far you want to take this, but at the terminals of the coil you have a 'spectrum of sensing vs frequency', ie. the effective reactance [inductance], the effective resistance, AND a bit of voltage generated by the motion. Think generator, or motor kicking back, that type of voltage generation. We used to 'quick step' AND 'slow step' a stepper motor by doing what you asked. Over time? don't know, but far more problems with variations caused by temperature. THAT can induce hysteresis and a LOT of non-linearity into your monitoring/control. From memory, doing this reduced power requirements and we could get the SM to run way faster than spec. ...without slipping a cog.
Flux, it's all about flux.

You need to know how much a normal stroke takes -- but of course, you need 
to calibrate for any of the things listed -- open/short, or if you wanted 
to know coil temperature (copper resistance, tempco), so I'm going to take 
that as a given.

A solenoid is reluctance driven, so its inductance goes up as it closes. 
If you drive with a constant current, you will observe a positive hump in 
the terminal voltage, with "area under the curve" (aka flux) corresponding 
to the actuation travel (not proportional, because all sorts of near field 
rules apply, but you could calibrate the flux vs. travel curve if you 
really wanted).

Mind to subtract I * DCR first, so your flux integrator isn't drifting 
constantly. :)

It'll be harder to do from a constant voltage supply, but current will dip 
by the corresponding amount; the result is, instead of V = I * DCR, I 
drops and EMF rises momentarily (or vice versa, since this all works in 
reverse, too).

I suppose the limits of detection, as far as indicating position goes, 
will be on magnetic hysteresis, since they probably use crappy mild steel 
in these things, not like, high grade nickel-iron...

Tim

-- 
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

"Don Y" <this@is.not.me.com> wrote in message 
news:mcu68a$909$1@speranza.aioe.org...
> Hi, > > What (and how!) can I deduce about the operation of a > particular (e.g., previously empirically characterized) > solenoid by observing it's electrical characteristics > UNDER OPERATION? > > E.g., I can tell if the coil is *open*, obviously enough. > Likewise, I can tell if it has been shorted. > > [Of course, these assume you can resolve the differences > with whatever detection means you employ] > > But, can I determine if, for example, the actuator has > traveled it's normal length (without encountering some > mechanical interference)? > > On a related note, how much control (in the hammer driver) > can I exert over the actual *motion* of the actuator? > E.g., "slow" the motion vs. speed it up? What factors > are likely to complicate this, over time? > > Thx!
Hi Tim,

