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Author Topic: Push vs. Pull and continuous vs. intermittent actuators?  (Read 4258 times)

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Offline corrado33Topic starter

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Push vs. Pull and continuous vs. intermittent actuators?
« on: October 18, 2009, 06:00:33 PM »
Hey guys, I was wondering a couple things.  I'm going to use a couple actuators in my robot.  This is what they have to do, extend to their greatest length (with no real resistance, or very very little resistance), then pull back tight against something.  I'm guessing I need pull actuators (and I really don't feel like springing for the combination push/pull ones.)  Will the pull ones be able to push their bar to the end with a little resistance?  It'd be dumb if they couldn't move themselves both ways.  Plus aren't they just motors connected to threaded rods? 

Secondly what's the difference between continuous vs. intermittent actuators?  From what I've read intermittent ones are generally used when the actuator has a long break after moving.  Meaning it'll move, then wait a while, then move again.  Continuous can move as (often)much as they want?  Then why in some cases are continuous cheaper than intermittent?  (According to the McMaster-Carr catalog) 

Thanks in advance!

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #1 on: October 18, 2009, 08:43:44 PM »
How fast and how much torque? Size constraints?

What about solenoids?

Offline waltr

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #2 on: October 19, 2009, 07:32:00 AM »
Could you give us a link the McMasterCarr parts.

Offline corrado33Topic starter

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Offline waltr

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #4 on: October 19, 2009, 12:02:46 PM »
That link is very helpful.
These are solenoids which are a coil that produces a magnetic field when energized. This magnetic field then pulls on the movable iron based core to retract the plunger or push a extension out the other side (pull or push). The standard solenoid can not do both and has no ability to return to the non-energized position without an external force such as a spring. However there is a Combination Push/Pull that can do both either by using two coils or a magnetic core.

When a voltage is applied to a coil the current flow produces a magnetic field. The current flow and the coil resistance also produces heat from I squared * R losses. Its this loss and the ability of the solenoid to dissipate this heat that determines whether it is rated for continuous or intermittent duty.

However there is a trick that can some times be used. Many times more force is required to move whatever the solenoid is pushing or pulling than is needed to hold that item in position (very common with relays). If this is the case the coil drive circuit can deliver the full voltage (and current) to the coil on activation then after several (or hundreds of) milliseconds reduce the voltage to the coil. This reduces the current and thus the heat generate losses so the coil does not over heat.

Offline corrado33Topic starter

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #5 on: October 19, 2009, 12:42:59 PM »
Hm, interesting.  Shouldn't technically you be able to reverse the current through the actuator to reverse the magnetic field and therefore reverse the direction of the travel?  Is that how combination push/pull ones work? 

Offline waltr

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #6 on: October 19, 2009, 02:15:17 PM »
A piece of iron (or steel) doesn't care what the magnetic polarity is, it will just be attracted to the magnetic field.
If the steel is magnetized, like a compass needle, then it will have a magnetic polarity and can be repelled with the same polarity. Do you ever try to push two magnets together, North to North?
The combination actuators at McMaster seen to be of the type with a magnetized core as there are only two wiring connections.
Another way to build a bi-directional solenoid is with two separate coils and the core to pulled into one or the other.

Offline corrado33Topic starter

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #7 on: October 19, 2009, 03:07:09 PM »
That makes sense.  Seems like I'm going to have to get a combination actuator after all.

Offline corrado33Topic starter

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Re: Push vs. Pull and continuous vs. intermittent actuators?
« Reply #8 on: October 24, 2009, 10:19:16 AM »
Sorry for double posting but I lied...  I'm not going to get a combination actuator.  I'm going to put a spring to push my "pull actuator" up.  Seems like it'll work well enough.

 


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