Roomba sweeps low-cost parts

To view an on-demand seminar featuring the Roomba Discovery robotic vacuum, click here.

The flying-saucer-shaped Roomba Discovery robotic vacuum, at the upper end of a range of Roomba devices from iRobot, features autoreturn to a home charger base when the unit is finished or low on juice. A wireless remote control included in the Discovery package allows operation from up to 20 feet away.

Cleaning aspects of the Discovery rely on a primary beater bar and brush design along with a perimeter whiplike brush to get out to the edges of the floor boundary. A bagless dirt bin collects all debris. Though lacking the high suction of a conventional upright, the Discovery did a decent job of collecting grime in our very limited test run.

Figure 1: Inside look at the Roomba (click on image to enlarge).

For mobility, two drive-wheel mechanisms combine with a third idler wheel. All three have basic spring suspensions (likely used to deal with automatic height adjustment). The drive-wheel assemblies provide locomotion and are based on simple dc motors, but wheel control is a bit more sophisticated than a simple on or off. An optical chopper within the wheel assembly housing detects rotation, monitoring wheel speed by way of counting the interruptions a wheel vane makes in an emitter/detector optical circuit.

Along with providing the feedback needed to control steering (by spinning one wheel faster than the other), the monitoring may form the basis for creating an internal “map” showing which areas of a room have been cleaned and which still require attention. By tracking its effort, the Discovery could ensure that autonomous vacuuming is not just a random walk.

Much of the science in making a usable robotic vacuum rests in knowing the surroundings. An edge-detection “bumper” and switches on the wheels flag when the Roomba is up against an obstruction and sitting on a surface, respectively. Perhaps more important, the device uses a set of four IR emitter/detector pairs around the perimeter to detect when the Roomba might be hanging out over a “cliff,” thus enabling it to avoid a damaging tumble down a flight of stairs. When the unit is on solid footing on a floor, light from the emitters will bounce back up to the detectors. If an open space appears below one of the emitters, however, no light will bounce back to the detectors, signifying potential trouble and letting the Roomba back up or stop to save itself. Simple, cheap and clever.

Also clever is a “dirt detect” feature. Because some areas of flooring are inevitably dirtier than others, the Roomba tries to spend most of its time where the most dirt exists. Once dirt is adequately removed from a specific location, the detect feature lets the Roomba know it can move on.

The sensing solution here is a wonderfully low-tech affair. The Discovery incorporates a pair of what are basically piezoelectric microphones, in this case applying the cheap piezoelectric disk speakers (the same kind found in toys and talking greeting cards) in reverse. The piezos' surface faces outward into the cleaning cavity on the unit's bottom, where they “listen” for dirt, generating small voltages when the surface is pelted by dirt and particulates. When these dime-scale pickups and their associated op amps generate a flag in the cleaning area, it means more cleaning is needed. When things go quiet, the Discovery infers that all debris has been collected and moves on.

For all of the sensor monitoring, motor drive, algorithm processing and general control needed in the design, the electronics are surprisingly concentrated. A single advanced 16-bit microcontroller from Freescale, the MC9S12E128CPV, runs the show. Along with the CPU core, the part contains 128-kbit flash, 8-kbit RAM and a number of peripheral blocks, such as timers, D/A converters, multichannel A/D converters and several serial interfaces. An on-chip voltage regulator generates the internal digital supply voltage (2.5 V) for the MCU. Aside from a few signal-conditioning op amp packages and a liberal set of power transistors for motor drive, only discrete components populate the 22.5 mm x 5-cm controller board.

The Discovery package also comes with a charging dock containing an IR emitter. The emitter serves as a beacon for the Roomba that lets it automatically find “home base” to park and recharge when the time comes.

Two battery-powered pods are also included with the Discovery, creating a virtual wall that keeps the vacuum from working in areas considered off limits.

All of the peripherals contain small circuit board assemblies of their own that make use of low-cost chip-and-wire assembly. The charging base also contains the extra circuitry needed to handle charge control for the vacuum's 14.4-V NiMH battery pack, which contributes all system power and a sizable piece of the Discovery's weight.

Much of the complexity in the Discovery design is electromechanical, particularly the integration of sensors, switches, motors and their associated cabling. To put it bluntly, the wiring complexity is ghastly. Cost-effective final manufacturing is only possible under the assumption of low-cost labor (the device is made in China).

The Roomba Discovery brings a primarily algorithmic value-add to the basics of motor drive, obstacle detection and dirt removal to create a very practical sub-$300 home robot. Inexpensive sensor solutions, multiple-role MCUs and low-labor-rate final assembly bring it all together.

Component Focus
The Roomba is a poster child for interconnect complexity. While semiconductor complexity is largely focused in the Freescale multiperipheral microcontroller, the rich set of sensors and outlying subsystem boards mean lots of cable harnesses with plug connectors back to the central circuit board. Because many of these cables serve the drive and vacuum motors-all of which have power drive back on the main circuit board-high-current noise management is an issue, and electrolytic filter capacitors from Samxon (a division of Man Yue Electronics of Hong Kong) keep things quiet.

Noise management is evident again on the dirt-detection board, where low-level signals from the piezoelectric pickups must be kept pure. The design uses a pair of pricier, but more effective, tantalum capacitors from AVX for keeping the dirt out of dirt detection. Overall, the strong mixed-signal nature of the design creates high demand for other passives throughout the design.

David Carey is president of Portelligent (Austin, Texas;, which produces teardown reports and related industry research on wireless, mobile and personal electronics.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.