Solenoid operating frequency limits - Page 1
Forum section: electronics / beginners

author: Muffins
author_rank: Regular Contributor
timestamp: March 17, 2020, 08:26:00 am
content: Hi guys.I'm trying to interpret the datasheets for DC rotary solenoids. I'm thinking of using one to actuate a mechanism on a machine that will need to cycle around 17 times per second. The duty cycle of the solenoid will probably be around 20% or less but I am having trouble figuring out if it is possible to drive them at the required frequency.I'm currently looking through this datasheet:https://www.magnetschultz.co.uk/wp-content/uploads/GDA1.pdfAnd this guide:https://www.magnet-schultz.com/fileadmin/Daten/Vertrieb/PR1/TechnErl/GXX_e.pdfWhat are the limits to how fast they can go?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2020, 09:29:59 am
content: You'd have to integrate torque(theta) times moment of inertia, and figure out how long a full turn (or however much you're using) takes. A hand-waving approximation may suffice, but beware as the torque drops off rapidly with angle, at least that looks to be the case for most of them. Don't forget to add the mass (as inertia equivalent) and resistance of whatever linkages it's driving!They give a typical curve; I might suggest parameterizing a curve to fit it, then best-fit it to the data points for the part you're using. Then integrate that, which might use maybe a hundred sample points, no big deal. Can all be done in a spreadsheet.Even just doing trapezoidal integration, along the few points given, shouldn't be too bad. Saves the bother of cooking up a parameterized curve.May also want to know about the spring constant of the end stops, so as to get some idea about how it will bounce and resonate when it hits them. At least if just to say that it's well enough damped and at a high enough frequency (100s Hz?) that it doesn't matter to the present application?Tim

author: Muffins
author_rank: Regular Contributor
timestamp: March 17, 2020, 11:34:33 am
content: Wouldn't the inductance of the coil also introduce a limit to how fast the solenoid is effectively energized? Is that perhaps assumed to be much faster than the mechanical limitations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 17, 2020, 01:49:09 pm
content: Yes, forgot to add that. You can drive it like you drive a stepper, and for the same reasons. Typically a constant current source, switching to take advantage of coil inductance, having a high compliance voltage. This gets the electrical response arbitrarily fast, more or less. At rated voltage, the L/R time constant may be substantial, and can be a limiting factor; this is easily measured with signal relays for example, but harder to do with, say, contactors with so much more moving mass that even the slowest possible electrical response hardly matters.Tim

author: Renate
author_rank: Super Contributor
timestamp: March 18, 2020, 12:36:17 am
content: The inductance can be a killer.In ye olden days news teletypes used to work on 20 or 60 mA loops.You were running at 110 baud, maybe 9 mS per bit.On long lines or with distortion you used to run a ton of voltage.The more voltage, the less tempermental the "sample point" (I forgot the TTY nomenclature for this) was.Voltages up to 100 V were used.Compare this to engine injectors, another tight electromagnet application.They (usually?) only run on 12V, but the steady state voltage is much (?) less.You could use one circuit to drive this stuff or one high voltage and one low voltage "maintain" circuit.

author: Renate
author_rank: Super Contributor
timestamp: March 18, 2020, 12:38:39 am
content: Oh, and to make the solenoid release quickly you need lots of voltage too.You could use an active negative voltage.A diode right across the coil would give you theslowestrelease.You might use a diode/resistor or zener to make the decay as quick as possible.

author: langwadt
author_rank: Super Contributor
timestamp: March 18, 2020, 01:36:11 am
content: Quote from: Renate on March 18, 2020, 12:36:17 amengine injectors usually come in three flavours, high impedance ~12R that is just driven with 12V, low impedance ~4R opened with 12V then current limited with pwm and then the fancy high speed ones for direct injection ~1-2R open with ~60V then drop down to 12V and pwm current limiting

