Lego NXT-Servo Controlled Pnuematic Switch

Sparramc has a video of how he is using an RC servo motor to operate a pneumatic switch. 

I think we are all craving a really good way to control pneumatics with a NXT.  It would really add a whole new dimension to our robots.  The way that I do it is rather than using a servo as seen in the video, I use a medium PF motor.  Then I control it using the HiTechnic IRLink.  I have tons of luck with it, but one drawback is that I cannot stop the motor in the center position as sparramc has done. Then again, I haven't found the need to do so.  Check out an older video of mine.  You can see how I did it at about 0:38 into the video.

I am currently using four of these switches to operate the grippers on my Flexpickers.  The bad thing in my case is that each robot is run by its own NXT and I had lots of problems because I only had two IRLinks sensors to operate four grippers.  But after I ordered two more, things went very smoothly. Just turn the motor on for about one quarter of a second, and it works great.

Two Motors-One Rotation Sensor

As I mentioned several times before, I really like the PF motors. I like everything about them except the fact that they don’t have built-in rotation sensors. So I thought long and hard about how to incorporate the old RCX rotation sensor into the drive wheels of a bot. I am not crazy about using up the input ports with RCX rotation sensors. The idea I came up with is to read the rotation of two motors with a single rotation sensor.

(Click to Enlarge)

I did it by running a chain from the driving axles of each wheel back to the axles of the differential gear. Then the outer gear of the differential gear turns the rotation sensor. As each wheel turns, the rotation of the driving shaft is transferred back to the sensor through the differential gear. There is no power “leakage” from one wheel to the other because the rotation sensor easily turns and doesn’t transmit through the differential. The lightweight chains can be used because the power transmitted is very small. I ran it around the floor some and it seems to work.

Pretty nifty, but interpreting the sensor output takes some thought. First, when both wheels are turning, the readings are twice as fast as if only one wheel is turning. This is because of the nature of the differential gear. Not really a big deal as long as the programming takes that into account. Second, if the wheels are turning in opposite directions as in a “zero radius turn,” the reading will not change because the axles on the differential gear are turning in opposite directions. Again, you have to take that into account with your programming. Third, if both motors are turning at different speeds, the reading doesn’t make sense. I think that this condition can be avoided most of the time. So for a general reading of straight forward and reverse, left and right, this crude solution should work just fine.

I like this solution for a few reasons. It means that the driving wheels can be controlled by the NXT and the HiTechnic IRLink. You will have used only two input ports while leaving three motor ports open for other tasks! They can also be controlled by the IR remote control although the sensor readings won’t make sense. So you have an intelligent robot that can drive itself and you also have a robot that you can drive remotely without making any hardware changes.

Lego Stewart Platform

Here is my version of a Stewart Platform. Stewart platforms have applications in machine tool technology, crane technology, underwater research, air-to-sea rescue, flight simulation, satellite dish positioning, telescopes and orthopedic surgery. (from Wikipedia).

It is powered by six medium PF motors with six linear actuators and three IR receivers. It is controlled by a single NXT with the HiTechnic IRLink. My version doesn’t have any feedback, so the motors linear actuators are turned on and off based on time. It took a few hours of research to get the linkages and joints down, but once I did that, building and programming took only one night.

Movement is sloppy because of the PF motors. I want to try to build this using servo motors, but mounting the huge motors is a problem.

UPDATE:
Here is what a well built Stewart Platform should look like...