Let's Make Robots!

Video of a homemade 5-DOF manipulator with tactile feedback made from styrene


 

 

 

 

 

 

 

 

 

 

 

 

Materials List:

Polystyrene sheets:       0.5mm, 1.5mm and 2mm thick (Evergreen Models).

Polystyrene tubing:        Various

Brass sheet:                   0.5mm thick

Brass tubing:                  Various  

Servos & Motors:              2 x HS-805BB (Shoulder yaw & pitch joints)

1 x HS-485HB (Elbow joint)

                                            1 x HS-322HD (Wrist joint)

DC motor with 300:1 gearbox (Gripper)

Tactile Feedback:         Quantum Tunnelling Composite (QTC)

1 x custom-made Electrical Rotary Union (Elbow-to-Wrist joint)

1 x SKF Thrust Bearing (AXK 2542) + 2 x SKF Thrust Washers (AS 2542)

Various springs

Polystyrene glue

Epoxy glue

Heat glue

Multi-strand wire

Heat-shrink

 

Tools List

Scalpel

Studding knife

Small saw

Small files – various cross-sections

Sanding paper

Dremmel

Hand-drill

Vice

M3 & M4 taps

Heat-gun

….generally speaking the more the better!

 

This is a project I’ve been working on, during the last month. This is a preliminary post, is work in progress since its not fitted with any electronics yet - hence no video, but very very soon. I must have this ready for Spain - Campus Party Europe. I will be constantly adding context.

A few words about Styrene

The main construction material used is Polystyrene (or simply Styrene), which is widely available on most hobby stores. For bulk quantities or for those coming from the States, you can contact Evergreen Models – they will stock every styrene sheet / tubing you will ever need.

Styrene is a soft, normally white plastic, which is very easy to handle and will bond very nicely when glued with polystyrene glue. I normally design the part I want to make directly onto the styrene sheet and cut it off using scissors. For internal cut-outs I use my Dremmel with a 2.5mm diameter Carbide cutter. If I need my part to be thicker than the sheet’s thickness, I make multiple identical parts and glue them together. When you make parts like that, make sure you leave the drilling for the end - this will ensure holes are aligned across all glued sheets.

This manipulator arm has been built without CAD drawings; entirely with the method described above. It comprises 5 DOFs: a yaw and pitch root (shoulder joints), an elbow joint, a wrist joint and a gripper. The wrist joint is fitted with a custom-made electrical rotary union that will allow for continuous rotation of the end-effector (gripper) without electrical connectivity problems. Basically the electrical rotary union eliminates the need for cables between rotating mechanical assemblies.

rotary union on the wrist

The gripper comprises two fingers each fitted with a Quantum Tunnelling Composite (QTC) pressure sensor. These sensors are basically pressure sensitive resistors that under pressure change from insulators (R >10 MΩ) to conductors (R < few Ohms). The two sensors are wired in series and the signal is fed directly to the controller of the wrist joint.

The QTC pills on the fingers of the gripper

 

I am currently waiting on the controllers, but more on that soon....!

 

Any comments welcome!

 

30/3/10

The preliminary design on V2... in AutoCad this time :))

 

5/5/2010

Added the video.... from a resisting Greece I salute you!

 

7/5/2010

A few words on the control just to clarify some details ;)

Each axis of motion is controlled via a 'Supermodified' combo. The arm was built using the first generation of Supermodified controllers which work in conjunction with a magnetic encoder (1024ppr) (http://letsmakerobots.com/node/18470). The second generation comes with 4 times the resolution (4096ppr) (http://letsmakerobots.com/node/18615) so i am really looking forward to try out those babes on the second generation 6DOF manipulator. Supermodified controllers are direct replacement boards for the original servo electronics, that allow full PID speed and position control of the servo. Based on the ATMega 368P (fully Arduino compatible with half the footprint of Arduino nano and 4MHz faster) also allow for controlling 4 digital IOs, as well as 4 analog inputs.

