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The Raven


Beyond the mechanical design and construction, I have spent an inordinate amount of time trying to work out the electronics for actuating the steering system. This has been no easy task thanks to my unorthodox cable idea. The simple fact is the lead screw driving each end of the truck requires in the neighborhood of 7.5 to 8 turns in order to move the steering from lock to lock [please see page 10 for details on this]. Servos that have been modified for continuous rotation are far too slow, considering the quickest only turn a 60-degree arc in 100mSec. I need speeds in the neighborhood of 350-450RPM for direct drive, and something like 2080-3200RPM for a worm-drive unit. Yikes!

Initially I wished to use a stepper motor and controller coupled to a microcontroller. The MC would handle the task of receiving the PWM signal from the RC receiver and determining the calculations for advancing the motor beyond what a servo was capable. Of course, this is no easy task for someone with little or no machine language experience. I experimented with both a Basic Stamp and an Arduino. Both proved easy to work with, however neither processor had a clock speed capable of stepping the motor up to 20kHz. Details... next page.Let me back up a bit to just after realizing the Basic Stamp was not going to work.

I spoke with a coworker and he had the idea of picking up an inexpensive RC servo to see what I could modify. The servo is very simple... the closed loop system uses a pot geared off the motor to let the little controller know of the shaft position. The radio tells the motor where to go and the differential between the motor's position and where it needs to go is corrected. With 6V or 7.2V systems, the servo moves very quickly. At that point a wonderfully simple idea popped into my head: why not disassemble the servo and put a gear reduction on the pot? It is just a voltage reference. With the pot turning slower than it was designed, the motor would be forced to spin further to arrive where instructed. Great idea, right? No...

An RC servo's power comes from combining a motor with a fair amount of torque with a hell of a reduction gear. Changing the gearing means losing torque, but it would need to be changed so the motor speed can be increased. (This could possibly work with a giant scale servo, and I may still visit that idea.) Soon after coming up with the geared-pot plan, I tossed it. The steering system on this vehicle needs much torque and I don't want to half-ass the thing. I have already spent tons of money trying to find the best method. I must be completely confident that the system will work before moving forward with gearing and mounting the motor.

The Arduino idea was promising, if convoluted. My crazy idea was to use the RC receiver to drive a servo directly connected to a potentiometer which would instruct the Arduino to spin the stepper a predetermined number of steps. This actually worked (and hopefully I can get the little video up here soon) but I found the limiting factor to be the calculation the Arduino was required to complete. As the voltage reference from the pot changed, the Arduino took that number and multiplied it by something like 2050 and then sent information to the stepper controller. This was to cause the motor to step as instructed and the result should have been similar to a servo, just much faster. Because of the lead screw, I needed some decent speed on the motor so that the steering wouldn't take all day to move lock to lock. Now, either my programming or the processor on the Arduino was the issue.

For whatever reason - and I exhaustively researched this - the stepper would not turn beyond 100RPM. I was able to tweak the code to cut the speed in half, but nothing I tried would move that motor any faster. Also, the motor speed was constant, meaning no matter how quickly I rotated the pot, the motor would remain steady. The only difference was the number of steps. With an RC receiver and servo, the PWM is relative to the radio's control. When the steering wheel (or stick, as it were) is rotated slowly, the servo does as well. Turn that steering quickly and the servo moves at what seems like lightning speed.

After working with that experiment for several days, I decided to shelf it and look in another direction. That was a bit of a shame because for me, just getting the stepper to turn at all was an accomplishment. I even posted the video on Facebook due to my pride in getting something to work. The system was not what I needed, but I succeeded nonetheless.

Cut to months later, and I switched gears yet again. After futher reading about servo motors I believe I have located a controller and motor combination which will work. The controller is programmable via software and can operate independently of any on-board computer. The motor itself has an encoder on its shaft which is desgined to communicate position to the controller. I am hoping the software will allow me to program some multiplication for advancing the motor speed beyond the PWM signal of the RC receiver. Also, the motor is geared from the factory to provide plenty of torque and that means it can likely drive the steering lead screw directly. Coupling the motor shaft to the lead screw will be simple.

So, that is where things stand right now. I have some of the equipment required for a test setup, and once I locate a triple-output power supply I can begin to program the controller and run the motor.

To be continued.


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Content Updated: Sunday, September 04, 2016 at 10:25 pdt

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