|
|
|
|
|
| |
Posted by pogo on April 23, 2008, 8:41 pm
Please log in for more thread options
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
|
|
Posted by John Nagle on April 24, 2008, 1:32 am
Please log in for more thread options
pogo wrote:
> 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.
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
|
|
Posted by pogo on April 24, 2008, 1:57 am
Please log in for more thread options > pogo wrote:
>> 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.
>
> 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.
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!
|
|
Posted by cadcoke4 on April 24, 2008, 11:49 pm
Please log in for more thread options 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
|
|
Posted by pogo on April 25, 2008, 12:43 am
Please log in for more thread options > 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.
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
|
| Similar Threads | Posted | | Re: Using DTMF to drive solenoids | July 26, 2005, 3:46 pm |
| New to servos | January 18, 2006, 8:54 pm |
| How do servos work? | July 23, 2005, 3:58 pm |
| Futaba FP-S29 Servos | November 1, 2005, 2:23 pm |
| control servos through TCP/IP | January 12, 2007, 4:16 am |
| what frequency to drive servos at? | July 2, 2005, 9:21 pm |
| Whats best place to buy servos from in uk? | September 14, 2005, 3:24 am |
| want to couple 2 servos in parallel | December 10, 2005, 2:47 am |
| Need a controll board for 3 servos | January 30, 2006, 3:36 am |
| hobby servos "amplifier" | August 13, 2006, 10:51 pm |
|
|
|
XML site map
> 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.