Vrije Universiteit Brussel

MACCEPA: The Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator


Besides the development and implementation of the Pleated Pneumatic Artificial Muscles, our research group developed a second actuator with adaptable compliance. The MACCEPA (The Mechanically Adjustable Compliance and Controllable Equilibrium Position Actuator) is a straightforward and easy to construct rotational actuator, of which the compliance can be controlled separately from the equilibrium position. Each of these parameters is set by a position controlled servo motor. Moreover, the torque is a linear function of the compliance and of the angle between equilibrium position and actual position. Thus this actuator can be seen as a fully adaptable torsion spring, where one motor sets the stiffness of the torsion spring, and another set the equilibrium position. Since this actuator has a spring it can store and release energy. This makes this actuator perfectly suitable for dynamic walking and human-robotic interfaces.

Figure 1. MACCEPA demo

Figure 2. Picture of the Slimline implementation of the MACCEPA concept

How does it work?

In figure 3 the essential parts of a Maccepa are drawn. As can be seen there are 3 bodies pivoting around one rotation axis. To visualize the concept, the left body in figure 1 can be seen as an upper leg, the right body as the lower leg and the rotation axis, which goes through the knee joint. Around this rotation axis, a lever arm is pivoting, depicted as a smaller body in figure 3. A spring is attached between a fixed point on the lever arm and a cable running around a fixed point on the right body to a pretension mechanism.

Figure 3. Working principle of the Maccepa

The angle f between the lever arm and the left body, is set by a classical actuator. When a, the angle between the lever arm and the right body, differs from zero, the force due to the elongation of the spring will generate a torque, which will try to line up the right body with the lever arm. When the angle a is zero--this is the equilibrium position--the spring will not generate any torque. The actuator, determining the angle f actually sets the equilibrium position. A second actuator, which pulls on the cable connected to the spring, will set the pretension of the spring. This pretension will vary the torque for a certain angle a, thus controlling the spring constant of an equivalent torsion spring.

Figure 4. CAD Drawing of the Maccepa prototype

In figure 4 a CAD drawing of a first realization is shown. In this prototype a classical 1D joint with roller bearing is built. The servomotor on the left, which is placed on the left body, has the same rotation axis as the joint and thus sets the angle between the left body and the lever arm, which is the equilibrium position. A number of different mechanisms can be used to pull on the cable to pretension the spring. As is shown in the right of figure 4, we opted for a second servomotor with lever arm in this prototype. By rotating this motor�s lever arm, the pretension of the spring is changed and thus also the compliance of the equivalent torsion spring in the joint.



The applications can be divided into groups, depending on the primary use of the compliance.

Using the adaptable compliance to adjust the natural dynamics of a system. This can be done to save energy or dampen a certain movement by storing energy.

  • This actuator with adaptable compliance can be used to combine the advantages of active and passive walking. Compliance can also be used for shock absorbance during touchdown of the feet.

  • The focus of the prostheses research lies on the enhancement of convenience for the user. Most prostheses nowadays are still passive, but the more advanced ones have an actuator and a compliant element. This compliance is fixed during construction and set to an average value. Depending on the stiffness of the ground and the walking speed this will give a more comfortable feeling to the user. When the compliance is adaptable, this can be made closer to optimal for all cases.

Using the compliance to make the interaction with humans safer and more natural.

  • Industrial robots nowadays are heavy and stiff machines, placed in a human free environment, due to safety reasons. In some applications it is useful to have robots and humans jointly fulfilling tasks, this requires safe robots. This could be done by making the joints of the robot compliant, but with a compliant joint it is harder to place the tool center point in an exact position or to track a certain trajectory. In this case an actuator with adaptable compliance can be stiff for exact positioning and low speed (grasping and placing an object) and compliant when positioning is not that important or moving at higher speeds (moving from one position to another).

  • Most robotic toys are actuated by stiff electrical drives. This results in the typical robotic way of moving. Especially for cuddly toys [Aibo, Robosapiens] a compliant movement is preferred. When children play with these toys it tends to happen that they impose another motion than the toy is doing, which can result in damage to the driving mechanism. Compliance in the actuation can prevent this from happening and give the cuddly toys a more natural feeling.

  • Step rehabilitation is done by a number of physiotherapists, resulting in expensive sessions limited in time, which extends the overall rehabilitation process. Recently robots were built, [lokomat, Autoambulator] imposing gait-like motion patterns to, for instance, the legs of a patient. When the patient suffers from spasms, which is not uncommon when not having full control over the muscles, the stiff actuator can hurt the patient or the spasm can destroy the actuators. Compliance could prevent these problems. Additionally, in the beginning of the rehabilitation process it is preferred to have a relatively high stiffness, which could gradually be lowered when the patient has regained a certain level of control over the legs.


VERONICA (Variable joint Elasticity RObot with a Neuro-Inspired Control Approach) is the first biped equiped with the MACCEPA actuator. In passive walkers, the natural frequency of each limbs is defined by the mass distribution in the legs. So, it is fixed during design and cannot be changed during walking. This results in the fact that passive walkers have one predefined walking speed. Veronica will walk using Controlled Passive Walking. Here the control strategy is to change the natural frequencies of the limbs by using actuators with adaptable compliance. This allows to change the walking speed of a passive based walking robot. Some pictures and one movie can be seen here. (The pictures can be enlarged by clicking on them.)


    Review of Actuators with Passive Adjustable Compliance / Controllable Stiffness for Robotic Applications
    R. Van Ham, T. Sugar, B. Vanderborght, K. Hollander & D. Lefeber
    IEEE Robotics and Automation Magazine, nr 3, vol.16, pp.81 - 94
    abstract and pdf

    MACCEPA, the mechanically adjustable compliance and controllable equilibrium position actuator: Design and implementation in a biped robot
    R. Van Ham, B. Vanderborght, M. Van Damme, B. Verrelst & D. Lefeber
    Robotics and Autonomous Systems, Volume: 55, N� in volume: 10, pp: 761 - 768, published by: Elsevier, ISBN-ISSN: 0921-8890
    abstract and pdf

  • Compliant Actuation for Biologically Inspired Bipedal Walking Robots (PDF�15,6MB)
    Van Ham R.
    PhD Dissertation, Vrije Universiteit Brussel, July 2006.

More info

More info about this actuator and its applications can be obtained from Ronald Van Ham (email). Up

©2019 • Vrije Universiteit Brussel • Dept. MECH • Pleinlaan 2 • 1050 Elsene
• Tel.: +32-2-629.28.06 • Fax: +32-2-629.28.65 • webmaster