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For whatever reason(s) Scooba has been fitted with closed-loop speed control systems for each drive wheel. The function(s) provided by these subsystems seems not to be for controlling various speeds while navigating, i.e., once beyond the start-up spiral, as done for the Roombas, but may very well be associated with determining room-size. In this section, we take a look at only the 'front-end' of the closed-loop, motor-speed controller, viz, its armature-speed monitor, a.k.a., 'tachometer' assembly.
Strictly speaking, a "tachometer" should transform the rotational rate of a shaft into some value of revolutions per unit time; but, the term is used loosely in this document to mean only the device which provides a variable-rate continuum of pulses to a processor that may convert that pulse-train to a rate value. The purpose of this section is to show you where the tachometer resides, and what it contains.
Upon first approaching the tachometer, you will find it hidden from view by an opaque, plastic cap as seen in Figure 1. That is a view of the interior (i.e., towards Scooba's left/right center) end of a drive-wheel assembly.
Figure 1. Tachometer Assembly at Inboard-End of Motor/Gear-head is Protected
The Cap is retained by three features, there is a single TFS(2.6x1.4x8LG, #1 Phillips, pan-head), and two barbed legs formed as part of the Cap. Those legs are seen in Figure 2, where a removed Cap is shown at left; and the slots which the barbs engage, are visible in the housing-view at right.
Figure 2 also gives a first glimpse of the tachometer's parts, but a much better view of them is in Figure 3!
Figure 2. Tachometer-Cap is Retained by One Screw and Two Spurred-Flexures
Figure 3 reveals two of the tachometer's three parts, the optical-beam-chopper (wheel, with bent-over tines, or blades), and the LED (at the RHS, with anode "A", and cathode "K", marked on the PWB). The other electrical components on the PWB pertain to suppressing the motor's generated EMI. Just left of the LED (item "D1", and printed on the board, you can see markings "C" and "Q1" through two blade-slots. Those markings tell you the third part, the photo-transistor Q1, is mounted behind the chopper-wheel. That "C" stands for the transistor's "collector" terminal.
Figure 3. Tach's Electro-optical Parts and EMI-Filter Parts Share Same PWB
Figure 4 provides a partial peek at the photo-transistor; about half of its lens is visible.
Figure 4. D1-LED-Device Mounts Outside of Chopper-Blades, Q1-Photo-transistor is Inside
These side-looking LED and PT packages are identical with those used in the Roomba wheel assemblies; in fact, they probably are the same parts. Scooba owners should not be concerned about this design likeness, since there is almost zero possibility that the lenses on these devices will become dirtied by floor-debris while working. There are several reasons for that prophecy, but the major reason has to be "wet-cleaning" -- the floor-debris is solution-sprayed before the wheel-assemblies pass over debris, and wet dust won't be blown into the air to wend its way into the tachometer chamber.
Besides, it would be a safe guess that iRobot did not spend any resources toward installing anything like the Roomba-tachometer's self-cleaning routine (that resulted in Roomba-body rotation activity named 'The Circle Dance') into Scooba's processing.
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