Mobile Base Theory of Operation

I feel compelled to include this section, beyond a simple "you plug in the motors and -- hey look! It really moves!!". Bear with me or skip to the next section or whatever you feel you must. When you read through this section, it's helpful to have the parts list or the drawings from the Design Description page handy.

The basic design is this; two Matsushita 24VDC 12.5:1 geared motors are used at a desired load of 200 oz/in of torque (using a maximum of 2A of supply current). One motor runs the steering, the other motor the drive. At the desired load they turn 180 RPMs. Each motor is connected by a cable chain (14CCF-90E) to a primary shaft (either steering or drive) using a 3:1 sprocket ratio (the motors have 20 tooth sprockets (14LC32A-20) and the shafts 60 tooth sprockets (14LC32A-60)). These shafts are referred to as the primary drive and primary steering shafts. Each primary shaft actually holds two 60 tooth sprockets. The remaining drive and steering shafts (secondary drive and steering shafts) have a single 60 tooth sprocket; a cable chain (14CCF-270E) connects the three sprockets on each shaft so that one revolution of the primary shaft gives one revolution of both secondary shafts.

Each steering shaft passes through a bearing (F-600-3) on the middle and top plates of the base (the plates are connected and separated by stand-off spacers (J218-ND, J178-ND)). The bottom end of each shaft is connected to a U-shaped wheel housing; between the middle plate and wheel housing is a thrust bearing (B5-10). One rotation of the steering shaft results in one rotation of the wheel housing.

Each transmission shaft passes through two needle bearings (NK 8/12 TN) located inside each steering shaft. The transmission shaft is held in place within the steering shaft using retaining rings (Q1-31). At the bottom end of each shaft is a 2:1 bevel gear (M32P-2) which transfers the power to the wheel shaft while reducing the speed further. Opposite the bevel gear on the wheel shaft is the wheel.

(There. That didn't hurt so much, did it?)

The "Steer Drive" Phenomenon

Someone who'd used this basic design discovered -- to their surprise -- that there were two side-effects caused by this design. Actually, I knew about one of these, but thought I had explained it (or warned readers) about it. They termed it the "steer drive" effect. It is caused by offsetting the wheels inside the drive housing. It manifests itself in two ways:

When the robot is stationary (drive motor off) and the steering motor turns on to rotate the wheel housings, it causes them to turn relative to the drive shaft bevel gears. The net effect is that when steering while not driving, the wheel itself "sees" one rotation of the drive shaft (i.e., relative to the wheel housing, the drive shaft makes one revolution). This results in the wheels themselves rotating. Since we use a 2:1 bevel gear ratio, they turn 1/2 revolution.
To solve this problem, you simply offset the wheel on the shaft at a distance which is 1/2 of the wheel's radius. When the wheels turns now, they're rolling in a little circle about the center axis of the wheel housing/drive shaft, so the motion is cancelled.
When the robot is in motion (drive motor on) and the steering motor turns on to rotate the wheel housings, it again causes them to turn relative to the drive shaft bevel gear. However, in this case, the motion may not be cancelled. It seems (although I haven't been able to verify this yet), that the robot might speed up or slow down, since the wheel is in fact turning faster or slower depending on the steering and drive directions.

So, the first problem is easily solved by placing the wheels in the correct place on the wheel shafts. The second might also be solved by this, but I haven't verified. There are other solutions as well:

Redesign the drive train such that the wheel can be centered below the drive shaft.
Compensate by coupling the drive and steering motors so that a "steering" motion drives both motors as needed.