Posts Tagged ‘Hexapod’

1976 – OSU Hexapod – McGhee (American)

Earlier 1976 version sans stereo cameras.

See a few seconds of McGhee's OSU Hexapod in motion in my walking machine compilation video clip.

Stop Press! 20 Oct 2010: Just found fabulous footage of this walker plus others. 50meg download. mp4 runs for 16 mins. see here.

McGhee used Electric drill's to power the legs, similar to his earlier 'Phoney Pony'.

From Basic Robotic Concepts by John M. Holland, 1983.

The OSU Hexapod
At Ohio State University, Robert McGhee and his associates have spent a great deal of effort in perfecting a six-legged insectlike drive system called the "Hexapod" . This system is not the first of its kind to be constructed. That distinction probably belongs to the General Electric "Quadruped Transporter," which was a manually operated four-legged walker developed in the late 1960s. In fact, McGhee himself developed a four-legged walker at the University of Southern California in 1967 [RH- see 'Phoney Pony' link above]. The OSU Hexapod is, however, far more sophisticated in its control than any of these earlier projects. At this time its only rival exists in the Soviet Union, but a similar machine is under construction in Japan.
The Hexapod is being used to develop and test control hardware and software. The system is not intended to be independent, and so it is driven by ac line power through the use of triac controls. The Hexapod is kept tethered and is made to walk short distances over obstacles. One of the forelegs is instrumented with a set of strain gauges and is used for the more complex functions. This is in keeping with the very well proven research axiom of simplifying the problem to the basics.
As with most basic research, the most promising thing about the Hexapod is not the device itself, but the new concepts that are being generated and/or verified during its development. One of the most fundamental concepts that has been studied is "active compliance." This concept arises because position control alone is not sufficient for a walking machine since the sensor systems of the robot will not likely know the exact level of the surface of the ground at the point of contact of each foot. Additionally, the surface firmness may vary greatly under different feet. For these reasons, the control algorithm must include the force being exerted on the surface as part of the feedback loop. This is done by adding a second term to the error feedback signal of the control loop.5 For a rotational joint, the simple position error is

     ERROR = K X (θc — θ)
     θc is the commanded angle, θ is the actual position,
     K is the feedback gain.
     This can also be stated as
     ERROR = K X Ea
     Ea is the angular error.
With active compliance, a torque term is added and the equation becomes:
     ERROR = (Ka X Ea) + (Kt X Et)

In this case, the angle and torque are both commanded, and the total error is taken to be a combination of the torque error and the angular error. To understand this consider the case of the vertical plane hip joint of a leg as the leg approaches contact with the ground. If the angular error reaches zero and the foot has not yet touched the surface (the ground is lower than the robot expected), the leg will not stop moving. This is because there is still an error signal to drive it. This signal is supplied by the torque term. Since the foot has not touched the surface, this term has an error contribution to the whole error. Conversely, if the leg touched down prematurely, it would not move all the way to the commanded position as the load torque term would go positive after the torque target was passed. This means that the robot trades off torque for position. This same process is used in the velocity and acceleration control loops of the robot. The ratio of the gains of the two terms (Ka and Kt) gives the compliance ratio. McGhee has found that this factor should best be adjusted for the roughness of the ground. It should be noted that compliance is necessary in the horizontal plane of control as well as in the vertical plane.
Other interesting facts have come to light during the development of the Hexapod. According to McGhee, a walking robot should be more efficient on soft surfaces (such as sand and mud) than a rolling machine. This is because rolling machines (treads included) generate a bow-wave effect. This continuous displacement of material all along the path of motion represents a significant power loss. In actual tests, however, walking machines are far less efficient. This is due to several causes, including the use of worm gears in the joints. McGhee has noted that for a walking robot to be efficient, it must recover the kinetic energy from a limb as it slows the limb's motion relative to the body (especially in the unloaded arc of its movement). Worm gears and most common hydraulic controls are not capable of doing this. Additionally, neither of these is very efficient in the first place! Direct-drive motors with back emf braking and power recovery may provide a partial answer to this problem in the future.
It should be noted that the translation between the desired cartesian forces, motions, and positions and their angular counterparts is nontrivial. The term proprioceptive is used to describe the joint control, and the term exterioceptive is used to describe the vector ground reaction forces. McGhee used Jacobian transforms to develop the relationships between these two systems, but the explanation of these is beyond the scope of this discussion.

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1976 – “Masha” Hexapod – Gurfinkel et al (Soviet)

Masha being used in some force-feedback experiments. The experiment here to feed a cylinder into an inclined funnel.

See Devjanin-Schneider paper here.

The above three images show experimentation by Gorinevsky.  His paper is available here.

Gorinevsky produced a video of the walking machine. After many media transformations, the quality is poor. See here.  

The above  image, I believe, is incorrectly attributed to Bessonov.

Note also the due to incorrect spelling from the old “Walking Machine Catalog”, some sites know this walking vehicle as “Mascha”.

“Masha”, a hexapod walking vehicle an control system designed at the Institute for Mechanics at Moscow State University and at the Institute for Problems of Information Transmission at the USSR Academy of Sciences.

See other early Steam Men and Walking Machines here.

1969-72 – Six-Legged Walking Machine – Mocci, Petternella, Salinari (Italian)

Six-legged Walking Machine by Petternella et al. (Instituto di Automatica, Roma, Italia)

Mocci, U., M. Petternella and S. Salinari (1973), "Experiments with six-legged walking machines with fixed gait"

Vukobratovich M. Shagayuschie roboty i antropomorfnye mehanizmy / M. Vukobratovich. – Moscow : Mir, 1976. – 544p.
M.Peternella (Rome, Institute of Automatics) with team of colleagues created the six-legged walking machine with electric drives. There is the interesting constructive decision for legs: hip joint was made in the form of hinge with lateral axis (single degree of freedom), and telescoping of shank is made in area of knee joint. Thus, motion of leg can be the same as one of articulated leg. This model of walking machine is able to do the straight-line movement only. The further improvement is planned.

