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The Actuators


The Actuators:

The Cog Shop
MIT Artificial Intelligence Laboratory
545 Technology Square, #920
Cambridge, MA 02139

write to the Cog Documentation Project: cdp@ai.mit.edu

Details of the Actuators

Each actuator of the arm consists of a DC electric motor with a series spring (so called Series Elastic Actuator). The spring allows low-noise force control from the geared electric motor.

Essentially, electric motors operate well at high speeds and low torques, but the majority of applications (including this one) require low speeds and high torques. This is usually accomplished by using a transmission, the most common type being a planetary gearbox. These types of gearboxes introduce friction, noise and backlash to the motor output, making it hard to achieve low-noise force control. To overcome this problem you can use a low friction, zero-backlash transmission such as a cable drive, (which tends to be bulky and mechanically complex), or use direct-drive motors (which tend to be very heavy). The approach used here is to insert a spring between the motor drive and the load as shown in the figure below.

The spring is linear, so the output force is proportional to its twist. If the twist is measured, then a control law can force the actuator to maintain that twist, and so give a constant force output.

  • The spring converts the force control problem into a position control one, which is better suited to the abilities of the motor-gearbox combination.
  • The spring naturally low-pass filters the noise and backlash of the gearbox, giving a low-noise force output.
  • Shock loads are absorbed by the spring, protecting the motor gear teeth.
  • Overall system bandwidth is low due to the spring.
  • It is easy to make the actuator behave in a passive manner, making it stable while interacting with all environments. This means that an arm powered by this actuator will not go unstable when touching a hard surface.

You can see a short MPEG video (403Kb) of Cog's arm hammering a nail into a 2x4 piece of wood.

Similar actuator technology is used in a walking robot, Spring Turkey in the MIT leglab.

Spring-like Behavior

Human muscles have a spring-like property, which can be approximately modelled as in the figure below.


The joint has a springy behavior given by the two springs in the picture. The equilibrium point can be changed by moving A and B in opposite directions. For Cog's arms, each joint is programmed to behave in this manner, as a virtual spring with variable stiffness, damping and equilibrium point. This gives the overall arm some desirable properties.

  • It is robust to collisions - the arm consists of masses connected by these virtual springs. If any part of the arm is deflected, the loads are taken up by the springs. There is no need for any explicit computation or compensation for collisions.
  • Since the actuators can deal with shock loading there is no danger of damage from collisions either
  • The system is stable: it consists of masses and springs which are all passive devices, so no stability problems.
  • The system has a low frequency characteristic, meaning that commands to the arm at a low rate can still obtain smooth arm motion. This gives more time for computation.



Representatives of the press who are interested in acquiring further information about the Cog project should contact Elizabeth Thomson, thomson@mit.edu, from the MIT News Office,  http://web.mit.edu/newsoffice/www/ .


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