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Friday, 13 March 2009

Digital Speed Control

My design is based around three parts:

1. The controller board. This is a fully digital circuit that takes the 1ms to 2ms pulse from the receiver and converts it into a pwm train at 1Khz. It uses six cmos ics (74hc and 40 series) and a 4Mhz crystal clock. The only other components are one resistor and two capacitors to complete the crystal clock and a capacitor across the supply for smoothing. This was built on a printed board measuring 2 x 2.25 inches using standard components (on the boat there was no shortage of space). If surface mounted devices are used, the lot can be crammed into a much smaller space. The circuit can give a resolution of 128 steps (7bits). Some day I will expand it to have reverse function, but this is better done by a switcher circuit supplied from another channel (my reciever can give 7 channels and I am using only two at present).

2. The power board. This is the collection of mosfets and driver. It was built on a separate board so that I could experiment with different configurations and power densities, without touching the controller. My initial circuit used eight BUZ11 mosfets (4 each motor). The choice was because these transistors were easier to obtain. The circuit remains the same for transtors with higher current ratings.

3. A BEC circuit so that only the main battery is used. This is a switched mode power supply to supply the 5V to 6V needed by the receiver and servos, thereby eliminating the reciever battry. The circuit is based around the LM2576 and uses the minimum component count. This circuit can supply much larger current than ones based around linear devices, without getting hot, even on 14 cells.

Plans

Schematics and pulse diagrams in Adobe Acrobat (PDF) format

Schematics and pulse diagrams in PowerPoint 4 format

PCB layout in AutoTrax format

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