[Matti Kolu (2012.07.23.2140 CET)]
Does anyone have any thoughts on this approach of using servo motors and
kevlar straps with load sensors for the actuation of a robot?
The Robotis Dynamixel RX-28 isn't cheap at $210 per servo but the cheaper
AX-12A at $45 seems to give the same readings (speed, shaft position,
voltage, load, temperature).
Link to the paper:
http://iopscience.iop.org/1741-2552/9/4/046011/article
A physical model of sensorimotor interactions during locomotion
Theresa J Klein and M Anthony Lewis 2012 J. Neural Eng. 9 046011
Abstract: "In this paper, we describe the development of a bipedal robot
that models the neuromuscular architecture of human walking. The body is
based on principles derived from human muscular architecture, using muscles
on straps to mimic agonist/antagonist muscle action as well as bifunctional
muscles. Load sensors in the straps model Golgi tendon organs. The neural
architecture is a central pattern generator (CPG) composed of a half-center
oscillator combined with phase-modulated reflexes that is simulated using a
spiking neural network. We show that the interaction between the reflex
system, body dynamics and CPG results in a walking cycle that is entrained
to the dynamics of the system. We also show that the CPG helped stabilize
the gait against perturbations relative to a purely reflexive system, and
compared the joint trajectories to human walking data. This robot represents
a complete physical, or 'neurorobotic', model of the system, demonstrating
the usefulness of this type of robotics research for investigating the
neurophysiological processes underlying walking in humans and animals."
Excerpt:
"3.1. Mimicking muscles using a motor-strap system
For each muscle in the robot, a Robotis� RX-28 motor is attached to a
bracket, to which a kevlar strap is buckled. Muscle contraction is mimicked
by rotating the motor to pull on the strap; thus, when the motor is in a
fixed position, the 'muscle' is of fixed length. Meanwhile, internal to each
servo motor, we have a PID control loop that maintains the motor position,
or muscle length. This effect loosely models the behavior of the muscle
spindle afferents in providing muscle length control.
To measure the amount of load in each 'muscle', we designed a custom load
sensor, based on a FUTEK force sensor, with a custom-designed fitting so
that it can be buckled into a strap to measure tension. Each load sensor was
connected to a networked sensor board, which communicates back to a PC. An
additional load sensor is added at the Achilles tendon, which acts as a
calibration check and allows force to be measured directly at the ankle. In
addition, we have contact sensors on the heels of the robot and foot
pressure sensors on the toes, analogous to cutaneous afferents. We also have
angle-sensing potentiometers in the joints, to measure the angle of the
joint. These sensors thus model the main sources of afferent feedback used
in locomotion."
Short clip on Youtube of the the set-up in action:
Older article from 2009:
Achilles: A robot with realistic legs:
http://www.ine-news.org/view.php?article=1422-2009-01-16&category=applications%3Arobotic