Kozyrev Y.G. Promyshlennye roboty : Spravochnik / Y.G. Kozyrev. – [2nd edition] – Moscow : Mashinostroenie, 1988. – 392p.
The experimental electromechanical six-legged machine equipped by extremity, which have two degree of freedom – first is rotary (femoral joint), and second (knee joint) has the telescopic sliding structure.

Thanks to Vadym Shvachko in supplying the extra information.

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1980 – Hexapod – J.J. Kessis (French)

Teleoperations and robotics: evolution and development Jean Vertut, Philippe Coiffet – 1986

J.J. Kessis at the University of Paris VII developed an interesting vehicle with six articulated legs, with a pantograph, allowing coordination to be carried out mechanically in the plane of the leg. The high compliance of the chassis is required for turning and non-planar ground, which means that good mobility on ground is not possible.

Real-time control of walking By Marc D. Donner

Kessis has constructed a similar six legged machine in France and it is the subject of continuing research. This machine has six legs with two controlled degrees of freedom each. The problem of permitting the feet to move in the direction of the third degree of freedom in order to permit turning is handled by making the lower legs thin and flexible and letting them bend to comply with the terrain. Control is open loop, with the approach taken to gait control based on the analysis performed by Bessonov.

1983 – “Six-Legged Hydraulic Walker” – Ivan Sutherland (American)

1983 "Trojan Cockroach", a Six-Legged Hydraulic Walker by Ivan Sutherland.

The Sutherland Walker was a six-legged all-terrain robotic designed by Sutherland Sproull Associates with the Robotics Institute at Carnegie Mellon University under contract to the Defense Advanced Research Projects Agency (DARPA). The robot used a gasoline motor to power its legs and required a driver to operate it via foot pedals. The machine would keep three legs on the ground at any given time, eliminating the need for balancing systems, though also limiting the speed of the system. Each leg could move forward, backwards, and side-to-side, allowing it to navigate across uneven surfaces.


Image found at Adam Megacz images here.


Marc Donner driving the walker "hands-free!"

See the complete Scientific American article pdf here.

From D. J. Todd's book "Walking Machines" (1985)

Sutherland's Hexapod
This machine, designed by I.E. Sutherland of Carnegie-Mellon University and Sutherland, Sproull and Associates, is significant in being the first man-carrying computer-controlled walking machine (Raibert and Sutherland 1983; Sutherland and Ullner 1984). Its design is also interesting for its use of a leg geometry and hydraulic circuit design intended to reduce the control burden on both computer and driver by automatically coordinating joint motions in ways suitable for walking.
The hexapod, whose basic geometry is shown in Figure 6.7, is about 2.5m long and the same width. It weighs about 800kg and is powered by a 13kW gasoline engine driving four variable displacement pumps. The walking speed in the alternating tripod gait is 0.1m/s. It can also walk sideways at rather more than half this speed.
It has an unconventional arrangement of its hip actuators. Two cylinders are mounted in a 'V' above the leg. It is possible to set the valves so that as one shortens the other lengthens in such a way as to produce horizontal movement, whereas if they both move in the same sense they move the leg vertically. (A third actuator for the knee produces sideways movement.) This hip arrangement is one instance of what Sutherland calls a 'passive hydraulic circuit'. Such circuits achieve joint coordination in two ways. First, actuators are sometimes connected together in series so that as oil flows out of one it must flow into the next. This forces the actuators to move the same amount. Second, if two or three are connected in parallel they will automatically share any applied load equally. In this connection their collective movement is actively controlled by a pump, but their differential movement determined only by the relative loading of the actuators.
The hip connection for horizontal movement consists of putting two actuators in series so that as oil flows out of the fixed end of one it flows into the fixed end of the other. The motion is exactly horizontal only if the plane of the cylinders is inclined at 45° (Sutherland and Ullner 1984). A series connection is also made between the front and back actuator-pairs on each side to coordinate their movement during the propulsion stroke. This arrangement is shown for one tripod-set of legs in Figure 6.8 [not shown-RH]. In this illustration legs 2, 3 and 6 are being driven together. Each side has a separate pump so only legs 2 and 6 are connected together; the coordination between this pair and leg 3 is achieved by non-hydraulic means. Other series connections are possible.
The parallel connection is used for various purposes such as raising and lowering a set of legs, and to connect the knee actuators. If the three knee cylinders of a supporting tripod-set of legs are connected in parallel then although their collective sideways movement can be controlled by a pump, their differential movement is free and compensates both for the movement of the knee in an arc during forward rectilinear walking and for the larger sideways knee movement which must occur during a turn. This knee coordination is perhaps the most successful application of a passive hydraulic circuit.
The valves are all directional, not proportional or servo, spool valves, which are relatively cheap and simple but cannot control speed. Speed is regulated by manual control of the displacement of the pumps. The combination of the main propulsion pump and sideways motion pump flow rates governs the speed and direction of walking and the rate of turn. The pump displacements are controlled by pedals and a joystick.
The role of the on-board computer is to switch the valves on and off in the sequence appropriate to the specified gait. It can interrogate joint angle and leg force sensors, and the driver's controls. Several types of program have been written to test different methods of control, and a special language (OWL) has been developed (Donner 1983). The robot, which was built as a way of learning about hydraulic actuation, has now been scrapped.

See other early Steam Men and Walking Machines here.

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