# Thread: Making your own Linear Actuators

1. ## Making your own Linear Actuators

Greetings,

Has anyone tried to Make your own Linear Actuators? I am working on plans for a version of a SW gonk droid. I am wanting it to walk on its own power. The biggest hurdle I for-see so far is weight, I am wanting to make it semi usable with power inverters to run small AC devices. In doing so, the projected weight is estimated at 30-40 pounds (14-18 kilograms). The batteries taking up the largest part of that weight. I am thinking of using the lead Screw method with Dc motors. Using a potentiometer at the joint as feedback and connected to the brains of a gutted servo for control.

T
hanks

2. ## Re: Making your own Linear Actuators

I don't think lead screws will be fast enough to make for walk-capable linear actuators, unless you go for some pretty significant leverage type coupling from the actuator to the legs. And, if that's the case, then why not use a rotational actuator in the first place?

3. ## Re: Making your own Linear Actuators

Does anyone have a formula to figure out the torque rating needed for the legs?? I do have some info to share, from the ankle to the knee is 6 inches and from the knee to hip is 6 inches. my original idea was leg construction with 4 DOF. Simular to this 2 DOF type in the video but with added servos at the hip to twist and move the leg in and out. please remember the projected weight is estimated at 30-40 pounds (14-18 kilograms).

T
hanks

4. ## Re: Making your own Linear Actuators

The formula is simple:
1) How much force is needed? This is gravity times the full weight of the robot if the robot is a biped that lifts one foot from the ground. Call this F.
2) How far from the pivot is the gravity force applied? You can measure this with calipers or a tape measure. If you only move the legs a little bit, you only need to consider the length orthogonal to gravity, but with legs turning at least 30 degrees, you typically want to use the entire length. Call this l.

Now, multiply force by length, and you get required torque to stand still! If you need to actually push against gravity to do work, you need to add additional torque.

Once you have a more complex assembly, the torque will get distributed through the assembly, but you can model most of it with proportional division. If you have two motors helping out, they can each have half the torque for the static case. Note that, when you try to move the leg forward/backward, the torque for one of the motors "disappears" so the other motor needs to take a lot higher load.

5. ## Re: Making your own Linear Actuators

jwatte,

two questions,
how do I get the gravity variable?
and I have been reading up on many topics here and there, I plan to use springs to the effect that they will hold the leg full extended as in standing, so there would be little torque needed to stand. I am planing on this week, to do a study model of a leg. having a physical model will help me understand the mechanics used.

Thanks

6. ## Re: Making your own Linear Actuators

how do I get the gravity variable?
9.81 meters per square second.

If you have springs helping with standing, then you need less torque for walking, as long as you don't need large acceleration values in the legs. Quick reactions are going to need a lot of torque anyway.

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