Leg mechanism

• horizontal speed as constant as possible while touching the ground (support phase)^[1][6]
• while the foot is not touching the ground, it should move as fast as possible
• constant torque/force input (or at least no extreme spikes/changes)
• stride height (enough for clearance, not too much to conserve energy)
• the foot has to touch the ground for at least half of the cycle for a two/four leg mechanism^[1] or respectively, a third of the cycle for a three/six leg mechanism
• minimized moving mass
• vertical center of mass always inside the base of support^[1]
• the speed of each leg or group of legs should be separately controllable for steering^[6]
• the leg mechanism should allow forward and backward walking^[6]

Another design goal can be, that stride height and length etc. can be controlled by the operator.^[6] This can relatively easily be achieved with a hydraulic leg mechanism, but is not practicable with a crank-based leg mechanism.^[6]

The optimization has to be done for the whole vehicle – ideally the force/torque variation during a rotation should cancel each other out.^[1]

Richard Lovell Edgeworth tried in 1770 to construct a machine he called a "Wooden Horse", but was not successful.^[7][8]

Patents for leg mechanism designs range from rotating cranks to four-bar and six-bar linkages.^[9] See for example the following patents:

• U.S. Patent No. 469,169 Figure Toy, F. O. Norton (1892).
• U.S. Patent No. 1,146,700, Animated Toy, A. Gund (1915). A leg mechanism formed by an inverted slider-crank.
• U.S. Patent No. 1,363,460, Walking Toy, J. A. Ekelund (1920). A leg mechanism formed by a rotating crank with extensions that contact the ground.
• U.S. Patent No. 1,576,956, Quadruped Walking Mechanism, E. Dunshee (1926). A four-bar leg mechanism that shows the coupler curve forms the foot trajectory.
• U.S. Patent No. 1,803,197, Walking Toy, P. C. Marie (1931). Another rotating crank leg mechanism.
• U.S. Patent No. 1,819,029, Mechanical Toy Horse, J. St. C. King (1931). A crank-rocker leg mechanism with a one-way friction mechanisms in the foot.
• U.S. Patent No. 2,591,469, Animated Mechanical Toy, H. Saito (1952). An inverted slider crank mechanism for the front foot and crank-rocker for the back foot.
• U.S. Patent No. 4095661, Walking Work Vehicle, J. R. Sturges (1978). A lambda mechanism combined with a parallelogram linkage to form a translating leg that follows the coupler curve.
• U.S. Patent No. 6,260,862, Walking Device, J. C. Klann (2001). The coupler curve of a four-bar linkage guides the lower link of an RR serial chain to form a leg mechanism, known as the Klann linkage.
• U.S. Patent No. 6,481,513, Single Actuator per Leg Robotic Hexapod, M. Buehler et al. (2002). A leg mechanism that consists of a single rotating crank.
• U.S. Patent No. 6,488,560, Walking Apparatus, Y. Nishikawa (2002). Another rotating crank leg mechanism.

Stationary

• 
  Jansen linkage
• 
  Klann linkage
• 
  Eight-bar leg mechanism ^[10]
• 
  Tokyo Institute of Technology walking chair^[11]
• 
  2 DOF pantograph leg mechanism^[12]
• 
  2 DOF leg mechanism of the RPRPR type.^[13]
• 
  Strandbeest (applied Jansen linkage)
• 
  Ghassaei Linkage^[1]
• 
  Tchebyshevs plantigrade machine^[14]
• 
  TrotBot Linkage (without heel linkage)^[15]
• 
  TrotBot^[16] Linkage Speed Variability as Ground Height Changes
• 
  Strider^[16] Linkage Speed Variability as Ground Height Changes
• 
  Strider Linkage^[17]
• 
  Foot-Paths of Strandbeest, TrotBot, Strider and Klann Linkages

Walking

*	4 legs	6 legs
Strandbeest
Ghassaei
Klann linkage 1
Klann linkage 2
Plantigrade Mechanism
Trotbot[18]
Strider Linkage[17]	Strider Prototype, 4 legs/side

Shown above are only planar mechanisms, but there are also more complex mechanisms:

• Hexapod (robotics)
• Jansen's linkage
• Kinematics
• Kinematic pairs
• Klann linkage
• Chebyshev's Lambda Mechanism
• Linkage (mechanical)
• Machine
• Mecha
• Mobile robot

Wikibooks has a book on the topic of: Comparison of crank based leg mechanism

• Media related to Leg mechanism at Wikimedia Commons