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Hydraulics System

Posted by JohnnyCooper 
Hydraulics System
March 17, 2010 01:48PM
I've been thinking about the Cyberdyne Hydraulic Control System.


The simplest solution is to use small diameter cylinders to directly drive large diameter cylinders. This reduction in diameter should result in an increase in control.

Alternatively: the linear actuators which are used to drive the robot could be composed of two cylinders:
-one large diameter for precision control
-one small diameter for rapid traverse

By using medical syringes as piston pumps, a hydraulic control system can be drastically reduced in complexity.

However, up until recently: I had not really explored more complex hydraulic systems as I knew little about them.


The issue falls to the ability to fabricate "proportional valves" aka: "hydraulic servo valves."

There was a discussion of this on the arduino forums:

Here is a common design I've seen using an eccentric cam on the shaft of a DC servo motor to control a spool valve like from the above howstuffworks animation.

This is a pneumatic/hydraulic "brake servo" from a car. Perhaps some inspiration can be drawn from it as well.

This is as much research as I've really done, as it seems like the designs only get more complicated from here.

IE: I saw a paper written on the subject of [en.wikipedia.org] as a drive mechanism for a servo valves.

For the immediate future: I should probably focus my attention on the large/small diameter piston combo because it should be the easiest to get off the ground. In the long run however: after success with the former, I think it would be fun to get in to the business of rapid prototyped proportional valves actuated by hobby servos.

(of a spool design most likely)

Edited 3 time(s). Last edit at 03/17/2010 01:54PM by JohnnyCooper.
Re: Hydraulics System
March 17, 2010 02:23PM
If I'm reading your ideas correctly, you would have a single high power pump to create a pressure reservior, and then direct flow to individual cylinders, correct? This reduces the overal cost/complexity because you replace individual motors with electronicly controlled valves..?

One other possible branch would be to adapt something along the lines of the peristaltic pump to drive a larger bore cylinder directly. I'm not sure a design of this nature would have any direct advantages over a simple mechanical linear drive.
Re: Hydraulics System
March 18, 2010 04:52PM
There's a couple of options being explored. The most recent being the use of reprapped servo valves to distribute hydraulic pressure.

For simplicity sake, it still may be advantageous to use a piston pump(paste extruder stylie) to directly drive individual syringes. One pump/linear actuator combo per syringe.

The advantage of all them them over a mechanical linear drive is weight vs. grunt.

With robotic arms: using hydraulics means the heavy pumps/motors do not have to be able to lift themselves in addition to their task. It's a weight savings thing in the specific critical areas.

The advantage of the valve route is speed.
Re: Hydraulics System
September 14, 2010 07:33AM
The main issues with replicated hydraulics are

1. Sealing, precision isn't very high so sealing will be an issue

2. Stiction or Static friction, Cylinders have very high static friction issues. This will cause significant precision issues.

3. Pressure, due to the materials used and for safety pressures will have to be kept low, which means more flow for the same power. This results in more oil and bigger actuators to provide the power. Which equals more weight. (you need to add the weight of the hoses and the weight of the fluid).

4. You will need more than just proporitional control valves, Pressure control valves, load holding check valves, anti-cavitation valves, Shock valves, there are more!

Not to mention this system will need closed loop control of the positioning. Which adds sensors and cost to the electronics.

How would we replicate hoses in long enough lengths?

Proportional valves are simple to look at, but extremely complicated in their engineering.

Servo Valves more so.

Fixed displacement hydraulics are very inefficient in terms of power usage to work output.
Re: Hydraulics System
September 14, 2010 07:43AM
... maybe this old thread can give some ideas winking smiley

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