So This week I didn't end up getting much time to work on the project. I focused on final testing of the arm and finding an alternative to running the grinder from the Pi Face.

 

For the final testing of the arm I simply wanted to run both motors at the same time to test the movement of the arm. This was successful but I found the movement to be quite jerky. The circuit I used is pictured below:

dual servo circuit

 

The purple and brown wires run to pins 17 and 27 respectively (BCM numbering), the pink wire runs to the ground on the Pi at pin 39 (Board numbering). The green and white wires run to a 5V 3A power supply. The grey, red, and black wires connect to the second servo in the same configuration as the visible connector (PWM input, positive and negative from left to right).

 

With the initial testing out of the way i moved onto getting an alternative to the relay on the Pi Face running the grinder. The Pi Face was unsuitable as i dint want to try and solder wires below the header connection, I haven't looked into the schematics in detail but according to this post some are free PiFace Digital 2 - Free GPIO pins? .

 

In my 3rd post i mentioned that i bought a mosfet which i tried to use to run the grinder but i didn't have any success I came back to this to give it another try The mosfet I purchased was an  STP60NF06LSTP60NF06L which is probably overkill for this project(the factsheet can be found here). When I initially looked at the fact sheet when ordering I selected this part as the transfer characteristics curve (figure 6 in the previous link) showed that at a gate voltage of 3.3V 40A would flow for a Vgs of 15V. This was much higher than I needed but as long as a gate voltage of 3.3V would be enough I assumed it would work [my lack of electrical engineering knowledge is showing here ]. The circuit I set up is pictured in the diagram below:

 

Mosfet circuit diagram

The physical circuit on the bread board is shown below:

Mosfet circuit picture

The purple wire connects to the raspberry pi gpio 13(bcm), the pink to the ground on the pi @ pin 39 (board), the yellow to the grinder motor and the black to the negative of the grinders batteries. This worked well and it will be the circut i use in the final build. I'm not actually sure what i did wrong last time I tried to make this circuit but at least i was successful this time.

 

Next week I aim to connect the final dots and have a complete working prototype