The Arduino has arrived! I chose the Arduino development platform due to its ease of interfacing with the components necessary for control of the arm and its massive user support base. The Arduino is based on an Atmel ATmega328 microcontroller, has a USB interface for programming, can control servos and integrate with sensors.

The current setup can take in analog values from our bend sensor and display the input as a graph.

Here is a photo of the current setup:



The bend sensor is a variable resistor. I have an 18kΩ resistor in series with it to create a voltage divider. The analog reading comes from the node connecting the bend sensor to the resistor (orange wire). The bend sensor has a resistance that varies between 8kΩ and 50kΩ.

Pressure Transducers are very expensive.

Combine a pressure transducer (analog sensor) with a proportional valve (analog output) and you have an electronically controllable pressure regulating machine. Beyond those two pricey components is the need for a microcontroller, and a DAC (Digital to Analog Converter). There’s also a significant programming overhead to tell the microcontroller how to map the signal it’s getting from the transducer into something the DAC will send to the valve.

This is all doable, but is it worth it?

In industry, this problem is solved using position controllers. They effectively bundle up all those components into a tidy box that you can plug into a computer to do the control with. These controllers cost hundreds of dollars.

I contacted Initek again, and I’ll be meeting with Joe to play with a PPC (Proportional Pneumatic Controller) valve from Mac Valves. I think this is going to work well for many purposes.

Update 6/25: I met with Joe and he was kind enough to leave me an older Mac Valve to test out. It operates at 24V and takes in a DC voltage between 0-10V (4-20mA) to control to output pressure.

The applications for all this work are many: Regulating pressure in links, (0-5psi range), regulating pressure in bellow/expansion links (0-5psi), controlling pneumatic muscles (0-60psi)...

Tuesday and Wednesday were spent familiarizing myself with ordering the various components that are required to start testing the vacuum cup designs.

Tuesday:
Component Research: I investigated all manner of vacuum generation and came up with two viable options:

• The first viable, and less expensive route is to use a Venturi type vacuum generator to create a vacuum from the compressed air present in the lab. This method of creating vacuum is somewhat inefficient, but since there is an abundance of compressed air available, I do not see efficiency being a concern at the moment.
• The second viable route is to purchase a standalone vacuum unit that simply has a single port that pulls vacuum. This unit can be quite small and would just sit on a bench top plugged into an AC outlet. This option also removes the need for many of the fittings that would be necessary to connect up the first option.

I also met with Sid and presented my vacuum cup idea.

Wednesday:
I got approval to place orders directly with Lynnetta Miller.
I had phone calls with-

• Clippard: I called up Clippard technical and learned about the proportional valve that I thought I would be interested in. I settled on part ET-P-10-25A0.

• Intek Systems: I am in contact with an engineer or technician at Intek who has previously worked with the RI and NREC on integrated systems. I ran my vacuum system idea by him and he seemed to think it would suite the purpose of the project. He also suggested looking into a vacuum unit, which could be convenient for many reasons. He said he will be consulting his “Vacuum Guru” and be getting back to me tomorrow by email.
• Keystone Components: The Clippard tech I spoke with recommended that I order through a distributor instead of online to avoid additional handling fees, and Keystone Components is the distributor listed on the Clippard site as serving the Pittsburgh region. I ended up spending a long time on the phone with them putting together a substantial order including the proportional valve, all necessary fittings, tubing and vacuum generator and cups. Once I have the quote finalized I will elaborate on its contents. I expect that to happen Thursday.
• BouncerLand: At Sid’s request I also contacted BouncerLand to see if they might be capable of creating a custom inflatable arm for us. Initially they seemed very excited about the prospect of a custom order, but upon further discussion of our needs it seemed that they were not set up for the scale that we require. The company specializes in large, simple structures, but if the designer thinks the job is possible then I will receive a call back in a day or two. I am not optimistic about BouncerLand.

I’ve spent this weekend and today researching and designing simple, compliant and inherently safe grippers. My initial thought was to use a donut like shape that inflated to grip whatever is in the donut hole. I started by researching inflatable artificial sphincters used in the medical industry to treat stress incontinence.



