# Comparison of lifting systems

What kind of systems can be used to make a torso lifting system like the one used by this robot (the black part) :

• Rack and pinion
• scissor lift
• can a triple tree help ?

What are the pro and cons of each system ? How do they ensure stability ? And finally, is there a way to draw current when lowering instead of drawing current when lifting ?

So this is summary...Open to discussions or edits...

| Mechanism          | Precision | Friction | Max length | Max force |
|--------------------| ----------|----------|------------|-----------|
| rack and pinion    | low       | high     | unlimited  | high      |
| lead screw         | high      | mid      | limited    | high      |
| ball screw         | high      | low      | limited    | high      |
| scissor lift       | mid       | low      | limited    | high      |
| linear direct drive| high      | low      | unlimited  | mid       |
| 4-bar mechanism    | high      | low      | limited    | high      |
|--------------------|-----------|----------|------------|-----------|


It may worth noting:

• that the scissor and 4-bar mechanisms have a non-linear dependency between input and output velocity

• not all 4 bar mechanism's motions are linear

• the scissor mechanism (if designed like this) provides a good lateral stiffness, all others would need some form of linear motion guide to cope with lateral forces that bend the structure.

You can use a spring (mech/pneu/hydro) to invert the direction of effort, or a simple counterweight.

You may want to use a non-backdrivable gearbox to drive these (not the linear direct drive of course)...

• You can use a pulley and counterbalance to invert the direction of effort. You can also use 4-bar mechanisms (crank-slider, etc) to accomplish the motion. – SteveO Dec 3 '15 at 19:28
• good point, might be better then a spring... – 50k4 Dec 3 '15 at 19:28
• There are 4-bars that provide exact straight-line motion over their entire range (such as a crank-slider). You just need to design the right type. See Wiki's en.wikipedia.org/wiki/Straight_line_mechanism and its references. – SteveO Dec 3 '15 at 20:25
• I wouldn't call a crank slider a four bar mechanism... but others in the link migh be considered 4 bar mechanisms...hence the edit... – 50k4 Dec 3 '15 at 20:32
• That edit works. A crank-slider (or slider-crank) is one of the primary types of 4-bars. See Hall's book or the wiki page for four-bar linkage. But this is tangential to the OP's question so I think it's good how you have it. – SteveO Dec 3 '15 at 20:34

Regarding the lift mechanism, I suggest you look at the semiconductor wafer-handling robots. You can see the insides of one version here: https://www.ifixit.com/Teardown/Hine+Design+Inc.+Automated+Wafer+Handling+Unit+Teardown/1651

In that robot they used a lead screw vertical drive mechanism along with (it looks like) two cylindrical shafts for stability and linearity. When I've built systems using parallel rails or shafts, it is very difficult to keep them collinear, so we would usually fix both ends of one rail, and only one end of the second rail. The other end could float over a small range. That way we would prevent binding due to misalignment issues.

That industry has made vertical lift mechanisms for robots out of everything from pneumatics, to linear motors, to ball screws, and more. All of their designs require compactness and cleanliness, so some of the designs are very clever. You can search for patents by companies such as Genmark, PRI Automation, Brooks Automation, Applied Materials, KLA Tencor, Hine Design, and Adept to find good examples.

• But they do not seem to have the same constraints as this robot. What's the weight of a wafer ? probably not that heavy... And the load is always the same, while In that robot, the arm may grab various object (having different weights) even if the weight of the torso+arm+head of the robot makes this negligible. – LiohAu Dec 4 '15 at 19:04
• Absolutely the design constraints are different. But the design concepts relate. I am not suggesting you copy, but rather study those designs. – SteveO Dec 4 '15 at 19:20