Modlab The Modular Robotics Laboratory at the University of Pennsylvania

Module Brake
Module Brake
We have designed a joint-locking mechanism for a new chain-style modular reconfigurable robot. This mechanism will enable the robot to perform a wide array of tasks such as dynamic motion and bio-inspired locomotion while consuming less power.


There are two competing factors towards maximizing capability, that is specific torque (the ability for modules to lift other modules by maximizing motor torque while minimizing module mass) and power consumption. Larger torque typically requires more mass and, in turn, more power for actuation. In addition, reduced size follows as a desired property, as smaller modules lead to shorter lever arms (more mechanical advantage) and typically lower mass. Therefore researchers in the modular reconfigurable robot community have been working to reduce size of the component modules. Control and configuration possibilities grow exponentially as the size of the modules is reduced and the number of modules grow for a given sized task.

Locking a joint will allow modules to passively hold a configuration, ideally with much larger holding torque than with a joint actuator. A typical chain-style modular robot configuration may have dozens of active modules in an arbitrary shape. The system will correspondingly have dozens of DOF somewhat like an octopus. This is usually many more DOFs than needed for a given task which can actually be detrimental in some cases. Like a boneless octopus, the system has difficulty applying forces to run or jump — the octopus can only apply local forces as it has no rigid structure in it’s body to push against. Bones in mammals also enable the exploitation of mechanical advantage in parallel structures or lever-like action. Locking joints of some modules allow us to create bone-like structures that correspondingly consume no power.

There is a variety of commercially available brakes including those based on electromechanical actuation, magnetic particle brakes, and piezoelectric transducer (PZT) brakes. Electromagnetic disc brakes are very common. They can apply large holding torques and are relatively fast without excessive size and weight. We refer to the torque-to-weight ratio as specific torque. The magnetic particle brakes and PZT brakes tend to be large with poor specific torque. Despite these shortcomings, they consume much less power and, in the case of PZT, can be extremely fast, but require high voltages, which complicate the electronic control.

Our mechanism is electro-magnetically actuated. It is bistable, consuming no power to maintain a torque, yet has better specific torque than the low power magnetic particle, PZT-based and the electro-magnetic disc brakes. The addition of this component will enable a variety of applications for modular robot systems including increased load bearing capabilities, bio-inspired control (acting as bones), and a longer battery life for remote untethered tasks.


Property Value
Mass 40.5 grams
Size 6(W) X 60(L) X 60(H) mm
MCU dsPIC30F4011
Sensing AS5040 (X2)
Actuation Brushless outrunner
Holding Torque 250 Nmm


Related Projects:

Self re-Assembly after Explosion: CKbot is a modular robot that can put itself back together.
Dynamic Locomotion of CKbot: Demonstrating the different types of dynamic locomotion gaits of CKbot such as rolling and legged locomotion.
Isomorphic Configuration Recognition: How CKbot can figure out its shape.

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