Images of inflatable artificial sphincter

This solution seemed promising at first, but I ran into complexities in packaging and manufacture so I continued my search. I next researched another biologically inspired soft gripper: the end of an elephants trunk. Again, I ran into issues with creating a simple pneumatic version of this shape. I attempted to design a complex sequence of seams to press in the plastic sheet currently in the lab, but again the system felt too complex for a simple tool changing operation.

Instead of looking to nature for inspiration, I moved into the realm of industrial automation. In industry, particularly machine tools, have many interchangeable tools. The tools themselves can vary significantly in size and shape but each tools is outfitted with a unified grasping point: a collet.


Image depicting various tools in collets.

With this concept in mind I elaborated on simple clamping devices that would be compliantly mounted to an arm capable of easily interfacing with each tool. The main disadvantage of this approach was that it was not inherently safe. Making a soft, compliant gripper capable of grasping and articulating the tools again seems like a complex approach, although the automation achieved in CNC tool changing is something to admire.


This is a carousel type tool changer for a CNC mill.

My final thought is still on the industrial path, we’re definitely on to something. I think that the best bet at this point is to use vacuum bellow cups to compliantly grip each tool. Each tool would be outfitted with a sort of collet, made simply of a disc of smooth, flat plastic. This disc would allow a pneumatic bellow to quickly and securely grasp anything outfitted with that attachment point. These bellows are also entirely soft except for the nozzle at the top of the bellow. These devices also come in a wide variety of sizes, working capacities, materials and colors to compliment their very low cost. The pneumatic control for a bellow is very simple: vacuum on to grasp, vacuum off to release.


An assortment of vacuum bellow cups.

Now, we can go further with this as I have some preliminary ideas about how to (add complexity, and) increase the capabilities and functionality of the setup. Elaboration on that is planned for tomorrow.

At today’s lab meeting I gave a presentation to gain some insight on the direction of the project. That powerpoint can be downloaded here:

6_4_10 Lab Meeting

The overall impression I got was that it was not worth pursuing a complex hand design, but instead focus my energy on creating a ‘system’ that can accomplish precisely the list of tasks that were originally set out. It was determined that if necessary abandoning the concept of a wholly soft gripper would be okay.

Some ideas presented include:
Tool Changer
Nerf Robot
Iris Mechanism
Extremely light-weight gripper

Today I met with Sid to discuss the ideas that we’ve been coming up with to achieve the goal of creating a compliant, soft gripper.
The main contending ideas are:

1) A pneumatically driven, jointed hand.
This hand would contain a ‘skeleton’ surrounded by a padded layer to reduce contact forces. Each joint would be driven by a pneumatic bellow with a spring return.

2) A tendon driven, inflatable/soft jointed hand.
Similar to the hand described above except driven by a series of tendons. This is essentially the Shadow Hand but made soft.

3) A tendon driven continuum hand.
Each finger of this hand would be a typical three or four tendon continuum ‘arm’ capable of grasping. The remote power source could potentially be pneumatic in the form of pneumatic muscles or typical pistons, or using electric actuation as in the two current arms.

4) A pneumatic chamber driven hand.
The fingers in this configuration each contain three air chambers running the length of the finger, arranged symmetrically about the center of the finger. The finger is actuated using a variable air/gas pressure in each chamber. The dynamics of this type of system are similar to a tendon based continuum arm.
The palm could be made of silicone as well and would ideally integrate all grasping fingers as a single cast silicone part. Eventually the air chamber approach could be implemented in the palm to increase the capabilities of the grasper.

Here is a snapshot of my whiteboard with the possible design paths set out.



At this point, the vote is for option 4. I am investigating pneumatic control systems and silicone injection molding manufacture techniques. The challenges ahead include fine tuning the pneumatic functionality, ensuring appropriate build quality and eventually dealing with tactile sensing. Integrating the finger into a grasper will be done with a central silicone palm at some point, but initially work will be done to test the feasibility of individual fingers.