# Linear Actuators

• posted
Hi,
I'm about to build a set (6) of linear actuators for my robot (AUV) and was
wondering if I could run a few design issues by the group?
1)
I'm thinking hat I can either use a rack and pinion or rotate a piece of
threaded rod. Would I be right in thinking that if you were to push on the
R&P method you would therefore be pushing on the motor and subsequently
motor draw would increase proportional to force applied? I'm not thinking
about using it as force feedback, but don't want to use up battery life if I
have a constant force pushing on it. And therefore would the threaded rod
therefore be the preferred method as the force is dissipated through the rod
and teeth as opposed to the motor itself?
2)
Sticking with the threaded rod method, I presume I could calculate the time
to extend (I presume there's a better term) by multiplying under load RPM by
thread pitch? I've always heard 'slow' and 'linear actuators' in the same
sentence and using a 5000rpm turning a coarse metric thread (pitch 1.25mm) I
get 3000(if we say this is load RPM)*1.25=3750mm/min which sounds pretty
fast to me....
3)
Has anyone here constructed a linear actuator and got any
suggestions/comments/pictures etc that could help me design/make it?
4)
And finally, in order to have level of position feedback, I was thinking
about using a hall effect sensor on the rod/motor (depending which one I
choose) and using that combined with knowledge of knowing which way the
motor's turning fed into a 4029 to provide a value as to the current
position that a uC could then read.
Am I heading in the right direction here?
Cheers,
Michael
• posted
"Michael" wrote in message news:kRmBh.338332\$ snipped-for-privacy@fe3.news.blueyonder.co.uk...
I'm a bit confused as to if you want your linear actuator to drive something or to feedback something.
A screw will hold its place once powered down. A rack/pinion will not. If you put an LVDT in conjunction with either, you will have very accurate feedback for a servo system like in an RC circuit.
What is it that you are trying to do?
Be specific.
• posted
Hi,
I'm planning on using a set of linear actuators to drive the control surfaces (sternplanes and rudders) on an automated submarine I'm building.
Michael
• posted
I've just built something similar for a FIRST robot.
notes:
1) Run the screwed rod in tension if possible.
2) You can run the screwed rod fast, but you may well see a lot of vibration etc - support at both ends if possible.
3) As a general principle I would always try to connect feedback sensors directly to the item in question - you don't need much accuracy here, so a simple pot would probably suffice.
4) It is kind of obvious - this kind of actuator can produce very large forces - think about limit switches and end stops - experience suggests that hitting a solid end stop at high motor speed can be spectacular.
Dave
• posted
Thanks Dave,
I seem to remember somewhere reading that you can't use 'normal' microswitches as limit switches as they have a high fail rate....Is that right?
Michael
• posted
On Fri, 16 Feb 2007 18:37:04 GMT, "Michael" wrote:
If you are spending serious \$\$\$, you may want to look at commercial actuator suppliers like below. Some have feedback pots for position indication. If you want to go really cheap and DIY, you probably can hack a standard servo to do most of what you need using the threaded rod. I'm currently looking at a similar simple DIY setup, but I need a roller skate wheel bearing to use as a cheap thrust bearing for the threaded rod. WalMart no longer carries replacement skate wheels, so I'll have to expand my range of parts suppliers. ;)

• posted
.........but don't want to use up battery life if I Michael
__________________________________________
I used cable around a 1/2" diam' drum . If you do it right , it has same loss as expensive rack and pinion .
But the ultimate is linear motor and quick clutches/ locks . The ram has springs to recover the lost energy . Its very efficient , dont waste batteries .
• posted
Microswitches have an 'operating' travel and an 'overtravel'. My experience is that some fail fairly rapidly if you use up all the 'overtravel' every time. The very small ones seem particularly bad.
Another common mistake with limit switches is placing them so that they are the end stop ! Your first control glitch will demonstrate the folly of this arrangement. Mount the switch off to the side, operated by a cam or similar
Dave
• posted
Wow ! Thats REAL digital , using MicroSwitches !
I use lots of ARM 7 mcu's with 144 pins ,
ussually one can use 60 GPIO's , for stuff
like reading 1n4148 glass diodes , in a string .
But if you need more precise , the ADC can
read 8 channels and the Timer-Match can do
many more inputs , to transducers , your
robot has .
Ideally , a robot should have ARM7 mcu's
in many places of movement , legs , arms ,
and wheels .
Each arm,leg , can send
rate of its "position"
to another ARM7 , thats reading the
rate gyros and other "centralized" stuff .
So , if robot falls/crashes ,
the s/w can improve automatically .
Robots are exciting , but human replicas
are not optimal , wheels are much more
useful , beacuse wheels can climb and
go fast , you just gotta articulate the wheels
clever like .
I will make one , but now im perfecting
a free OpSys for ARM , that trashes all
the silly C++ , assemblers and Pythons
and other farm animals .
New bidirectional protocol for USB ,
Software is fun to simplify ..
• posted
Those are things that rotate. Why do you want to put a linear actuator into the system? If you just need more torque and can accept slower speed, add a reduction gear.
Read the catalogs of Berg and of Stock Drive Products to get some idea of what's available.
John Nagle
• posted