The main characteristics of the prototype, dubbed "Sixhoe", are described in the Patent Application "Walking Machine with Backhoe Legs" by J.H. Cocatre-Zilgien (1997). It consists of a relatively simple chassis equipped with 6 backhoes as legs. Each backhoe-leg can be fitted quickly with interchangeable feet, buckets, or any other implements specific to a particular trade. The choice of backhoes as legs provides the Sixhoe with decisive advantages over previous walking machines (and over wheeled and tracked vehicles). First, they provide a wide span for its stance, and therefore a great stability of shape that allows it to climb safely on slopes not accessible to other vehicles. Second, because of their reach, they can provide a high ground clearance for its chassis, which allows it to progress over large obstacles in unstructured terrains. The high ground clearance also enables it to carry external underslung payloads, as a walking crane. Third, backhoes have evolved into versatile tools over the years, nearly acting now as manipulators; also using them as legs greatly increases this versatility. The relative lack of strength of the swing of backhoes is compensated for by a walking method that maximizes the use of their powerful boom and dipperstick actuators.
The Sixhoe is envisioned as a relatively slow moving yet agile and forceful machine, for terrains not accessible to wheeled or tracked vehicles. For fast road transport the Sixhoe is either carried on a flatbed trailer or mated to a simple tracked cart that is hydraulically powered by the walking machine it carries. The Sixhoe is to use only statically stable gaits, which are inherently safe for hexapods, and which are not difficult to coordinate by the on-board computers now available.
The multidisciplinary design and construction of the Sixhoe relies on a new combination of well-known technologies, nearly all of which are in the field of expertise of a Company manufacturing hydraulically-powered earth-moving or similar equipment. The prototype is designed to be robust, of relatively low cost, and a fully operational machine, not an experimental research vehicle. Its intended market is the mechanized access to a significant part of the 50% of the land on Earth that is not accessible to wheeled or tracked vehicles, and to do so with minimal environmental impact. Target fields of operation comprise agriculture, sylviculture, oil & mining, transportation, general utility work, military, and many others.
1. Introduction 2. Problem and background 2.1. Need to access sloped and unstructured environments 2.2. Walking machines as a solution 2.2.1. Japanese research 2.2.2. Review of some significant walking machines 220.127.116.11. The General Electric "Walking Truck" 18.104.22.168. The OSU "Adaptive Suspension Vehicle" 22.214.171.124. The Plustech "Walking Harvester" 2.3. Problems of walking machines 3. Project description 3.1. Project goal 3.2. Project objectives 3.3. Project technical justifications 3.3.1. Choice of legs over wheels 3.3.2. Choice of wide-span legs 3.3.3. Choice of backhoes as legs 3.3.4. Choice of the number of legs 3.3.5. Choice of sensors 3.3.6. Choice of control system 3.3.7. Choice of "no-fall" and other performance objectives 3.3.8. Choice of actuator control 3.3.9. Choice of Patent Application 3.4. Project marketing justifications 3.4.1. Versatility by design 3.4.2. Choice of timing 3.4.3. Choice of Company manufacturing earth-moving or similar equipment 3.5. An analogy with aircraft design 4. Work plan 4.1. Work method and organization 4.2. Work schedule and evaluation 4.2.1. Phase 1 - Team constitution 4.2.2. Phase 2 - Machine design 4.2.3. Phase 3 - Prototype construction and static testing 4.2.4. Phase 4 - Mobile two-legged version testing 4.2.5. Phase 5 - Mobile six-legged version testing 4.2.6. Phase 6 - Customer evaluations 4.3. Estimated budget Bibliography Appendices 1. Patent Application "Walking Machine with Backhoe Legs" 2. Short Description of Autopod 0.33 (built in 1996) 3. Plustech Advertisement for Walking Harvester
The main problem related to working on slopes is the ability to climb and descend on them, and to do so in a stable and safe manner. The main problem related to progressing in unstructured or rugged environments is the ability to pass obstacles such as boulders, trees, and various gaps, sometimes filled with water.
FIG. 1. Survey of the national origin of walking robot US patents.