author: wizard69
author_rank: Super Contributor
timestamp: March 18, 2020, 03:29:29 am
content: First off I don't have a lot of experience with rotary solenoids only using a couple of Ledex neither of which could be considered a high performance application. Ledex, reference for engineers:https://www.johnsonelectric.com/en/resources-for-engineers/solenoids/principle-of-operation,doesn't appear to be written by engineers. The link may help you or maybe not.Quote from: Muffins on March 17, 2020, 08:26:00 amI'm going out on a limb here and will say no but keep in mind my only uses where low speed. This really sounds like a place where you need to talk to an applications engineer. Even the way you drive the mechanism can be an issue, you can use rotary cams or linkages. The reason I say this is important is because return on most of these solenoid installations is spring based. You simply have more issues than just the solenoid burning up, if the mechanics can't handle 17 times per second then the cycle rate of the coil doesn't matter.QuoteWho knows? Can the coil be heat sinked? Can the return of the solenoid happen fast enough (spring rate, gravity & etc)? What is the inertia of the moving part? Lots of questions to consider and frankly I'm not sure they are even considered in some design ins. Often it look more like lets buy the biggest they have that will fit or lets tweak the springs afterward type solutions. Yes I know that isn't engineering but some products have the appearance of that amount of thought being put into them.So if this is a critical piece of equipment that requires a good fit of actuator capability then you will likely need to work with a mechanical engineer or work out the mechanics yourself for the part being moved. Also consider that a rotary solenoid might not be the best solution.

author: wizard69
author_rank: Super Contributor
timestamp: March 18, 2020, 03:48:13 am
content: What is ideal is often tied close to what an electromechancial marvel is doing. The more out of the norm your drive solutions are, the more one should consider if you are on the best path. I don't have a lot of experience with the application of rotary solenoids but in every one that I have had experience there was always another way possible to achieve the same mechanical results. Maybe not as elegant but more than one way to skin that cat.Quote from: Renate on March 18, 2020, 12:38:39 amInteresting that you bring up diodes as it is common to use them to snub relay coils. This usually works fine even if the relay is a little slower. Rotary solenoids also suffer from the same pikes due to field collapse so you have to be careful about noise mitigation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 18, 2020, 05:03:40 am
content: And as far as alternate methods go, you may consider a rotary (or linear + linkage) motor. Magnetism shines when the velocity is high. Solenoids are kind of the worst possible application for magnetism(which is apparent when you think about just how low their power density is).You're still at pretty low frequencies, and I suppose the amplitude isn't much if you are considering a solenoid, so this may not be so viable. But at some point it does become attractive to consider. For example, a small DC servo motor plus light gear reduction. Bonus could be, if a worm drive gear train is used, it will have holding force without needing a return spring.And there may be smarter approaches, far more complicated to design but more realizable with off-the-shelf components.Is the repetition frequency actually constant? Could you get away with a CV shaft and a crank or cam? Perhaps use a (much smaller?) solenoid to move the cam follower between options; that could get you an electromechanical pulse code (PWM, PPM, whatever) with far more power output than a direct solenoid can offer.Can the waveform be decomposed into periodic and variable components? Perhaps linkages can be designed to superimpose those components, for a modest power savings over direct drive. (Analogous to generating power supply ripple directly, versus coupling it into an existing supply.)Is the frequency not constant, but varied over a small range? (PFM code for example.) Perhaps the motor can be modulated for less power consumption (only the drive train inertia needs to be accelerated to the difference in velocity, not fully stopped or reversed).Linkages, cams and other geometry provide all manner of nonlinear opportunities; given enough time you might synthesize a complete linkage where a CV motor and a couple variable position levers are able to produce the entire desired waveform! This may quickly spiral into mechanical computer territory where an electronic solution is far more preferable, but don't discount the ingenuity that just a handful of linkages can afford!Tim

author: Muffins
author_rank: Regular Contributor
timestamp: March 18, 2020, 06:39:32 am
content: Okay, thanks for the responses guys. I appreciate the enthusiasm. Seems like I'll need to run down a few feasibility calcs with one of the mech guys but simple enough.My application is that I have an arm that needs to rotate 20 degrees or so, moving a small object from its starting position and just kicking it out the way really. The movement doesn't need to be careful, no dwell requirements or anything, just a solid whack. I discovered the rotary solenoid yesterday and it seems like it could be just the right thing, otherwise I'll be going for a motor with a crank mechanism and converting it's full rotation to partial rotation on the whacking arm. Unfortunately pneumatics are not allowed.

author: Muffins
author_rank: Regular Contributor
timestamp: March 19, 2020, 06:50:48 am
content: Just an update. I found values for solenoid energizing times for rotary solenoids by Johnson Electric on this page:https://www.johnsonelectric.com/en/product-technology/motion/solenoids/rotary-dc-solenoids(requires user details for download). They are in the order of milliseconds.