Make Umbilical for Robot
You will need to power your motor and receive signals from your thermistor. Since we are keeping our board on the bench and outside the water, we will need a umbilical to connect your robot to the board.
1. Making your Umbilical
First, cut 11 ft of red wire, 11 ft of black wire, and two 11 ft pieces of white wire, (all stranded wire).
Figure 1. Wires needed for umbilical.
First you will have to twist the two pairs separately: the white pair of wires for the thermistor, and the red and black wire together to power the motor.
Clamp your first pair of wires in the vice as in Figure 2.
Figure 2. Clamping wires in vice.
Pull your wires taut so they twist securely. You may have to stand across the lab, so be careful!
Figure 3. Pulling wires taut.
Your wires may be uneven when you pull them taut, so just make sure to snip off the excess with wire clippers as in Figure 4.
Figure 4. Snipping off excess wires.
Now put the ends of the wire in the drill. Be sure to securely clamp the wire.
Standing as to keep the wire taut, run the drill until the wire is twisted as in Figure 5 (c).
Figure 5. Twisting the wire.
Follow the same procedure as before to get a twisted pair of black and red wire. Make sure that you twist the wire together in the same direction as with the previous pair.
Figure 6. Twisted black and red wire.
Before twisting the two pairs together, reverse the drill direction by toggling this button.
Figure 7. Flipping direction on drill.
Now you have two pairs of twisted wires. Bring these together like in Figure 8(a) and twist them together using the drill as before.
Figure 8. Twisting two pairs of wire.
You should now have about 10 feet of a cord made with these four wires.
Next, you should test the wires for continuity using a multimeter. Set the multimeter to ‘continuity’ mode (where it beeps when you touch the probes together), touch both ends of each wire with the probes and confirm that the multimeter beeps. This tells us that the two ends are connected with low resistance.
Now you need to add a pressure tube to your umbilical. Find one of our pre-cut lengths of tubing (or measure and cut a piece of tubing as long as your twisted wire cable if we're out of pre-cut tubes), and attach the tube to the cable using electrical tape. Make a connection between the tubing and the cable about once per foot of cable. Use 2 inches of electrical tape to make the connection.
Figure 9. Finished umbilical.
2. Attaching your motor to umbilical
Congrats! You've made your core umbilical. Now we have to connect it to the motor and thermistor in a waterproof manner (using heat shrink).
Get three pieces of heat shrink tubing and thread one piece through both wires, one through just the red wire, and the last piece through just the black wire (see Figure 10a). Some notes
There are various sizes of tubing and the size that works will depend on the size of your solder joint connecting the two wires. If you use the smaller pieces, keep your solder joints small!
The heat shrink will only heat a fixed amount so bigger pieces may not fully shrink over your joints. DO NOT overheat the tubing since it may melt and short you wires
Do not take too much tubing. You only need 2 inches or so depending on the amount of exposed wire (see the photos)
Solder the black wire of the motor to the black wire on your umbilical, and your red motor wire to the red wire on your umbilical.
After you soldered your motor onto the umbilical, test it by connecting the umbilical’s motor wires to a power supply. Provide 5V to the motor, and confirm that the motor spins as expected. If there is a problem with the motor’s behavior, check the connections you just made (you may have to redo them). If the connections are good, move on to heat shrinking.
Figure 10. Sliding heat shrink onto motor wires.
Do your best to align the soldered joint so that the heat shrink slides on smoothly.
Slide the red wire's and black wire's heat shrink onto their respective joints.
Use a heat gun to shrink them down.
Slide the remaining piece of heat shrink to the solder joint area, and use the heat gun to shrink this last piece.
See the Figure 11 for images of this process.
Figure 11. Heat shrink on joints of motor - umbilical.
3. Attaching your thermistor to umbilical
We similarly have to waterproof the connection of the thermistor and the umbilical . Recall that this thermistor is actually waterproof, but that refers to just the blue top, which is why we need heat shrink here too!
First, make sure you are soldering the thermistor onto the same side of the umbilical that you soldered the motor. The thermistor will go into the water with the robot and motor.
Slide two pieces of heat shrink on either wire. Then, solder the white wires of your umbilical to the ends of the thermistor as in Figure 12. Recall it doesn't matter which wire connects where, which is why we just used one color for both wires.
Be sure that you are only soldering to the silver ends of the wire. The copper portions of the thermistor legs are waterproof, and the waterproofing is insulating. Your thermistor will not work unless you solder to the silver tips at the end of the thermistor leads.
Once you’ve soldered your thermistor, test it using a multimeter to measure its resistance. Make sure you are measuring resistance, not conductivity With the multimeter probes touching the umbilical’s thermistor wires (doesn’t matter which is touching which), pinch the thermistor to heat it up and confirm that the resistance goes down, then let go and confirm that the resistance goes back up. If there is a problem with the thermistor’s behavior, check the connections you just made (you may have to redo them). If the connections are good, move on to heat shrinking.
Figure 12. Soldering thermistor.
Use the heat gun to shrink those two pieces of heat shrink. Then slide another piece of heat shrink to cover the copper part of the thermistor and use the heat gun to shrink it, as in Figure 13.
Figure 13. Heat shrink over copper part of thermistor.
Lastly, we want a large piece of heat shrink to slide over the whole joint we've made to ensure waterproofing and to tighten up the look of the cord.
Figure 14. Large last piece of heat shrink.
Congratulations! You've completed your umbilical plus the motor and thermistor. Figure 15 shows how the wire ends are put into the screw terminal, and the pressure tube in the pressure sensor.
The motor and thermistor will be mounted on your robot.
Figure 15. Completed umbilical!