In a survey conducted in 1996, the number of US Patents related to "walking robots" was plotted against time according to their country of origin. The results (FIG. 1) show that the number of Patents of Japanese origin appear have climbed exponentially since 1990, whereas those of American origin appear stagnant, and those from other countries are practically non-existent. Given the complication of filing patents in both Asia and the United States, this can be interpreted as a will and a readiness of the Japanese industry to "lock on" some of the key technology in preparation for selling Japanese walking machines on US soil. These Patents are indeed the tip of the iceberg, as the Japanese have dynamic research in the field of walking machines, historically led by Shigeo Hirose (1985). Other foreign countries involved in research and development of walking machines comprise Belgium, Canada, England, Finland, France, Germany, and Russia, but except for a very few, they are generally still far from manufacturing an end- product that is useful enough to be commercialized.
FIG. 2. Photo of the General Electric Walking Truck (Rosheim 1994).
FIG. 3. Side view of the General Electric Walking Truck (Rosheim 1994).
The Walking Truck was developed by Ralph Mosher at General Electric in the 1960s. This quadruped was quite spectacular (FIG. 2), especially considering it was built using technology that is 40 years old. In particular, it did not use any computer control; its on-board human operator had to directly and simultaneously operate 12 servovalves for the machine to walk. This proved too much of a burden and was enough to cause the termination of the project (Rosheim 1994). The Walking Truck was potentially one of the most agile (FIG. 3) of all the walking machines to follow, and one can only wonder what it could have achieved if the small and powerful computers and servovalves available today existed at the time.
FIG. 4. Photo of the Adaptive Suspension Vehicle (ASV) (Rosheim 1994).
FIG. 5. Side and front view drawings of the ASV (Rosheim 1994).
The Adaptive Suspension Vehicle (ASV) was developed by Kenneth Waldron and his team at Ohio State University from the late 1980s to until recently (FIG. 4). Unlike the Walking Truck, the ASV makes heavy use of computers, uses sophisticated vision and scanning systems, and has provided the walking machine community with many insights (Song & Waldron 1989). However, it can also be considered as unnecessarily complex, and expensive. Also, the lateral stability (FIG. 5) is limited by a total width of 5.2'. The pantograph leg design (FIG. 5) does not provide this machine with a clear advantage over, for example, the chassis of a typical front-end loader fitted with oversized tires. As a result, the future of the ASV is uncertain.
FIG. 6. Photo of the Plustech Walking Harvester (Plustech 1997).
The Walking Harvester (FIG. 6) was recently developed in Finland by Plustech Ltd. and is advertised as the "first working machine application of walking technology in the world" (Appendix 3), which is not far from the truth. The machine does walk and can accomplish a useful marketable work. It is still in its development stage, as technical specifications are not yet officially released, and serial production is still a few years away, according to a representative of that company. However, the Walking Harvester symbolizes the entry of private investment and industry in the field of large civilian walking machines. It may become the first successful expression of an old idea whose time has come.
In many research institutions, there is an overemphasis on the control aspects of walking (adaptive control, fuzzy logic, neural networks, etc.), elaborate classifications of gaits, and the many links with the study of animal intelligence. All are interesting from an academic viewpoint and worth investigating, but the hardware meant to be controlled in such ways just does not measure up to the same levels of effort. The fact is that a hexapod is inherently a stable structure and it can walk with a relatively simple control system. Advanced forms of control are only necessary to improve adaptability, motion smoothness, or energetic efficiency. While worth pursuing, they are only secondary to the act of walking itself.
By convenience, many walking machines are designed to walk first on a flat horizontal surface (where actually they cannot compete with wheeled vehicles), and then tested in more and more sloped and unstructured ones, where sometimes agility problems can reveal themselves to be without solution. In particular, the reaching envelope of their legs is generally too small compared to body dimensions, which defeats the purpose of legs in general. Short legs do not provide enough of a clear advantage over wheeled and tracked vehicles to warrant a new product. The Sixhoe Project expects to follow the opposite process to ensure the compatibility of the machine with its intended rugged environment. Only in its ultimate version is the walking machine of this Project optimized for flat horizontal surfaces (such as roads). This is done by partially reverting to some kind of "wheels", in the form of a removable transporting cart which is hydraulically powered and controlled by the walking machine it carries (see later part of Patent Application).
