Monthly Archives: March 2015

Gravis BlackHawk Joystick

The arm we are building will need some form of control. Since we are cost minded, and we happened to have an old 15 pin gaming joystick laying around, we decided to hack it for our purposes.

I was able to search up some pin-out guides for the joystick outputs:

http://www.epanorama.net/documents/joystick/pc_joystick.html

http://pinouts.ru/Inputs/GameportPC_pinout.shtml

Here is the setup I came up with.

IMG_20150311_192238072_HDR

 

I have some simple arduino code reading in the analog and digital outputs from the joystick. As the guide was not exact for the joystick we happen to have, I had to do some testing to determine which physical inputs were linked to each particular pin.

Here I tested the x-axis (arbitrarily named), of the joystick.

IMG_20150313_152605831

Here are the outputs I was testing and logging.

 

IMG_20150313_152608585

I found that two of the digital outputs were linked to two buttons, the third button seemed unresponsive (which is ok, because we don’t necessarily need it). In addition, the two axes on the joystick work fine, as well as a side wheel. These analog inputs give us ranges on outputs from ~500 through 1023. I used 100Kohm divider resistors in the breadboard circuit. I think changing these could help out with our current resolution/input range.

There are clearly a few kinks to figure out, but we should be able to create controls for our project using this joystick.

The Quest for the Perfect Power Connector

Our system involves a lot of actuators that are powered off a 12V source. If all actuators are stalled at once, you’re looking at about 20A of current. This demands some pretty beefy wires and power connectors if this worst case scenario ever occurs, otherwise we might burn something.

When I first started investigating power connectors, I thought of Molex connectors – like the ones you’d find in your average desktop power supply. The only problem is these connectors can actually really only handle 6-10A, at least according to MolexKits.com’s datasheets: (http://www.molexkits.com/Datasheet/76650-0176/)

On the same site, I found some connectors that looked a bit more up to the task:

http://www.molexkits.com/Type/HCS-125/

The HCS-125 connectors can handle 12-20A, according to their datasheet (http://www.molexkits.com/Datasheet/76650-0001/)

These seem to do the job, so I’ll stick with these. I realize I’ve only looked at one manufacturer, but frankly I trust Molex to make good quality power connectors.

Now the next issue is how to assemble the actual cables. On the side I’ve been looking into cable assemblies for the motor data lines, and manufacturers charge a LOT of money for crimping tools, which are designed for a specific series of connectors – they’re not universal. Take a look at this:

http://www.digikey.com/product-detail/en/58247-1/A1999-ND/117951

How ridiculous is that? $189 for a stupid crimping tool. I guess it makes sense – the demand for the crimper for this SPECIFIC line of power connectors is probably pretty low, so their profit margin has to be high.

Anyway, I expect to run into the same problem for the HCS-125 connectors. I can buy the connector housings, the wire crimp ends, but the crimper itself costs a fortune. Maybe I can get buy with soldering the wires to the wire crimps? I dunno. I’ll let you all know what I come up with in another blog post.

EDIT: It may not be the end of the world to just use pliers to crimp the wires…

Project Overview

Hey everyone,

This blog is where the Tufts Roboticists team will post updates on our progress on our Intel Cornell Cup Project.

So what is it we’re building?

The Tufts Roboticists team is building a robotic arm intended for use with any mobile robot. We intend to engineer a modular, lightweight, multipurpose arm that will allow almost any mobile robot to interact with its environment.

 

Making it Multi-purpose

To make the arm multipurpose, we take inspiration from the universal gripper arm, developed by Cornell University:

This arm is able to pick up almost any object, regardless of shape. The only constraint on the arm is the amount of weight it can pick up. Thus, this kind of arm is suited to many applications and will enable almost any mobile robot to interact with its environment.

Our goal, then, is to mount this universal gripper on a lightweight arm, better suited to mobile applications than the one in the video. Our design consists of two joints and a rotating base, allowing for horizontal extension up to 60cm. Here’s a video showing our design in motion:

Making it Power Efficient

Another goal of the arm is to reduce power consumption. Cornell’s universal gripper makes use of a vacuum to create negative pressure, allowing the balloon to grip objects. The problem here is that running a vacuum for the duration an object is picked up consumes a lot of power. We have redesigned the system to use a noncompressible fluid (water), a locking linear actuator, and a syringe to create negative pressure. This required that we changed the solid medium in the balloon from coffee grounds (what Cornell used) to fine glass beads, as coffee grounds will dissolve in water. This mechanism only requires energy to remove fluid from the balloon, and once power is cut from the syringe, it (or rather, the attached linear actuator) will lock in place. This means that once an object has been picked up, no more energy need be expended to hold onto it.

Making it Lightweight

Another important factor in the efficiency of the arm is how much weight it has to lift. Ideally, most of the work done by the arm will be put into lifting the target object. This means we must reduce the weight of the arm itself as much as possible. For this reason, the arm is being made from length of 1″ diameter PVC. Additionally, our larger servo motors will be offloaded from the arm, rotating the base plate and the first join of the arm. A smaller, lighter servo is mounted at the upper joint.

What Can It Do?

Our preliminary calculations show that the arm will be capable of the following:

  • Arm will be able to lift 2.3 kg before stall
  • Arm is 60 cm in length when fully extended
  • Base motor has ~50 kg-cm torque
  • Upper arm joint has ~45 kg-cm torque
  • Full range of motion reachable in 10-15 seconds

What are your designs?

Here is a system diagram showing all the actuators, how they are controlled, and how they are used:

2015012303091078

 

We are currently designing a shield for the Galileo which will have ports for all the actuators to plug into for data. Power will be delivered on separate boards, to protect the Galileo from high currents.

Where are you guys at?

At this point we have received all mechanical parts except a few we need to gear down the servo which will rotate the base plate. We have conducted a proof of concept demonstration of the balloon mechanism with water and glass beads, and found that the mechanism succeeded in picking up objects. We are finishing up a design for a coupling between the linear actuator and syringe. We are also designing the shield for the Galileo which will interface with all actuators. All in all, there is work to be done!