The fischertechnik Arm
This is the arm from the Industrial Robots II kit (part 96782), pictured here in its stock configuration. The model uses the 16-bit controller (part 93293), sold separately. It's a boom-style arm with 3 degrees of freedom - swivel, raise/lower and extend/retract. The gripper is also motor driven.
NOTE: Since this page was written, I've gotten the "Automation Robots" kit (part 511933). This kit is a significant upgrade over what is shown here. It includes 4 arm models and uses encoder motors with the Robo TX interface. This page will be updated soon.
For objects to move around, the kit includes 3 cylinders that are lightweight and conveniently within the operating range of the gripper. In general, a cylinder is the easiest geometrical shape to manipulate with this type of arm. It's always correctly oriented and it won't roll away, unless of course it gets tipped on its side.
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| Side View | Front View |
This kit does not come with stepper or encoder motors, instead it has only simple electric motors. Revolutions are counted by a cam-and-bumper system. You can plainly see one of them on the rear of the boom. This is a crude but effective method of odometry.
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| Rear View |
Fischer has included stepper motors in their kits for many years (part 32311) and currently manufactures encoder motors (part 135484). Unfortunately, those are not used here. Exactly why the cam-and-bumper method was chosen for this kit is somewhat of a mystery. There may be compatibility issues with the 16-bit brick. Also, keep in mind that Fischer caters to the educational market. One could speculate that this is a case of performance being sacrificed for the sake of educational value - it's visibly obvious how the device is doing the counting. All of that being said, it's satisfactory for this application and the arms works great. The extra clacking noise gives it a little character.
The cam has 4 bumps on it, in other words 8 transitions from low-to-high or high-to-low per revolution of the shaft. This gives it a resolution 45°, which is adequate resolution for this application. For example, the arm swivels with a range of motion of 180°. The base is turned by a worm gear, there are 100 revolutions of the drive shaft (= 400 clicks of the bumper = 800 transitions) to turn it through its range of motion. With this resolution, the end of the gripper can be moved within about 1/8" when the arm is fully extended.
You may have noticed a lot of wires in these photos. I believe the German word for it is "Kabelsalat" (cable salad). FT is "close to the wire" in the literal sense. For a stationary model such as this, it poses no problem and it makes the system more open. It's easy to cut wires to length as needed and it's easy to plug in non-FT devices, such as analog sensors.
The RoboPro software ships with a nice little "teach in" demo program.
This program allows you to move the arm into various poses and take "snapshots" of its state. Then you can play back the poses in sequence. This effectively trains the arm to do repeatable motions. The software automatically does basic "in betweening", filling in motion between the poses, but there is no intelligent route planning. If you're unhappy with the the route the arm took to get from point A to point B, all you need to do is add more poses in between. (Note the unusual text on some of the buttons...)
The only fault I have found with this model is that the gripper motor can easily go into a powered stall. In this case, a "powered stall" is when the motor is attempting to close the grippers more tightly than the object it's grasping will allow. This problem can be partially solved by padding the insides of the gripper with a spongy material and grasping more loosely. This is an adequate solution if you happen to know the size of the object(s) being manipulated in advance. A more general solution might involve pressure sensors or pneumatic grippers.
There is no corresponding problem when opening the grippers. When they reach the fully open position, a bumper is depressed, which can be used to signal the program to cut power to the motor. The fully open state is actually the "home" position for the grippers.
Starting from scratch, you can build a functioning manipulator arm in a few hours using only stock FT parts. The device is useful as-is, but of course can easily be modified to suit a specific project.