solenoids or servos ?

I've been curious for a while about this so I'm coming to you guys to get some experience-based answers!
The big question: which is more efficient - power consumption-wise - for pulling
the same amount of weight, friction, same amount of "throw", and a similar actuation speed - solenoids or servos ? I'm talking about 2 scenarios: that of momentary pull/release of some small weight (< 16oz) and that of continuously holding that amount of weight for about 30 seconds or so. I guess for purposes of this discussion, make the battery voltage the same for both.
Thanks for any input ! JCD
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pogo wrote:

Hold currents for solenoids are far lower than pull-in currents. It's common to use pull-in currents that temporarily overload the solenoids, then drop to a lower holding current. Pinball machines and modern piano players do this. You need two power supplies, or chopping with power MOSFETs.
There are special solenoid-driver ICs that make this easy to do. See "http://web.media.mit.edu/~dmerrill/mas863/assn3/solenoid_driver.pdf ". You just turn the solenoid on with a logic level signal, and the driver handles the high pull-in current, and the timed power reduction. Built-in overload protection, even.
                John Nagle
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When you say 2 power supplies are needed, you mean one for the initial "pull-in" and one for the holding ? I took a *quick* look at the data sheet you linked to and only saw one power supply. Having seen that it uses PWM, I wonder if PWM can be used to control how far in a solenoid is pulled in ?
But what about the comparison between servos vs. solenoids ? Which ones pulls less current throughout the overall range of motion - meaning which one would run down a battery first, all other parameters equal ? For example, lets say we're designing robotic gripper that uses some sort of spring to compensate for whether it is gripping something large or small, and just "hammers" it every time. Which would be better, in your opinion ? solenoid or servo ?
Thanks!
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Sorry, no experienced based advice from me. But, here is some thinking on my part... I observe that solenoids are rarely seen in robotics. So, I think that is your answer. The question is, why?
There are obviously some advantages to servos. By the simpicity of their design, they are inherantly more reliable, and cheaper.
While most solenoids are on/off type devices, there are proportional types as well. But, most are the "hammering" type you mention.
Some solenoids are latching... a magnetic force holds it in position without power until a reverse current is applied.
I just thought of a large scale installation of "robots" that uses solenoids. I believe the Tiki Hut in Disney uses solenoids for the birds mouths. This quick motion of a light-weight object is one good application for solenoids.
And another application comes to mind... actuators for ultra-light indoor R/C Aircraft. They should be considered rotary solenoids. These are often home made coils with a magned mounted to pivot in the middle. Not just, on-off, but proportional control by controling the current provided to the coil.
In the above two examples, the solenoids are fast acting, but don't use the mechanical advantage of gears, so they are weak.
If I were to take the rotary solenoid used in the R/C Aircraft, and attach a gear to the magnet, then repeadly turn it on and off, I will have turned into a brushless gear motor. Conversely, if i take a purpose-built brushless gearmotor, then remove the gears, and only actuate it by applying a DC current, it will function like a rotary solenoid.
The same thinking can be done with a linear solenoid. If I stack a bunch of linear solenoids, and put one plunger in it, and then actuate the coils in series, I will have made a linear motor.
So, with the above thinking, they two devices are essentially the same, and therefore are likely to share the same force and efficiency.
The difference is if they are designed to actuate over a large range of motion, or a short range.
Joe Dunfee
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I think you are probably right!
After I started this post, I realized that I really only had one application in mind (the gripper) and that everything else I was considering needed some kind of proportional control - more than an all-or-nothing hammering action.
Thanks! JCD
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Check out: http://users.rsise.anu.edu.au/~roy/SMA / Larry
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Solenoids are mostly applied for ON-OFF operations and SERVOS for position control of the valves. The question here is then, do you need position control of the plunger ?
In robotics, we do apply solenoids for gripper applications which are mostly ON-OFF and usually on pneumatic power for simplicity reasons. On the other hand, we do apply servo systems on hydraulic driven robots where the actuators (piston in cylinder) are moving the robot links. Hence, to obtain position control on the actuators, the pressure is balanced between both sides of cylinders using those hydraulic distributors which are fitted with servos.
Hope it helps,
LHR
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laurentien wrote:

Position control with solenoids is possible, but rare. It is sometimes used in proportional hydraulic spool valves set up for electrical actuation. See "http://www.continentalhydraulics.com/proportional_control_valves.aspx ". It's not easy to do, because standard solenoids have a pull-in force that varies with the position of the slug in the coil.
(Don't put too much effort into this; the original poster was more trolling that seeking a useful answer.)
                John Nagle
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Thanks! JCD
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wrote:

If you want linear position (or more properly, force proportional to current in which the proportion varies MUCH less with position than in a solenoid), you may want a voice coil actuator. I think all modern hard disk drives use rotary versions of this, and I took apart an old (really old) "full-height" 30MB AT-clone hard drive that had a linear voice-coil actuator to move the heads.
This brings to mind the idea of using disk-drive head actuators in a gripper. It would be light, fast, and easily able to apply a variable force, but would still need sensors to detect exact position and force.

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