More than one way to skin a Cat
The trouble with a bot having automotive style steering is that trying to drive it as if it were "tank" style steering is not very effective. That is, trying to drive the two rear wheels in opposite directions simply "drags" the front wheels sidways across the pavement. This not only produces excessive where on the poor little plastic tires but as the vehicle weight increases the motors don't have near enough umph.
Besides, Power Wheels has already provided us with a pretty decent steering mechanism
consisting of two King Pins, two Steering Arms, and a Connecting Rod. Very
much like you would find on a conventional automobile. Granted, the King Pin angle and the centerline
of the tire do not converge at the road surface, thus still having significant sideways tire drag
across the road surface when turning, but it's not all that bad while the vehicle is in motion. We can't
go too far wrong with the existing hardware, but we will need some easy way to move the connecting rod
side to side, about 5 inches total from lock to lock.
There is always more than one way to skin a cat. I prefer method number forty-seven, use a power sander. But that's getting a bit off topic... Off hand I envision several different ways to accomplish positioning of the connecting rod:
- A worm gear with a travelling nut to pull the connecting rod side to side. This is very similar to Rack and Pinion steering on a conventional automobile.
- A winch and cable where the winch pulls a cable, attached to the connecting rod, through pair of pullys located at either end of the travel.
- A sailboat servo that works basically the same as the winch but is controlled differently.
- Or a gearhead motor that directly couples to one of the king pins and twists it into the desired position.
They all have their benefits and their drawbacks. Let's look into each possibility...
Worm Gear
At a local Ham Radio Swap-n-Shop I chanced upon this worm gear assembly. It consists of a 12-Volt DC motor, reduction gearbox, and a 10-inch long threaded shaft. On this shaft rides a nut. A good deal for only $20 bucks!
The nut would be attached to the center of the steering connecting rod. But as the wheels are turned through their range, the steering arms follow an arc and thus the connecting rod will move closer to, or farther from, the front of the vehicle (see the Steering Geometry diagram, above). This means that the worm gear cannot be mounted too rigidly to the chassis but must move forward and back freely. Or, it must be able to pivot at the motor end as well as where the nut attaches to the connecting rod.
A concern with the worm-gear design is limit sensing. There is alot of mechanical advantage in a worm gear. If you drive the motor too long, thus extending the nut too far on the shaft, the steering linkage will reach its mechanical limits and be damaged. So you will need some method to detect when you approach these right and left extremes, turning OFF the motor BEFORE you do damage.
Another problem with a worm-gear drive is that the worm gear (screw shaft) is exposed to dirt and dust. Since it is lighly lubricated this makes it become a dirt magnet! As the nut travels across the screw the dirt gets clogged up in the threads of the nut, leading to excessive wear and eventual slop or loosnes in the system. With regular maintenance this can be avoided, but who wants to clean the worm-gear after every use.
Winch & Cable
One of the other members of the YAAARC had purchased some of these winches from All Electronics. His plan was to use them on his own GP-Bot. Perhaps later I'll be able to publish how his winch assembly is designed and how well it performs. But here is how I tried to do it...
Pittman # GM8712G595-R2. 10:1 gearhead motor attached via worm drive to winch
assembly. Motor rated 19.1 Vdc. Operates between 12-24 Vdc. No load ratings @ 12
VDC, 190 MA: Motor speed, 450 RPM. Winch speed, 30 RPM. Winch pulley diameter,
1.5" Will lift 2.5 - 12 lbs depending on voltage. Overall dimensions: 4.95" x 2.5"
x 1.95". 4" wire leads. There are 4 side mounting hooks on the winch. Removed from
equipment.
CAT# DCM-112
Available from All Electronics, Inc. for $17.00
A problem that I ran into was that the 19.1-Volt motor being driven by 12-volts was unable to deliver enough force. Even with all the gear reduction it was unable to reliably move the steering while the wheels were under the weight of the vehicle. But it worked GREAT when the wheels were jacked up off the ground!
Applying 24-volts to the motor delivered a bit more force but was still inadequate to move the steering reliably
. But I must say, it was a mighty pretty piece of hardware...
Sailboat Servo
2.3 x 1.1 x 2", 3.9 oz.. Huge power for big sails: TEN pounds of pull: 161 oz./in. torque! Heavy duty gears, ball bearing. 1-1/2" diameter drum, capable of 3-1/2 rotations. Each rotation can let out or take in 4-3/4" of line - over 16" of line movement! FAST - it turns one full rotation in 1.62 seconds. Has Hitec connector which will fit Hitec, JR and Futaba. Order HLAE259 connector converter to use with an Airtronics receiver or swap wires on the Hitec connector.
Available from Hobby Lobby for about $70
Gearhead Motor
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Third "Task" - Go Around a Pylon Contest
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