Selecting a backhoe for a leg may also be viewed as a voluntary constraint to get a useful end-product. The disadvantage of this choice is that backhoe-legs are generally not strong enough to bear the stresses of lateral swings, but this is compensated for by the way the control system employs them (see P.A. and section 3.3.6 below). The same limitation was actually encountered in Autopod 0.33 (Appendix 2). Offsprings of the Sixhoe Project may optimize the design of future backhoes made by the Company so that these are better suited to their double purpose of backhoe and leg.
Given the importance of the sensors of the Sixhoe, for safety purposes production machines may use redundant parallel sensory systems, with error reporting to the operator on a diagnostic screen. This could be a double system with automatic stopping or freezing if two readings do not agree, or a triple system with polling and rejection of any value not agreeing with the two others, as used in the aerospace industry.
Sensory input comprises about 62 analog channels, namely 6x1 foot "contacts", 6x3 joint angles, 6x6 actuator pressures, and 2 body attitudes (pitch & roll). Some of these inputs can be preprocessed by dedicated microcontrollers. Several fast analog- to-digital boards with 64 analog inputs and Direct Memory Access (DMA) are currently available to convert that information into a form directly usable by the controlling computer system.
As said earlier, the Sixhoe is envisioned as a "slow moving yet agile and forceful" machine. The relatively slow speed allows a statically stable walk, compared to what would be required for the dynamic stability of a biped. It not only minimizes the energy consumption for the acceleration and deceleration of the legs, but provides ample time for sensor data acquisition and for the actual "thinking" necessary to coordinate walking. Significantly, the motion of the legs is slow enough relative to computing speed that simple proportional control schemes can be used without risk of feedback instability and mismatched response times. In other words, the control, at least in initial implementations, is of a very conservative nature. The primary reason for this choice is that stability is the first duty of the control system, for safety reasons discussed later.
The algorithm is based on an endless loop of sequenced sensory, thinking, and motor phases as used in Autopod 0.33 (Appendix 2). Such a cycle would repeat at about 50 Hz, although the similarly slow Autopod 0.33 uses a 12 Hz refresh rate without problem. During the sensory phase, the computer collects all the data from the sensors and collects the commands from the operator. It may also display relevant information in a multimode synoptic way for the operator on a computer screen. During the thinking phase, the computer first analyzes the current forces and stability margins, and then, as a function of the commands of the operator, draws itself in a virtual "phantom" position and attitude in space in the immediate future, and re- analyzes what the new stability margins would then be. At that stage it would refuse motions that would narrow stability margins beyond prespecified safety thresholds. The algorithm then focuses on where the body-leg "hips" are desired to be positioned relative to the ground in the immediate future. This determines how each backhoe-leg is to be employed, that is either as an outrigger or as a propulsive unit, as explained in the Patent Application. Essentially, the algorithm would activate the actuators of a leg bearing weight (on the ground) so that it pushes or pulls its associated hip towards its desired phantom position, and those of a leg not bearing weight (off the ground) so that it positions its foot in a favorable location for the time it will bear weight again. During the motor phase, the computer sends off appropriate commands to servo-valves or variable-displacement hydraulic pumps used to control flow and pressure in the actuators.
FIG. 7. Maximum weight-bearing ability of a CAT 426C backhoe modified by replacement of its bucket by a foot, as a function of boom and dipper stick angles. Surface is saddle-like with a diagonal "cliff" (see text).
A double action cylinder actuator exerts more "pushing" force than "pulling" force, because of the presence of a rod on its extension side. This has consequences when using such actuators in multi-segment articulated legs. The weight-bearing force available, for example, to a CAT 426C backhoe as a function of boom and dipper stick angles is shown in FIG. 7. A clear "cliff" is apparent in that figure, delineating the combinations of angles beyond which the stick has to be pulled instead of pushed to bear weight. One of the tasks of the controlling system is to remain in a volume bound by the surface depicted in FIG. 7, both for the current positions and for the virtual positions of the immediate future, with appropriate safety margins to allow for perturbations.
