Team BeligerAnt's
Tech Talk
R/C Mods
If you are handy with a soldering iron, here are some mods for R/C gear that you may find useful. Note that all these mods will invalidate any warranty your equipment may have. These instructions are provided "as is" with no warranty express or implied - if you mess up your gear, it's your fault! If you are not confident in your ability to carry out these mods then you probably shouldn't attempt them!
Tx Mixer
To control the "standard" two-servo tank-steering drive used for ants, you will need a "channel mixer" to mix the fwd/rev and left/right signals from the transmitter into seperate fwd/rev signals for each wheel. The mixer can be at the transmitter or the receiver, but for ants - where weight is a big factor - a Tx mixer is preferred. Some (more expensive) transmitter have a mixer facility built-in, but mine didn't so I built this little circuit which works fine in my Hitec Ranger.
First, some theory: To go forwards, an ant needs the left-hand-side motor to rotate anticlockwise, whilst the right-hand-side motor rotates clockwise. To turn left, the left-hand motor slows down, and the right-hand motor speeds up. Conversely to turn right, the left-hand motor speeds up whilst the right-hand motor slows down.
So, if the Tx has Ch2 = throttle, Ch1 = steering, and the ant has Ch1 = left wheel, Ch2 = right wheel, we get the following requirements:
| Direction |
Tx Ch2 (throttle) |
Tx Ch1 (steering) |
Rx Ch1 (left wheel) |
Rx Ch2 (right wheel) |
| Forwards |
+1 |
0 |
-1 |
+1 |
| Backwards |
-1 |
0 |
+1 |
-1 |
| Turn left (spin) |
0 |
-1 |
+1 |
+1 |
| Turn right (spin) |
0 |
+1 |
-1 |
-1 |
| Forwards left turn |
+1 |
-1 |
0 |
+1 |
| Forwards right turn |
+1 |
+1 |
-1 |
0 |
| Backwards left turn |
-1 |
-1 |
+1 |
0 |
| Backwards right turn |
-1 |
+1 |
0 |
-1 |
| Notes |
|
|
|
|
| +1 means |
Full forwards |
Hard right |
Full clockwise |
Full clockwise |
| 0 means |
Stop |
Straight |
Stop |
Stop |
| -1 means |
Full reverse |
Hard left |
Full anticlockwise |
Full anticlockwise |
From the table, we can see that Rx channel 1 (the left wheel) needs to be driven by (minus) the sum of Tx channels 1 & 2, and Rx channel 2 (the right wheel) needs to be driven by the difference between Tx channels 2 & 1, with both channels limited to a maximum value of +/-1. (If you avoid the limiting by driving each channel with half the sum and difference, then it still works, but when going in a straight line the motors will only run at half speed, so your ant will be unnecessarily slow.)
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The circuit is fairly simple, but there are a few things to be aware of. The controls on the Hitec Ranger are wired via changeover switches to +5V and 0V, so one end of the pot' is at +5V, the other end is at 0V. Hence, U1 must be a rail-to-rail input and output amplifier - the LMC662 is cheap and stocked by Farnell. The resistors can be any type you like - I used 1/8W to keep the size down. RV1 and RV2 are used to trim out any offsets in the circuit. Start with them at the mid-postion then adjust them slowly if necessary. C1-C4 are to keep RF out of the circuit. C3, C4 could be reduced to 22pF or so, but I found 100pF OK.
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I fitted the circuit onto a small piece of matrix board which tucks into a handy space in the Tx.
| This is a view of the connections to the PCB in the Hitec Ranger Tx as you see them when you open the case. Disconnect the red wires from the pot's at the PCB end and connect them to the inputs of the mixer as shown in the schematic. Then connect the mixer outputs to the PCB. Some handy spare power connections are provided which are ideal for powering the mixer! |
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Troubleshooting
I will assume that you can build and troubleshoot a simple circuit like this without too much help from me! :o) However, there are some things to trap the unwary.
If you try to use a digital multimater to read circuit voltages, remove the crystal from the Tx first! Otherwise, your DMM will pick up the RF from the Tx and give some very strange readings! Alternatively, use a good old-fashioned moving coil meter.
Don't forget to set the trim pot's RV1 & RV2. Set any trims on your Tx to centre, then set RV1 and RV2 to centre, then adjust RV1 and/or RV2 to stop the servo motors. I used multi-turn pot's, if you use single-turn ones you may need to add 10k-100k resistors to each end of the pot's to reduce the amount of trim and give you finer control.
If your ant works in reverse or steers back-to-front or the throttle controls direction and vice-versa, you may need to switch one or both channel's servo-reverse switches and/or swap your servo connections to your Rx. Remember there are 6 possible combinations, so take your time and eventually you will get it right!
Email me with any comments you may have to make this article more useful.
gary.aylward@ntlworld.com
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