So for example the gripper’s pressure sensors need not be fed back to the main controller, since the Supermodified controller board of the gripper can handle the analog signal from the sensors and report back to the main controller (in the video that would be the PC). The Arduino merely plays the role of a USB to I2C bridge. Btw the gripper’s DC motor has been a recent addition since its predecessor - a tower pro 90 micro - proved inadequate. The tiny DC motor powers the dual parallel-link finger assembly by using a tiny pulley system having an extra-strong thread acting as its timing belt :))).   

The remaining servos have all been “Supermodified”. The joint hierarchy is as follows:
 
Shoulder (yaw) -> Shoulder (pitch) -> Elbow -> Wrist -> Gripper

The wiring is fairly simple and straightforward since each Supermodified controller can be ‘daisy-chained’ with other Supermodified controllers, minimising complexity and offering a neat end-result. The whole manipulator arm wiring effectively comprises a single 4 core cable (Power and I2C comms), running from one servo to the next according to the hierarchy above. I2C comms are then fed to the Arduino, which for the time being acts as a USB (com) to I2C communication-bridge between the host computer and the Supermodified servo controller network.

Thats it for now soon I will upload a number of high quality pictures of the assembly process for the electronics and the arm.

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How does it works in your robot?Could you show some pictures or give a detail describe? BTW,your robot is very beautiful,I admire it very much~

Anything that kan be squeezed or bent is called "compliant" by engineers like Anton. I had to learn that word myself when I started reading tech-english. Make sure your "element" unsqueezes or bends back when you release the pressure.

It seems that you are also not from an English-speaking Country?I've found it's hard to express my idea in English...

Hey Antonb,

The Pills that you used for pressure pads. Where did you get them, and more importantly where can one get them now?

I found them on one site but they are discontinued. These sensors whether pill form or sheet would be great for my feet of my robot so that I can balance properly.

 

Thanks

Dave

 

Hi Dave,

I used to buy them from MAPLIN electronics (UK)

I had a quick search after you pointed that out, but it seems they dont stock them anymore :(((

Unfortunately it seems that they are discontinued.

If you come across a supplier please let me know.

Anton

I got my QTC pills from Technobots: http://www.technobots.co.uk/acatalog/Shop_Front_Pressure_Sensitive_Materials_433.html

I haven't tried them out yet though. I hope I haven't lost them (they are too small ;-)

 

Thanks for the info nuumio well appreciated! These sensors always come handy. I just hope they do not become discontinued

 

Nuumio thanks for the link.

Anton

Since you have used these pills, can you tell me what is the max weight the pills can handle before they are maxed out giving a 0 ohm output?

Also when you get these pills are they ready to be used straight out of the box or do you have to place them on something?

When I was not able to find the pills I looked for pressure transducers. I found a company ( well many companies) that I have been talking with about their pressure dies. ( http://www.meas-spec.com/product/t_product.aspx?id=5045 ).These are piezo H bridge transducers measuring 1.72mm X 1.58mm X .6mm physical size . There are different maximum pressures I have chosen the 7bar which the link should get you to.

Once I get a price I will pass it on.

Another place to find pressure sensors very easily is any electronic scale one might find in a house bathroom. This is not economical in high volume but if you only need a few then it's an alternative that is easy to get.

 

Hi Dave,

The maximum force according to the datasheet for the 6x6mm sensor footprint comes to 100 Newtons. When installing you should provide two conductive surfaces making sure that these do not short when force is applied. If you need more sensitivity i suggest you employ a peak on one of the conductive surfaces. I normaly use copper plates for conductive surfaces (onto which I solder cables) and if i need a peak point i just use a solder blub.

In terms of the sensor you want to use i suggest you coat the sensing face with some polyurethane. this will account for linear results - remember that fluidic pressure is applied uniformly on the sensor face where mechanical pressure is not.

What are your future plans for this? Maybe you could add a router head and have it manufacture copies of itself?