I have a project in which I would like to produce a "smooth" reciprocating motion with an adjustable stroke length between about 1 mm and 2 mm and an adjustable frequency between 100 Hz and 150 Hz. I think there might be some advantages to using a solenoid for this (as opposed to a rotary motor and a mechanical design) but only if I can get good feedback control of the stroke and frequency. In a spring- mass mass system frequency can be controlled open-loop, but I'm not sure how well I could regulate the end and start position of each stroke or even what sort of sensor might be appropriate and have a fast enough response and a clean enough signal to work at that frequency. Wondering if anyone has any good electronic (or mechatronic) ideas as to how this might be done.
I'm taking the liberty of cross-posting this to the controls newsgroup
-- it needs some valid traffic, and this post is spot-on for the group.
This sort of question always boils down to questions like how much money you have to spend, whether you're using custom electronics, what sort of production volumes you're contemplating, how precise it needs to be, how long it can take to achieve precision, what sort of environment it needs to work in, how heavy it can be, etc.
_The_ indicated position sensor for this would be a short-stroke LVDT. It may be hard to get the accuracy you want from it at 100Hz without custom electronics (with custom electronics it's a snap). But LVDTs can be a bit spendy, they tend to be bulky, and most LVDT circuits have a serious bandwidth vs. excitation bleedthrough tradeoff.
Potentiometer feedback is cheap, but it's cheap in both senses of the word -- it won't cost much, but you'll have trouble getting noise-free operation at those speeds.
For that size of a stroke you may be able to get away with an optical sensor -- measure the amount that a transmitter/receiver pair is occluded as the thing travels. This has engineering issues, and therefore high initial cost, but it may be just the ticket for high-volume production. LEDs get funny at temperature extremes, so if you want this to work from Alaska to Algeria you can't just get it working on a lab bench and trust that it'll work everywhere, any time.
There are some linear quadrature encoders that may work -- 2mm is a pretty short stroke, but those things are getting better and better every day. I'd do an A/B comparison between that and an LVDT were I to launch a product doing this. An optical linear encoder suffers from the same environmental issues as the differential optical gizmo I suggested above.
Depending on the solenoid and your preferred engineering expense vs. per piece price, you may be able to do this by using the solenoid itself as a position sensor, as its inductance will change with position. For a one-off this is seriously unwise mad science; for production it makes sense as long as the level of accuracy you needs rivals the production volume: modest accuracy would work with modest production volumes, really high accuracy would require lots of engineering which would only get paid off with high production volumes, etc.
As for control:
Frequency is easy -- just drive the thing at the frequency you want.
Stroke is not too much harder: to a first order, if your solenoid and driver have a high enough bandwidth you can directly servo your position. If you can stand a few cycles of settling you can inject some drive signal through a stage that you adjust with feedback to get the amplitude and phase that you need.
Do you need to regulate the center position as well?
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
What does "smooth" mean? A solenoid is one way but they are non-linear and may be difficult to control the stroke length because of inertia etc. How much force do you need and what is the precision of motion and position necessary? The best way for precision motion, controlled velocity and position is a voice coil actuator. These can be designed with any force from few dynes up to tonnes if necessary. The feedback can be acceleration, velocity or position with various sensors, accelerometers, coils, linear pots or line encoders. There are many options depending on what you are trying to do.
Voice coils have the advantage over moving iron devices like solenoids of having low mass and a linear current to force function which makes them much easier to control. They have much wider band width as well. More information would be helpful.
I once had a similar project, and no budget, so I carefully removed the cone from a small bass loudspeaker, preserving the suspension and the electrical connections, and epoxied a take-off point to the circumference of the remaining dome over the voice-coil. Driving it open-loop with a D/A and a power op-amp, it worked quite nicely for the application, which was to simulate the pecking action of a bird. Another advantage of a voice-coil over a solenoid is that it can both push and pull.
My usual answer is to first look at car cams and find a good reason why a simple mechanical solution can't work. If the amplitude doesn't need to change on-the-fly you can put what ever needs to be moved between two cams that push and pull the object back and forth. If there isn't much mass only one cam is required and a spring can push the object back against the cam.
We have done custom electric projects like this. We were trying to make a fast cutting device using a solenoid. The cutting had to be precise so position was important. The frequency was only about 60 Hz for the whole cycle but cutter had to accelerate from a stop, accelerate to almost twice the belt speed then slow down to match the belt speed and then accelerate quickly then stop at the top. This was done in 16 ms. The amplitude of this motion was much bigger than 2mm.
I would use a glass scale encoder. The resolutions are now sub micron. I would not use LVDTs because of the phase delays involved. It is relatively easy to do this open loop using a signal generator and then monitoring the peaks to make adjustments to the open loop signal.
Make sure that the speaker center is ok sans cone. I did this several times and had one series where the coil started chafing after a while, obviously the centering wasn't quite stable sans cone and sagged.
The good thing is now you can switch to an MP3 file when the boss ain't lookin'. Tchk .. tchk .. *BOOM ... tchk .. tchk .. *BOOM* :-)
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Regards, Joerg
http://www.analogconsultants.com/
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Now that I think of it, I removed the cone so I could also remove most of the frame, achieving a more compact unit. If you have the room it would be better just to leave the cone (and frame) intact.
There is (was?) another option but I am not an audio guy and don't know if these are still available: In the 70's you could buy coils without cones. They had a pad attached that you'd screw or glue onto a surface that you wanted to "insonicate". This way you could turn a divider wall or other structure into a speaker. The then floating body of the thing had enough mass to give it a decent sound down to rather low frequencies.
