which is more efficient at this: motor or solinoid? - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: May 09, 2012, 07:11:59 pm
content: I should probably know this, but I think it would depend on a lot of subtle factors.I need a waterproof actuator that will cycle at about .5 to 4 hz (guesstimate). It should be capable of about 10 pounds of pull. I don't need to control position (full swing is fine) but I would need to control the cycle rate. The stroke would need to be about 1 to 2 inches.Obviously I can go with a electric gearmotor and cam (like an airplane servo arrangement), but I am thinking about maximum battery efficiency and also longevity. One would think the solinoid might win in that case, since it would be very easy to make waterproof...or wouldn't even matter if it was not as long as the wires were potted.What do you think?

author: bilko
author_rank: Frequent Contributor
timestamp: May 09, 2012, 08:22:51 pm
content: IMHO, pneumatic cylinder with mosfet operated solenoid would be your best option

author: Jimmy
author_rank: Regular Contributor
timestamp: May 09, 2012, 08:56:08 pm
content: I would use a motor because it will be easier to control the speed over the long term.

author: Unknown
author_rank: Unknown
timestamp: May 09, 2012, 09:02:42 pm
content: Solenoid is by far the simplest and the easiest to isolate. However it consumes power to hold and has (I presume) higher inrush current.A suitably geared motor could hold in position without consuming power an has lower inrush current. On the other hand it has more moving parts therefore more prone to fail, also difficult to isolate.

author: bilko
author_rank: Frequent Contributor
timestamp: May 09, 2012, 09:41:22 pm
content: Quote from: HAL-42b on May 09, 2012, 09:02:42 pmThe OP said "I need a waterproof actuator that will cycle at about .5 to 4 hz (guesstimate)"He did not say that he needed to lock the position. Even if he did the pneumatic option uses zero power in lock position using detent solenoids. It is also infinitely more waterproof and more robust than either the motor or electrical solenoid as it is under positive air/fluid pressure. That it is why it is used extensively in industrial applications that have to see service life in the millions of operations.

author: Kremmen
author_rank: Super Contributor
timestamp: May 10, 2012, 06:56:09 am
content: Sure, but the OP obviously thought between motor and solenoid. Pneumatics bring in the requirement for compressed air and that vastly complicates the solution. Now that may or may not be a problem we don't know. Stating that it is "the best option" however is a bit premature, don't you think? First one needs the criteria for "best". Waterproof is 1 and there a pneumatic cylinder easily beats electricals, functionally it is at least the equivalent of either of the others. Cost/complexity might be the killer however.

author: amyk
author_rank: Super Contributor
timestamp: May 10, 2012, 10:36:29 am
content: It's hard to say exactly without knowing the application, although I can think of one or two things which fit that description (reciprocating, waterproof, 0.5-4Hz, 10lbs, 2" stroke, and battery powered), one of which may be a bit harder to mention

author: Kremmen
author_rank: Super Contributor
timestamp: May 10, 2012, 11:15:19 am
content: Now who has a dirty imagination?The requirements and specs don't really match very well. For starters solenoids pull rather than ahem.. thrust. And that's what the OP asked - pull.But who knows, many things are possible...

author: KTP
author_rank: Frequent Contributor
timestamp: May 10, 2012, 12:16:16 pm
content: Ok, heh heh, thanks guys, but the application is strictly Disney audience rated. I want to keep it a bit secret for now as I am not sure it has been done this way before...not that I would patent it probably, but I want it to be a suprise at leastThe travel needed is actually more like a pendulum, but I was going to use a linear type of device connected to a linkage arm to obtain that. I did have an idea last night that I could construct a curved section of steel, maybe stick some rare earth magnets to it, then have a curved section of several coils to create a sort of brushless curvilinear motor. This would directly create the motion needed with the fewest moving parts and the only wear item being the bearing for the moving section. At these slow cycle rates, the bearing could probably be just teflon or bronze?Do you think I could hand make a few prototypes of this type of curved linear motor that would have that type of torque? I know they use linear motors in some cnc equipment to move pretty massive tables...The motor is the simplest idea...but has gear box and linkage issues and would need extensive waterproofing. The linear motor presented could just be coated with something protective and left to the elements.In addition, what I was wanting was efficiency. Say it takes 12V at 5 amps average current to cycle this actuator at 4hz with a DC brush motor/gearbox/linkage. I want the solinoid or brushless linear motor to be as efficient or better. I am guessing that homemade, it probably doesn't stand a chance vs the commercial DC brush motor though...

