Servo Pulse to Dual H-Bridge Interface continued...


click for a high resolution schematic

Technical Notes:
Current limiting is done by sensing the saturation voltage of the transistors in the H-Bridges. The higher the voltage, the more the transistor is hurting. If the voltage is high enough, the microprocessor senses a logic ď1Ē and shuts off the bridge until the next cycle. Simple, but quite effective. The threshold voltage is chosen by resistor value. In the above schematic you will see 22K ohms listed if Darlington H-Bridges are used. A MOSFET H-Bridge has lower saturation voltage, so use a higher value resistor, letís say 100K ohms. The 150pF capacitors spread around the circuit are for RF and other noise rejection. You might find 100pF an easier value to find, and works just as well.

Getting it Connected:
There can be some confusion getting the connections sorted out. Using Berg (0.025 square posts on 0.1 inch centers) connectors for the low-current signals is useful for switching and swapping connections around. When you first connect everything, donít connect the current limit sense wires. Plug in the throttle cable to the receiver, but not the steering cable, and make all other connections as shown. Power up and operate the throttle control on your transmitter. If nothing happens, probably the throttle channel is a different output from the receiver. Usually it is channel 2. When you find the correct channel, perhaps one or both motors will be running backward. Reverse motor connections as needed. Then plug in the steering cable and verify the steering operates correctly. If the steering works backward, exchange left and right cables between the interface and the H-Bridges. If the motors run backward, reverse the motor connections.

Connect the current sense wires to the outputs of the H-Bridges. The connection is to the same wires as go to the motors, but make the connections at the H-Bridges, not at the motors. Power up and operate the system. If you get a little sound but no real motion, you have the connections reversed. If you get operation in one direction but not in the other, you have left and right mixed up. At this point you are done. If you followed the block diagram it should all work from the start and none of this troubleshooting will be needed. The + and - symbols shown on the H-Bridge indicate the polarity when the robot moves forward.

Download:
Source code in C, version 0.7: sv2hb07.c
Binary file ready to download into a PIC16F876 chip, version 0.7: sv2hb07.hex

License:
The hardware design and software are covered under the GNU General Public License.

Warranty:
None.

Disclaimer:
Fighting robots are dangerous. You should expect this interface to turn into a killer at any time without warning. No assumed liability is expressed or implied.

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