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Regards, Joerg
http://www.analogconsultants.com/
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One could use an "open" solenoid so that the metal piston can move in either direction without bumping into an end plate. That piston ideally would not be magnetically biased so that the drive would then not need to have a DC offset if uniform mechanical movement WRT to solenoid is desired. Once working, this is "nice". Without springs and if centered, this is not going to work too well, so two solutions: a spring on one end to give mechanical offset (which then _would_ require DC offset on drive) or magnetically biased piston which would require DC offset. For open loop, adjust drive for desired motion. To close loop, one would need positional sensors (have fun - magnetic sensors would not exactly be the best).
No. Early speakers used a very crude suspension that looked like a spider web. The speakers were so crude that there were adjustments to center the voice coil.
Later speakers were made of stiff fabric that were glued to the frame & the cone so the alignment didn't change, as long as the cone wasn't damaged.
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More information would be really helpful. And I second the thought that a voice coil would be better than a solenoid, unless you have strong reasons to want one.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Thanks for the responses. Lots to think about. I have a controls background, but most of my work has been on the algorithmic/academic end of the spectrum so I don't really know much about technology unfortunately.
I guess I can share a few more specifics about what I'd like to accomplish. It's a hand tool for an artist friend, but it has a potentially niche-sized market if it's an decent improvement over what he's using now, so I'm thinking in terms of production.
Minimum requirements:
1) Pretty good frequency control. Any oscillation not noticeable to the user.
2) Very precise control of one end point of the stroke and reasonable control on the other end.
3) Little if any noticeable off-axis vibration.
Really nice things to have that would be probably be necessary for commercial success: A) Low enough power drain to operate off a small battery (3 cm^3 max???) for a decent about of time. Sorry, I don't have more specifics on that yet. B) Production costs for accuator/sensor/electronics of $200 at the very most. Probably needs to be less than that unless this turns out to be so great that people are willing to pay substantially more than the current technology costs.
Other things that would be cool but aren't absolutely necessary: a) Shaping of end effector trajectory. Lower velocity on out stroke then on return stroke. b) Quick adjustment of stroke length.
Force requirements: Pretty small I think. The reciprocating tool is very light, on the order of a couple paper clips maybe and it doesn't have to apply much force to the material you're working on. Force requirements are probably dominated by the friction needed to hold the tool in place.
That's all I can think of right now.
(2) and (a) make me want to use a cam. The potential problems I see with this are that cutting an arbitrary cam profile on something small is not that easy, so (a) may not be that achievable. Expense is also an issue. Small motors are pricey and even a 16 line encoder that small seems to cost as much as the motor. Easily over $100 each total right there which is pushing the limits of what this thing can cost. Accuracy and settling time of speed control is not going to be as good as I would have hoped for, but it's probably acceptable. Power drain? I don't know if a solenoid would be any better, but that voice coil has me intrigued. (1), (3), (a), (b), and maybe (B) and (A) are what got me thinking about a solenoid, provided that (2) could be achieved via some combination of physical mechanism and feedback control.
FYI: engineers don't use democratic words like "big", "little", "better", "worse", "decent", "reasonable", "pretty". Engineers use numbers. Before you have the numbers, there is nothing to discuss.
I respectfully disagree. In the early stages of a project we most certainly do -- and it's never long before we realize that we need to define them in the context of that project, before all go astray.
In the end those smoky words are the 'real' specifications that you are working towards -- you can hit all the numbers on the nose, but if those numbers don't look decent reasonable and pretty to the customer, if the value of the product you engineer isn't better, then your product will make little money, whoever hired you will thing they could have done worse than to never start things, and everyone will have big problems.
But yea -- you don't want them hanging around for very long at all before you pin them down.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Brandon wrote: > I have a project in which I would like to produce a "smooth" > reciprocating motion with an adjustable stroke length between about 1 > mm and 2 mm and an adjustable frequency between 100 Hz and 150 Hz. I > think there might be some advantages to using a solenoid for this (as > opposed to a rotary motor and a mechanical design) but only if I can > get good feedback control of the stroke and frequency. In a spring- > mass mass system frequency can be controlled open-loop, but I'm not > sure how well I could regulate the end and start position of each > stroke or even what sort of sensor might be appropriate and have a > fast enough response and a clean enough signal to work at that > frequency. Wondering if anyone has any good electronic (or > mechatronic) ideas as to how this might be done.
Swiss precision motors are pricey; if you can make a Mabuchi or a Johnson motor work reliably they'll come pretty cheap. I think that such a motor, properly used, would last for a good long time. For a motor/cam assembly all you need is a once-around sensor on the motor -- there are a number of cost-effective ways to do this; the two that leap to mind are a magnet and a hall-effect sensor, or a slotted disk and an LED/phototransistor pair.
A speaker coil is probably more efficient by far than a solenoid, and will probably be lighter as well. Better yet, it's something that you can fabricate yourself with a bit of wire and some easily obtained magnets. A motor/cam is going to be a power hog if you really just need to overcome inertia -- but a speaker coil's power requirements go up significantly with load, so if you have to actually _push_ on anything or maintain rigidity, that motor/cam may start looking good.
Of course, if you want to change the motion profile easily, the speaker coil beats the pants off of the motor/cam.
If this is going to be a hand-held gizmo, then an LED/phototransistor position sensing arrangement will have no temperature range troubles -- and such a position sensor is something you can probably womp up fairly cheaply.
You'd have to play with this to find out about power drain, but 25cc is enough volume for a lithium-polymer battery to hold a significant amount of energy.
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Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
Good engineering is being able to guesstimate ballpark numbers then verify once requirements are known. Often the customer has no real idea what's involved, but they usually know what they want? Unless you happen to work in a field where the customers also understand the technology they ask for.
Isn't the design process just that? Pin down the numbers and make some thing real from some loose specification. Otherwise the design task may as well be handled by a technician, as no engineering involved.
But then, lotsa people like to call themselves an engineer these days.
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