Scooba's DRIVE-WHEELS

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INTRODUCTION

Consideration will now be given to Scooba's driving wheels. This page reviews the wheel-assembly's independent suspension features, and discusses what must be done to dismount the assembly from the robot's lower-chassis assembly.

Suspension Action and Mechanics

Upon initial inspection of each of the robot's external features and characteristics, it requires only brief consideration of the drive-wheel assemblies to detect several obvious design elements:

  1. Drive-wheels are independently suspended (well, yes... but so are Roomba's, so what?), no biggie, they are just different. Unlike Roomba's design, Scooba's suspensions have pivot-axes running fore and aft rather than transverse. Any advantage gained by that change, may very well relate to an overall 'packaging' optimization.
  2. The wheel-assembly appears to be composed of a motor, its reduction gearing, and a wheel, all in a common package.
  3. The assembly's package is of such size and shape that it cannot, sensibly, contain any belt-driven shafting.
  4. No evidence of any 'wheel-drop' sensing tactics can be seen from outside the robot.

Moving on then, we see in Figure 1 the left-wheel has been allowed to 'drop' (actually assisted down by a leaf spring) to the 'wheel-down' position; and it is obvious that the assembly's pivot point is off toward the middle of the robot.

Figure 1. Left-Side Drive-Wheel Limit-Stopped at Wheel-Drop Position

Downward travel of the wheel-assembly is limited by spring-arms which have been formed in the forward and aft sides of the cast-plastic motor-housing. One of those arms can partially be seen in the photo (just aft of the rectangular notch). The aft-side spring-element, along with a shorter type, will be seen in entirety in later figures.

Figure 2 illustrates the normal position of the wheel while Scooba is cleaning a floor. Essentially, only the tire protrudes below the belly-pan.

Figure 2. Wheel Forced Into its Operating Position

The weight of an empty Scooba is sufficient to overcome the force of the wheel-drop spring and cause the wheel assembly to tuck up inside the robot.

Wheel-Assembly Dismount and Other Details

Upon removal of the robot's upper-chassis (separating the chassis), the entire Wheel Assembly, and its pivots, can be seen in the following figure (3).

Figure 3. Interior View of Wheel-Assembly in the Working Position

The leaf-spring on top of the gear/motor housing is the Wheel-Drop Spring. Normally the spring's upper tips react against the upper-chassis casting.

Next, removal steps take the Wheel Assembly out of the lower-chassis. The first three steps partially lower the assembly and disengage pivots:

Figure 3 shows the resultant position of the Wheel Assembly after those actions. Notice the short spring-arm which is in contact with the inner surface of the lower-chassis. That one, and it forward partner must then be bent towards each other so they clear the chassis opening.

Figure 4. Wheel-Assembly Position After First Three Operations

Prior to further action, another view of the dislodged motor-assembly is given in Figure 5, to better see how much of the assembly is below the lower-chassis at this point.

Figure 5. Same Condition as in Fig.-4, Viewed from Rear

Upon pinching the two smaller detent tabs toward each other, the motor-assembly is set free from its chassis mount and can be lowered to the table, as in Figure 6. Both types of detent, or limit-stop spring-tabs be be clearly seen here.

Figure 6. Wheel Assembly Lowered Out of Chassis

In addition, the image in Figure 6 clearly illustrates the sprung, stub-pivots, and the various steps / jogs which constitute the edge-profile of the aperture through which the motor-assembly just passed. Notice that the wheel had been removed at some point. That condition is incidental, and not critical to the removal process just outlined.

Now that the Drive-Wheel Assembly is out of the chassis, we can turn it around and have a good look at it. First, pay attention to the fact that is remains tethered to the chassis by its six-wire harness. There is no getting around that, without lifted the harness all the way to the main PWB -- where a pigtail plug-cap will be found under the sealed-cover of the Control Module.

Figure 7. Motor/Gear-Head Assembly -- Wheel Dismounted

Breaking the Control-Module's Cover-seal should be reserved until necessary, so additional examination of the wheel-assembly will be done while it remains tethered. Figure 7 gives us a nice view of its aft face, and from that the following features, from left to right in the picture, have been noted:

  1. A short stub-axle is seen poking out of the cylindrical, final-reduction-gear housing, and with a four-screw flange (two of which are visible) interface to the main-housing.
  2. An intermediate flange, sandwiched between the final-stage and high-speed reduction-gears housing, will be seen (in Motor-Speed Reduction Gearing) to support the final stage of compound gearing.
  3. From that intermediate interface over to the first vertical slot, is the section of housing which contains the higher-speed compound gearing.
  4. The motor-mounting bulkhead is at the RHS of the gears' housing.
  5. Two vertical slots in the housing profile the wheel-drop limit-stop / release-tab.
  6. A little farther right, angled slots define the secondary limit-stop / release-tab.
  7. The I-W-suspension's pivot-bearing hole is next.
  8. Within that hole, a peek at the wheel-drive motor is provided. It appears to be one of the Mabuchi designs, but is sightly different than those used to drive Roomba's wheels.
  9. Just below the pivot-hole, and a little right there is a short, vertical slot. That is a detent-slot, which receives one of two retaining-clips for the black-cap at the far end of the assembly.
  10. Black-cap with harness wires emanating from it. The cap covers the Motor Speed-Control Tachometer assembly. The harness contains two conductors for the motor-power, two LED leads, and two photo-transistor leads.

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This page is currently maintained by G. Plews