On 3/1/2015 11:50 AM, Tim Williams wrote:
> Flux, it's all about flux. > > You need to know how much a normal stroke takes -- but of course, you need > to calibrate for any of the things listed -- open/short, or if you wanted > to know coil temperature (copper resistance, tempco), so I'm going to take > that as a given.
... and there's the rub! :< Open/short are easy to detect (for a particular "class" of coil). Beyond that, I think there is too much "finesse" required -- esp if you have environmental issues that factor into the observations.
> A solenoid is reluctance driven, so its inductance goes up as it closes. > If you drive with a constant current, you will observe a positive hump in > the terminal voltage, with "area under the curve" (aka flux) corresponding > to the actuation travel (not proportional, because all sorts of near field > rules apply, but you could calibrate the flux vs. travel curve if you > really wanted).
But what if it encounters an obstacle in the way? Something that prevents it from "continuing its travel" -- yet still results in power being consumed, etc.?
> Mind to subtract I * DCR first, so your flux integrator isn't drifting > constantly. :) > > It'll be harder to do from a constant voltage supply, but current will dip > by the corresponding amount; the result is, instead of V = I * DCR, I > drops and EMF rises momentarily (or vice versa, since this all works in > reverse, too). > > I suppose the limits of detection, as far as indicating position goes, > will be on magnetic hysteresis, since they probably use crappy mild steel > in these things, not like, high grade nickel-iron...
I can accept a result that tells me "yes, the actuator made its full stroke -- even if it was sluggish or impeded along the way" vs. "no, the actuator got hung up mid-stroke". And, handle open/short elsewhere. But, I suspect -- short of adding a limit switch or other physical detector -- getting any sort of reliable result will be tenuous?
On Sunday, March 1, 2015 at 7:44:23 PM UTC, Don Y wrote:
> Hi Tim, > > On 3/1/2015 11:50 AM, Tim Williams wrote: > > Flux, it's all about flux. > > > > You need to know how much a normal stroke takes -- but of course, you need > > to calibrate for any of the things listed -- open/short, or if you wanted > > to know coil temperature (copper resistance, tempco), so I'm going to take > > that as a given. > > ... and there's the rub! :< > > Open/short are easy to detect (for a particular "class" of coil). > Beyond that, I think there is too much "finesse" required -- esp > if you have environmental issues that factor into the observations. > > > A solenoid is reluctance driven, so its inductance goes up as it closes. > > If you drive with a constant current, you will observe a positive hump in > > the terminal voltage, with "area under the curve" (aka flux) corresponding > > to the actuation travel (not proportional, because all sorts of near field > > rules apply, but you could calibrate the flux vs. travel curve if you > > really wanted). > > But what if it encounters an obstacle in the way? Something that prevents > it from "continuing its travel" -- yet still results in power being > consumed, etc.? > > > Mind to subtract I * DCR first, so your flux integrator isn't drifting > > constantly. :) > > > > It'll be harder to do from a constant voltage supply, but current will dip > > by the corresponding amount; the result is, instead of V = I * DCR, I > > drops and EMF rises momentarily (or vice versa, since this all works in > > reverse, too). > > > > I suppose the limits of detection, as far as indicating position goes, > > will be on magnetic hysteresis, since they probably use crappy mild steel > > in these things, not like, high grade nickel-iron... > > I can accept a result that tells me "yes, the actuator made its full > stroke -- even if it was sluggish or impeded along the way" vs. > "no, the actuator got hung up mid-stroke". And, handle open/short > elsewhere. > > But, I suspect -- short of adding a limit switch or other physical > detector -- getting any sort of reliable result will be tenuous?
L changes with position. R changes too but is a separate property, measurable separately. I'm not clear why that would make a measured L value tenuous. NT
Don Y wrote:
> Hi, > > What (and how!) can I deduce about the operation of a > particular (e.g., previously empirically characterized) > solenoid by observing it's electrical characteristics > UNDER OPERATION? > > E.g., I can tell if the coil is *open*, obviously enough. > Likewise, I can tell if it has been shorted. > > [Of course, these assume you can resolve the differences > with whatever detection means you employ] > > But, can I determine if, for example, the actuator has > traveled it's normal length (without encountering some > mechanical interference)? > > On a related note, how much control (in the hammer driver) > can I exert over the actual *motion* of the actuator? > E.g., "slow" the motion vs. speed it up? What factors > are likely to complicate this, over time? > > Thx!
After some characterization, it is fairly easy to electrically determine actuator position (static mode).
On 2/28/2015 10:55 PM, meow2222@care2.com wrote:
> Inductance increases as it closes, so with ac drive or a small ac component > you can measure v,i and deduce L.
But, you're having to do that "quickly" -- i.e., the stroke takes a small fraction of a second to complete.
> You can measure its cold R & present R & > work out how hot it is. If you drive it with high enough f you can deduce > where it is in its travel from L. A suitable controller can then give you > complete control over its position - within its mechanical and control loop > limitations at least.
I suspect that would be hard to do reliably over large temperature ranges (e.g., 100F). I would be happy with a *static* appraisal of its "final position" (actuated and released). The goal here is to determine when it (or the mechanism with which it interacts) has failed (fully or partially). Doing so without "mechanically" instrumenting it.
> You can speed it up by driving it with a whacking > great spike or overvoltage - ratings are for continuous operation and > there's masses of scope to exceed them briefly. By monitoring spikes you > could also tell if the sometimes included diode is faulty, fwiw. You could > monitor switch contact waveforms to detect deterioration & predict failure.
No switch contacts. It's not a relay/contactor.
> I don't know whether you could also initiate a switch cleaning cycle with > the load otherwise disconnected. And finally you could simultaneously use it > as a radio receiver for a suitably low frequency and a moving iron speaker > driver coil.
On 3/1/2015 4:06 AM, RobertMacy wrote:
> On Sat, 28 Feb 2015 21:57:58 -0700, Don Y <this@is.not.me.com> wrote: > >> Hi, >> >> What (and how!) can I deduce about the operation of a >> particular (e.g., previously empirically characterized) >> solenoid by observing it's electrical characteristics >> UNDER OPERATION? >> >> E.g., I can tell if the coil is *open*, obviously enough. >> Likewise, I can tell if it has been shorted. >> >> [Of course, these assume you can resolve the differences >> with whatever detection means you employ] >> >> But, can I determine if, for example, the actuator has >> traveled it's normal length (without encountering some >> mechanical interference)? >> >> On a related note, how much control (in the hammer driver) >> can I exert over the actual *motion* of the actuator? >> E.g., "slow" the motion vs. speed it up? What factors >> are likely to complicate this, over time? >> >> Thx! > > Haven't done this with solenoid, but with stepper motor control to get much of > what you asked for. Don't know how far you want to take this, but at the > terminals of the coil you have a 'spectrum of sensing vs frequency', ie. the > effective reactance [inductance], the effective resistance, AND a bit of > voltage generated by the motion. Think generator, or motor kicking back, that > type of voltage generation.
Yes, one of the firms where I worked used back emf as a means of sensing when/whether the motor had physically taken its step. Allowed us to use steppers as DC servos without requiring an encoder.
> We used to 'quick step' AND 'slow step' a stepper motor by doing what you asked. > > Over time? don't know, but far more problems with variations caused by > temperature.
Exactly. That's a ~100F range of temperatures *before* factoring in any self-heating effects. I'd be leary of any conclusions beyond "failed open" or "failed shorted" in that environment. OTOH, a *relative* measurement could be useful. Note characteristics over time and watch for "changes". E.g., yesterday, it behaved thusly. Today, it is behaving...
> THAT can induce hysteresis and a LOT of non-linearity into your > monitoring/control. From memory, doing this reduced power requirements and we > could get the SM to run way faster than spec. ....without slipping a cog.