how to exert spring like pressure by using actuators or solenoids

[Robert Milton]

Hi Kinarfi,.....Thanks for the advise and drawing, I never worked with servos before so I will be calling around to see what kind of force a small size servo can deliver[as you maybe noticed in my photos i have limited space available to me in that box it is approximately 2"X3"]

 

[Robert Milton]

i forgot to ask you guys earlier, my power supply is a 12v DC powertool battery, and I will need to cut the power off to the servo after it assumes its position, does servo need a continues supply of power to hold the 3 positions we have discussed.

 

[alec_t]

The servo's gearing friction may well be enough to hold position when powered down. Obviously it depends what force is trying to move it out of position. A solenoid would not hold position. BTW a standard servo will generally need ~5V supply, not 12V.

 

[jpanhalt]

One big difference between so-called "coreless" servos and servos with 3 or 5 pole motors (most are 3-pole) is that coreless servos do not hold position. The simple unbalanced weight of a control surface is enough to move them. Cored servos will hold based on the resistance of the gear train and motor, but I would not rely on that for anything critical. The idle drain of a servo is quite low in the absence of a force to move it. With an opposing force, the drain goes up considerably. One way to avoid both the need for continuous power to the servo or the high drain is to design the servo mechanism to operate a latch.

There are probably 12V servos around, but none of the ones I mentioned were designed for that voltage. Most standard servos are spec'd for 4.8 to 6.0 V. Higher voltage ones designed to operate from 2S lithium batteries are becoming much more common. If you must operate at 12V, I suspect a small switch mode converter plus standard servo would cost less than a 12V servo.

John

 

[kinarfi]

Or, just buy a linear servo for less than $4 USD.

**broken link removed**

It is also quite easy to convert a regular servo with rotary motion to a linear servo. In fact, EMS used to sell a purpose-designed part that replaced the top case on a typical servo to give a liner motion. The range of motion was about 3 cm.

How much force do you need? How fast does it have to move?

John

That's a pretty cool unit, John

**broken link removed** The EMS servo looks like a dandy, then with some electronics to send it the correct PWM the 3 different positions, you may have your solution, provided it has the force you need.

Do you need to turn the power off between position changes? If so, you may need to go to a worm gear drive, they generally hold their position.

 

[jpanhalt]

The key to the EMS device is the "linear drum" that fits within the modified case.

View attachment 66846

That is not really much different from a pushrod on the servo arm, but it is quite compact.

I bought one several years ago for a project, but before using it, I redesigned the mechanicals to use a latch that maintained position without power and had a spring overdrive to allow it to be reset without having to use servo control.

Note quite sure what you are asking:

Do you need to turn the power off between position changes? If so, you may need to go to a worm gear drive, they generally hold their position.

The linear drive only operates under power. Its holding ability without power is not bad.

John

 

[kinarfi]

On all the servos I've played with, if the power is removed from the servo, they can be turned, so depending on the force that Robert need, it may not hold, where as a worm gear such as this **broken link removed** will hold it's position, solidly! Using this unit would require limit switches to stop the motor in the correct position, be it mechanical or electronic such as hall effect and a selector switch and maybe some logic circuits.

 

[jpanhalt]

As an alternative to a worm drive, a jack screw will also hold its position and is mechanically less demanding to construct. It just needs a nut on a screw. Continuous-rotation hobby servos are now quite common.

John

 

[Robert Milton]

After i get 1 small unit working successfully, i then plan on building medium and large size for the same purpose. for now i can say approximately 10kg worth of push/force against that lever would be sufficient.

 

[alec_t]

10kg worth of push/force against that lever would be sufficient

That would require quite a chunky servo. Methinks a screw-jack type would be needed to hold position against 10kg.

 

[Robert Milton]

Is an actuator not a advanced screw jack, i still would have to rotate/extend the screw jack some how to do the job and able to push?

 

[Robert Milton]

Hi John,...would continues-rotation hobby servo have enough power to drive a screw that needs to deliver 10kg. of push to a lever? and what about a smaller apparatus that would need 5 kg of push I assume we can still use same solution for both? [what ever the final solution]

 

[jpanhalt]

I am trying to achieve this goal with under 20 dollars if my idea is to survive,....

As for the desired speed to achieve the 2 settings [and plus back to beginning no-pressure mode], my guess would be that when the remote button is pressed calling for any of the 3 settings it should deliver immediately.

Hi John,...would continues-rotation hobby servo have enough power to drive a screw that needs to deliver 10kg. of push to a lever? and what about a smaller apparatus that would need 5 kg of push I assume we can still use same solution for both? [what ever the final solution]

Combining those specifications, i.e., a device able to provide a push of 10 Kg to move an object immediately over a distance of 18mm at a cost of less than $20 and be able to hold that positon without power presents a problem that a simple model servo cannot solve. Servos with torques of greater than 600 oz-in and a transit time of 0.17 second at 7.4V are available, but they are not cheap (Hitec HS-7980TH is about $135). Moreover, 0.17 second is still not "immediate."

You can get power-off holding, 10 Kg push, three positions (or more), and <$20 with a much slower transit time using a jack screw or worm gear. For faster speed, servo torque or cost may be the limiting factor. I believe some trade-offs have to be made. The speed requirement is the most limiting and still somewhat obscure specification.

Perhaps you can give more details of the original solenoid system you had that did meet those specifications. What solenoid, voltage, and current did you use?

John

 

[Robert Milton]

I was using 5 inch long / 10mm diameter spring pulling on the lever from the right side fixed in 1 continues setting constant pressure and when i wanted to lift the pressure off totally and put lever back to a relaxed position then i used a strong bi-stable solenoid pulling from the opposite end or the left side of the lever [as seen in photo] which pulling on the spring did of course relieve the pressure exerted by the springs on the lever.[and solenoid did lock in that retract the shaft position not needing any more power after retraction,..... but this design was only able to provide me with 1 setting of pressure on lever now that i need 2 different amount of pressure the spring is not a good option any more.

 

[Robert Milton]

an as for the "immediate" part,...i believe i can get away with a delay from 0.5 to 1 second,...especially if it would cost less, testing it at that speed will help decide,... which is same for the force/pressure against the lever i will have to go with and test 1kg-2kg-3kg,...and on and on up to 10kg pressure to realize the results hence learn to perfect the machine that maybe down the road be made with in different physical sizes and abilities.

 

[atferrari]

Linear actuator stepper

Something like what you could find **broken link removed**

Google for linear actuator stepper. Variety? Yes. A lot.

 

[kinarfi]

Hi Kinarfi,.....Thanks for the advise and drawing, I never worked with servos before so I will be calling around to see what kind of force a small size servo can deliver[as you maybe noticed in my photos i have limited space available to me in that box it is approximately 2"X3"]

can you post an actual photo? is it 2 X 3 inches or feet? is that length and width or width and depth, what's the 3rd dimension?

 

[alec_t]

I was using 5 inch long / 10mm diameter spring

How did you fit that into a 2" x 3" box

 

[Robert Milton]

Hi Alec, On page 1 /August 27 reply, i had attached this photo., View attachment 66920

 

[Robert Milton]

Here are the dimensions of my box

View attachment 66922