A series of watchdog features would monitor the progress of the legs as a function of hydraulic pressures to detect slippery or sticky conditions without tactile information. The algorithm would have the ability to branch out in insect-like reflexes such as trial and error methods to step a leg over an obstacle, for example. For the prevention of falls, the control system would possess a "bracing reflex" (Appendix 2) that pressurizes actuators in a predetermined way if too few of the sensors that should reveal a load are stimulated, a condition that most likely anticipates a fall. Originally, the algorithm would lift only one leg at a time, the most convenient way to progress safely in a very rugged environment, but it would ultimately lift three at a time in the classic alternating tripod gait, which is the fastest on easy terrains. Computer control enables completely different methods of walking by merely changing software. Only as a complementary option, further avenues for control (fuzzy logic, neural networks, genetic programs) can be explored. Eventually this can be done by collaboration with academic research institutions, which undoubtedly will be interested in the machine as a support for their own investigations. Advanced control methods are not necessary for statically stable hexapods, unlike for dynamically stable bipeds, where they are actually required.
In agriculture, a Sixhoe can walk above the top of corn, shrubs and small fruit trees, on sloped and disorganized terrain. In sylviculture, the Sixhoe can perform logging tasks similar to those of the Plustech Walking Harvester (Appendix 3). In mining and road construction, a Sixhoe can be a rock drilling platform prior to blasting of obstacles. In precision demolition work, a sixhoe can support simultaneously a hydraulic hammer and scooping bucket feet. A Sixhoe can erect utility poles in forbidding terrains, as described in detail in the Patent Application. A Sixhoe can progress on loose debris for post-earthquake or post-explosion cleanup work. A Sixhoe can walk in standing water for rice and cranberry culture. In shoreline maintenance, a sixhoe can wade in surf with its body above the waves. As described in Patent Application, a Sixhoe can act as a walking crane in warehouses and harbors. It can also simply act as a walking excavator. It can be used for plain patrol or reconnaissance in difficult terrains (including volcano craters). However, planetary exploration, of Mars in particular, which is often brought forward as justification for walking machines, is currently too narrow and restricted a field to be part of this list. It is important to realize that standard market studies are likely to ignore many of these specific trades because of the very novelty of the product. This is why versatility (mostly conferred by the multi-functional role of backhoes) is such an important attribute of the Sixhoe. It can be adapted to any specific task.
The target acquisition cost of a Sixhoe should be of the same order of that of a standard wheeled loader-backhoe. This is important as this walking machine will be called to work in countries with much less wealth per capita than in the United States. Low cost is here part of versatility.
It is worth noting at this point that the human appeal for walking machines is very real. This was witnessed at the University of Illinois Engineering Open House exhibiting Protobot (Price 1995), and for Autopod 0.33 (1996). Depending on their size, they are regarded as pets, horses, or elephants. As a consequence, the recreational use of a Sixhoe just carrying a couple of passengers is not to be underestimated. It is a fact that US manufacturers shunned the building of All Terrain Vehicles (ATVs). They usually carry only one person and a very limited payload, and yet they were a huge commercial success in the USA (incidentally for mostly Japanese companies). The Sixhoe can be envisioned as an "artificial horse" of the future. This human appeal can also capitalized on for self-advertising of the product (section 4.2.6.).
Human beings are said to evolve in a 3-D space; however, they usually roam on a 2-D surface, and all man-built structures include flat horizontal surfaces on purpose, including roads. Most, if not all, companies manufacturing backhoes, for eaxample, happen to be located in "flat" locations, so the need for the machine described in these pages may not be initially recognized. The same applies to the potential purchasers of Sixhoes; the sale of such walking machines in Illinois or in the Netherlands will be minimal; they are not intended for such lands, but nearly all the others.
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2. Description of Autopod 0.33, built in 1996, 26 pages, 258 kB
3. Plustech Advertisement for Walking Harvester, 1997, 1 page, 29 kB