The grabber of the robot arm is opened and closed by pneumatic pressure. Therefore, I needed an air compressor that can generate a pressure of approx. 20 – 30 Psi. But how do you manage this air pressure? If the motor keeps running, the air pressure becomes too high. If you switch the motor on and off manually, you need to stay alert and watch the manometer if the air pressure doesn’t become too low. The solution: build an automatic start/stop system. In this article you can read how I achieved this.
The first prototype was a a mechanical one. A piston is used to press against a shock absorber and that movement is used to control a PF switch. If the air pressure is high enough (~ 30 – 32 Psi), the piston reaches its top position and the PF switch turns into the off position. If the air pressure drops (~ 20 Psi), the shock absorber pushes the piston back to its lower position and the PF switch is turned into the on position. The PF switch is connected to the motor that drives the air pump. In the photos below you see a working prototype and the principle of the mechanism.
In total, I created two working prototypes. In the video below, you can see the first prototype.
Both the prototypes worked fine. Apart from one major issue: how do you fine tune the mechanism that the pneumatic pressure is always between 20 – 32 Psi? I extended the lever, I added an extra shock absorber, I used different types of shock absorber, etc. But in the end, I didn’t get it right. Sometimes, the pressure came above 35 Psi resulting in the air hoses to pop-off. But the main problem was that the air pressure first came below 10 Psi before the pump was switched on again. With such a low air pressure, the grabber didn’t have enough power to get a good grip on the candy container.
I bought the PPS58-Nx pneumatic sensor of Mindsensors. This sensor is able to read the actual air pressure and feed it to a NXT or EV3 brick.
The motor driving the air pump should be switched on at the start (no air pressure yet), then building up the air pressure until it reaches the maximum of 35 Psi. When the pressure drops below the 20 Psi, the motor should be switch on again. In the picture below, the state diagram is sketched:
The air compressor is build around a Mindstorms NXT:
You can view the working air compressor with digital auto start/stop at Youtube:
With this solution, I always have enough air pressure for controlling the grabber. To avoid an additional investment of an EV3 brick, I used one of my ‘old’ NXT bricks. The solution is far more better than the mechanical version. The price is a big disadvantage if you don’t have enough bricks on your shelf.