need to create 0 to 30 pounds linear force

in addition it needs to be able to operate at peak force indefinitely.

maybe something like a speaker coil would be good..

You can buy "electromagnetic transducers" that work on the voice coil principle and will do what you want. (But they are not cheap!)

Over what kind of distance does force generator have to generate this force?

Speaker coil design trades off force against travel - the force produced by the interaction of the cylindrical speaker coil and the permanent magnet does vary as the coil moves with respect to the permanent magnet, but you can trade off force against throw.

There is a solution to to this problem in the linear stepping motor - which has two or three sets of coils which translate across an array of permanent magnets. Two coil motors are usually designed so that as you move the coils along the magnet array, the voltages induced in the coils vary as the sine and cosine of the displacement; drive the right currents into the coils at any given position and you can generate whatever force you want (up to the limits set by coil dissipation and demagnetisation).

The Sawyer motor is the best known example

If you don't need much travel the solenoid/speaker coil approach can be fine. Force will vary as the coil moves with trespect to the permanent magnet, and as the permanent magnet heats up, but you can monitor temperature and position and correct for both.

-- Bill Sloman, Nijmegen

But you haven't bothered to tell us what the travel (if any) required is. I"ll assume you just need a force and the actuator doesn't move at all!

Yes you are. Solenoids don't generate a nice linear force with respect to POSITION. they are fairly linear with respect to drive current. OF course 30 pounds is a LOT of force and that implies an iron core solenoid and that means that the iron will hae hysteresis that will reduce linearity even of drive current.

Fast could also be a problem speakers (moving coil) are fast because the moving parts are light. High forces are a problem. Such things are made (to drive shake tables and the like) but $$$$. Iron core solenoids get lots of force but the linearity problems and the mass of the iron cores can reduce response speed.

YOU are thinking of solenoids that PULL things. IN other words they are not only providing force but motion as well. It is the force/ position curve that is way nonlinear in a solenoid. That is why they are rarely used when linear positioning is needed. For example, large hard disk head positioners used to be of the speaker type (only MUCH larger).

You have not mentioned how far it has to travel. If it is just a few milli-meteres its one thing, if it is a foot, it's quite another. Furthermore, the mass you need to push will be important in the response time.

You are correct about solenoids, they are non-linear devices who's force is a strong function of the distance pulled in as well as current in the coil. However, they can provide the high forces in the range you require and are cheap.

One possibility is to use a solenoid, either a pusher or a puller depending on the direction you want with feedback from a force sensor to linearize and control the force. You did mention closed loop. A simple strain gauge force transducer or load cell provides the the force sensing feedback. This is applied to an ampifier to deliver current to the solenoid. A Pulse Width Modulation scheme, PWM, may be in order to keep power dissipation in check.

As mentioned in other posts, a voice coil actuator would probably be the best, though expensive choice. Force can be quite linear over a moderate throw distance and the direction depends on the polarity of the drive current. Lower mass makes it fast compared to a solenoid. Feedback can also be applied here to improve performance.

A piezo electric (ceramic) transducer would probably be the cheapest way to do it but the throw distance is very short. But, it can be very fast. Ultrasonic tranducers work this way.

Yet another way is the linear induction motor. Basically it is like a transformer with a shorted single turn secondary free to move. AC current in the primary causes the shorted secondary to repel away. Forces can be vary high. Depending on design, mass can be relatively low, but accurate force would have to be controlled with feedback. Throw distance can be as long as you want. Maglev trains work on this principle.

A small DC or other motor working on a ball screw, jack screw or rack and pinion may be the simplest and and relatively cheap method of getting the motion you want. With proper design, the speed reqirement can probably be met. Again throw distance must be considered in determining speed, motor size and gear ratios.

snipped-for-privacy@e23g2000prf.googlegroups.com...

Yea verily -- almost.

Not all linear motors are linear induction motors -- there are linear brushless motors, and I wouldn't be surprised at a linear brushed motor.

The OP wants a force, and a 100ms response time. He may be able to do this with a very free-moving rack and pinion (it depends on just how much parasitic force he can stand), but I'd approach this option carefully.

Other respondents have mentioned that voice coil actuators are $$$ -- they are, but if you wind your own coil and place your own magnets the purchase price is low. The _labor_ is high; that plus the fact that voice coil actuators are generally low-volume items* is what drives up the price, but rolling your own is a quite viable option.