author: Kremmen
author_rank: Super Contributor
timestamp: May 10, 2012, 12:37:53 pm
content: One thing that comes to mind is a motor driving the kind of reciprocating lead screw you have e.g. in fishing reel line guides. That would keep the mechanical solution relatively simple, you might not even need a gearbox if the pitch was right. Unfortunately i have no clue as to sources of such screws.

author: Unknown
author_rank: Unknown
timestamp: May 10, 2012, 01:08:39 pm
content: Quote from: KTP on May 10, 2012, 12:16:16 pmIt sounds like you are describing a hard disk arm movement. If built chunky enough such a mechanism could fit all your needs except perhaps the current draw. I can't think of a viable way to decrease current draw without lowering your forces and acceleration requirements. It is unlikely that a solenoid / linear motor would be more efficient than a geared motor. As an alternative you could consider using timing belts, like the tiny ones inside printers, which will give you the chance to move the motor away from the wet area.

author: KTP
author_rank: Frequent Contributor
timestamp: May 10, 2012, 01:26:34 pm
content: Quote from: HAL-42b on May 10, 2012, 01:08:39 pmHey thanks, yes! it does remind one of a hard disk arm movement. Funny how one misses those obvious electro-mechanical real world examples when brainstorming. Now where did I put those VAX780 hard drives...But seriously...why would a properly constructed linear motor be less efficient than a DC brush motor and gearbox? The gearbox itself would probably only be 90% efficient, and coupled to the 50 to 60% efficiency of the motor (just tossing out guesses here), I don't quite understand how that system would require a smaller energy input. If you smeared out a regular brushless DC motor it would look almost exactly like the linear motor setup.Is it that the gearing of a higher speed regular motor and a cam system allows the motor to operate at a constant speed to generate the pendulum motion, thus reducing the I^2R losses compared to a soliniod or linear motor setup where the peak currents would be very high at each end of the swing? Is it that dang squared term that is going to bite me in the caboose?

author: Unknown
author_rank: Unknown
timestamp: May 10, 2012, 01:57:23 pm
content: It's just that electric motors are more efficient at higher rpm. In your case you have a tiny motor with gearing operating at perhaps 8000 rpm, or a big motor operating at 4 rpm (linear or rotary, no significant difference). Which is going to draw more current? A tiny motor at its most efficient range or a chunky motor in its most inefficient range?

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 10, 2012, 03:05:07 pm
content: Quote10 pounds (4.5kg) at 4rpmHz is not tiny.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 10, 2012, 03:19:52 pm
content: How long does it have to last? 2" travel isn't a tiny solenoid, and at 4 Hz or even 0.5 Hz, your cycle count will ramp up pretty fast. I think a rotary motor with an appropriate gearbox and linkage would be more reliable than a solenoid in this case, but I am by no means an expert.Pneumatics would be a strong contender if you already have compressed air, but probably not worth adding that for a single piston.

author: Unknown
author_rank: Unknown
timestamp: May 10, 2012, 03:26:23 pm
content: Quote from: Mechatrommer on May 10, 2012, 03:05:07 pmOops, it should have been 4 revolutions persecondie. 4Hz or 240 rpm. To drive this from a motor at 8000 rpm we need a 1/33.3 reductor. So compared to direct drive, this motor only needs to have 1/33'th of the torque. Add gearing losses and other fudge factors and you get something like 1/20'th of the torque compared to direct drive. This is what I meant bytiny.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 10, 2012, 06:08:03 pm
content: things usually got mixed up (corrected)2" stroke reciprocating engine = 1" radius circle... calculating... torque for 4.5kg = 1Nm / 33 = 30mNm.. 100% efficiency... go figure.if you can lift 5kg 4 times a sec at 2" stroke for sometime, i believe you can win Olympia Bodybuilding contest alreadyand i believe 8Krpm is special type "tiny" motor.

