Advanced Robot Arm Project
May 23, 2013
This is a long term project to develop a robotic arm that is more advanced that traditional hobbyist arms using RC servos without being so expensive that only well funded universities can afford it.
The first prototype gripper used standard RC servos while we experimented with the configuration as shown in the first video. Once I had a good gripper configuration I built a prototype arm for it as shown in the second video. This arm used a differential drive system with the goal being to keep the weight of the motors as close to the shoulder as possible.
Although the differential drive did work, it's accuracy was poor. Even with the highest quality differential gears it was never going to have the precision I wanted.
Since then I have focussed on the gripper as this is the most difficult part with a lot of precision moving parts in a small space. The second prototype of the gripper still used RC servos but I tried using steel cables to link the servos to the fingers. Unfortunately the steel cables stretched more and snapped easier than expected.
You can see in this photo of the second prototype that pressure sensors were fitted to the finger tips and palm. These sensors used QTC as the sensing element. The finger joints use magnetic rotary encoders to measure angles. The third video shows you a simple test of the second prototype.
The second prototype was also the first to use molded rubber grips. The first prototype simply used a sample rubber sheet cut and stretched over the fingers for traction. The biggest problem here is that soft rubber is needed for good grip but when handling heavy objects the screws holding it in place can pull through the rubber. The rubber skin is not a major issue right now so it has been put on hold until the mechanical design is complete.
Recently I have been working on the third prototype. Unlike the previous prototypes this gripper uses 3-phase brushless motors and a minimal gear set. These motors are more efficient and have higher torque than standard DC brushed motors. I have kept gears to a minimum to reduce play between the gears. The gearboxes also have adjustments to minimize play further. All aluminium parts have been hollowed out to reduce weight as much as possible.
The third prototype uses an I2C bus for everything. Each motor controller and all magnetic rotary encoders are on the I2C bus to reduce the wire count and make the gripper easy to control using anything from a PC to an Arduino. In fact, each motor controller is a dedicated Arduino compatible controller to make the gripper easy to hack.
I have now started work on the 4th prototype. Unlike the previous versions this will be molded from polycarbonate rather than milled from aluminium. The polycarbonate is lighter than aluminium and it quite tough. It will bend and spring back into shape in situations where the aluminium would dent.
These images from Sketchup show the basic configurations of the gripper. This first configuration is ideal for pressing a button or flicking a switch.
This second configuration uses opposing fingers as a pair of tweezers for picking up small objects.
The third basic configuration is better for picking up cylindrical objects such as cups, cans and bottles.
The fourth configuration can be used to pickup round objects such as a ball.
This is a work in progress so there will be updates.