• Except for disk drives, but those are in-house items.

Try Google on "linear actuators". Lots of options. Dan Thomas

stroke/displacement length?

OKAY:

sorry i left out the throw

i imagine the throw will be less than 5mm..

let me explain the application:

i want to make a programmable adjustable fuel pressure regulator for a car

you take a normal fuel pressure regulator, which is basically a ball and spring valve where the spring sets the pressure

if you were able to push on the spring, then you can further increase the regulated pressure

SO, my regulator will have a spring to set the MINIMUM pressure, and some kind of electrically controlled "force" which will push on the spring to increase the pressure to whatever it needs to be

the "closed loop" will measure the RESULTANT fuel pressure, NOT the force of the actuator

my application would require a minimum pressure of about 30 lbs, with the ability to go up to 60 lbs, so the spring will give me the minimum, and this electrically controlled "force" device will supply the remainder, as needed

pneumatic systems arent an option because they add too much complexity to the system

i thought about using a normal rotary motor but it seemed like it would be too slow...i could be wrong though. a motor connected to a threaded rod in a tube might work, but i dont know how much torque the motor would need and it might need to be geared down, and it might be slow....but it MIGHT WORK..i may have to look into that

A stepper with a lead screw might work. But pneumatics are probably the cheapest and simplest.

If it were a really small throw then a piezo stack motor might have been an interesting choice. But I suspect that 5mm would be pushing it (sorry about the pun) for such devices.

Why do you need the 100ms response speed? If not for that, a geared-down motor driving the spring position my just be the bee's knees.

An even better way may be a mondo-big fuel injector valve feeding into an accumulator -- that way you'll be using known-good technology instead of inventing from fresh.

You may want to do a web search on industrial remote-controlled pressure regulators: I'm sure they're out there to inspect & dissect.

So for openers, you are talking about pressure, not force. The actual force is probably far less than a pound depending on the orifice area where the ball seats, right? No way do you need 30lb of force as originally claimed.

Simple solution: Place a screw behind the spring to adjust the force on the spring. Attach a gear to the screw. Drive the screw gear with a small DC motor with a pinion gear on its shaft. The pinion engages the screw gear. The purpose of the gear and pinion is to give a gear ratio to increase the motor torque on the screw keeping the motor small and the system simple.

When there is no current in the motor, the pressure will remain at it's setting and can remain forever consuming no power. Motor current in one direction will drive the screw toward higher pressure while current in the other direction will drive the screw to lower pressure.

Connect the motor in your pressure control loop with appropriate amplification and compensation using pressure as the control variable as you state.

Privide two limit switches on the screw travel to keep the screw from bottoming out and stalling the motor at each end of travel.

Don't make this a bigger deal than it needs to be.

replace the current spring in the regulator with one that'll give you

the reason i need 100ms response time is that this regulator is for a turbocharged car and the fuel pressure must increase immediately in response to boost or the mixture will lean out with catastrophic results.

controlling the fuel pump sounds interesting but i'm not sure if it could be controlled fast enough to respond to boost changes..

GOOD POINT:

whoever said that I dont really need 30 psi...you are probably right. i will do an experiment and see how much force it really takes on the spring to increase the regulated pressure...this may mean I could use a much smaller/cheaper/faster/easier solenoid!

How about using a rotary motor with an eccentric cam? You can make a quite good, low loss cam using a collar bearing with a center plug having an off-center hole bored in it to accept the motor shaft. Use Loctite cement (RC680) to attach center plug to motor shaft and to inside race of bearing, and have bearing outer race press against load point. Should be able to get the force you need with a modest sized motor. Force will not be linear with displacement, but, will be quite repeatable. And, since the motor only needs to rotate 90 degrees or so, response should be fast.

W Letendre

When I was a kid, we retired our old Bendix washer and my two brothers, my sister, and I got to take it apart in the vacant lot next door. It had some 115V solenoids about the size of your fist, with a T-shaped armature. When we energized it, it went "CLACK!" I presume it was for some water valve or motor clutch or something.

I have no idea if they make them that big any more, or in 12V, or anything.

If you use a 6V solenoid with a resistor in series that's equal to the solenoid's DC resistance, it pulls in a lot snappier, but won't burn up: see T = RL or whatever it is. ;-)

Or heck, just adapt an injector!

Have Fun! Rich