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2012, 06:16:26 pm
content: I just replaced a pair of pneumatic cylinders which were used for clamping. 10mm travel, and around 200N of clamping force. They lasted around 2.5 million cycles, pretty good for cheap cylinders ( cheap in a relative term, they cost $100 each, but were a lot cheaper than buying a unit from FESTO at closer to $500 with only double the life) but they have very good actuators, Numatics controls that were made in the 1980's and which are likely to do 100 million cycles. They probably are at the 25 million cycle mark, only ones I never have changed or rebuilt.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 10, 2012, 08:13:38 pm
content: Quote from: Mechatrommer on May 10, 2012, 06:08:03 pm9 watts, or ~1/100 hp. Not a super small motor, but that is certainly not huge either.QuoteYes, I agree it is a bad match for a human without some gearing (although 10 watt continuous power output is not so bad). How that is relevant to the problem at hand escapes me.QuoteNot really. Many small DC motors run at that speed. This motor:http://www.sparkfun.com/products/9608costs $2 and runs at 23 krpm, although it is not high enough power for this. 8-12 krpm is quite common in small motors.

author: G7PSK
author_rank: Super Contributor
timestamp: May 10, 2012, 08:59:00 pm
content: What about a central heating valve actuator. They are a small AC motor with a gear train with enough force to turn the central heating valves on and of as the boiler fires up and whether the hot water is required for radiators or the hot water tank. if the speed is wrong you just need to alter the frequency of the current driving the motor.http://www.screwfix.com/p/honeywell-v4043h-2-port-motorised-valve-28mm/40705Or this one it runs on 2 to 10 volt DC and has a force of 710 N or 160LBf over 12 to 25 mm for longer stroke just use a lever.http://customer.honeywell.com/honeywell/ProductInfo.aspx/ML7984A4009

author: KTP
author_rank: Frequent Contributor
timestamp: May 10, 2012, 10:21:22 pm
content: Hey thanks. As for gearmotors, I have a bunch, including some overkill wheelchair type motors that are probably 1/2 HP DC.A friend suggested windshield wiper actuator assemblies from cars...could work I guess...not sure of the torque rating there.I am not concerned really with finding an off the shelf item as much as I am with experimenting with the most efficient and reliable system (and small and light and inexpensive....easy huh?)

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 10, 2012, 11:41:42 pm
content: Quotei will be glad to hear suggestion about 'tiny motor' that can do 45N at 4Hz at cost somewhere $10-20, or even up to $50. i can harness even more power (load) 180N at 1Hz from it. 9watt is too optimistic imho. i have here iirc 15-20W motor geared to 34rpm that can barely struggle a few kilos of load. it was an adjurned/postponed project due to insufficient budget to gather all the necessary motors. though back to OP, i'm not sure if 10 pounds is weight or force pound. i assumed the worst case.

author: westfw
author_rank: Super Contributor
timestamp: May 11, 2012, 04:35:06 am
content: I doubt that you can find a solenoid with 10lbF and a 2inch stroke.Magnetic fields get weak very quickly with distance (r^4?), and a motor will usually do much better than a solenoid since (among other things) the magnetic gap is fixed and small.

author: sonicj
author_rank: Frequent Contributor
timestamp: May 11, 2012, 05:58:40 am
content: Quote from: KTP on May 10, 2012, 10:21:22 pm+1 for wiper motor. massive torque. cheap & reliable. not sure how efficient at your required load....-sj

author: KTP
author_rank: Frequent Contributor
timestamp: May 11, 2012, 02:49:12 pm
content: I am concerned that the windshield wiper motors actually might not have the torque I need at the speed I need.They seem to be rated around 30 watts and about 40rpm, with peak efficiency somewhat lower (around 20 watts).I am now considering a somewhat radical solution that might be pretty efficient and compact. A brushless motor coupled to a rolled ballscrew and nut. I think (based on some cnc equipment I have built) that cycling this at 1 or 2 hz would be doable. I have built 50 pound tables that move at 2000IPM using these rolled ballscrews...that is 33 inches per second. Of course you have acceleration issues, but perhaps cycling 2 inches twice a second is still pretty easy. There would be no gearbox needed...the motor would couple directly to the ballscrew and the ballnut would attach to the device lever arm.Cycle life: I would hope the ballscrew/nut could last around 2 million inches of travel (this is probably published somewhere). At 7200 inches per hour, this would be 270 hours. My application is about 5 hours per week during the boating season, maybe 20 weeks a year. So nearly a 3 year life (made a lot of assumptions here). Probably acceptable.Gearbox might still be better. I could built the ballscrew system with parts on hand though...

author: sonicj
author_rank: Frequent Contributor
timestamp: May 14, 2012, 01:48:46 am
content: Quote from: KTP on May 11, 2012, 02:49:12 pmcheck this:i could be wrong, but i don't think it will be trivial to homebrew a ballscrew drive that can match that sort of speed & torque + weather resistant....-sj

author: steve_w
author_rank: Regular Contributor
timestamp: May 15, 2012, 09:39:41 am
content: That's cool, a lot of work went in there.regardsSW