Mobility and Control

A.    Mobility System Design

The main considerations in the design in the mobility platform are maneuverability, controllability, traction and stability, environment impact and simplicity. The overall weight and ease of storage are also product design issues that concern not just the user, but those who take care of the user.

The Smart-cane mobility system uses two-wheel skid-steering drive because of its simplicity in construction. It has two individually controlled driving wheels and up to two passive casters. This configuration also has relatively good maneuverability in congested environments as it allows an on-spot spin. Each drive motor has an incremental optical encoder for motion control and dead reckoning.

While it is acceptable for the cane to have a non-holonomic system as it is relatively small. It will be highly desirable for the smart walker mobility system to have omni-directional mobility. This type of mobility would make it much easier for the user to maneuver a walker through congested areas in the eldercare facility. We have invented an omni-directional mobility design concept using conventional wheels, which is in the process of patient application.

Samrt-cane moblity system

While it is acceptable for the cane to have a non-holonomic system as it is relatively small. It will be highly desirable for the smart walker mobility system to have omni-directional mobility. This type of mobility would make it much easier for the user to maneuver a walker through congested areas in the eldercare facility.

We have developed an omni-directional mobility design using conventional wheels based on the active split offset castor (ASOC) []. Using conventional wheels ensures a simpler and more compact mechanism, a higher loading capacity and a greater robustness to poor environmental conditions. An ASOC unit consists of a pair of independently driven wheels separated at a distance and connected with an offset link to the platform. Each wheel will be equipped with a motor and an encoder for wheel velocity control. Each offset link joint can also have an encoder to measure the joint position relative to the platform for error compensation during platform motion control. By controlling the velocities of the two wheels, arbitrary velocities can be achieved at the joint of the link. With a minimum of two sets of such wheel assemblies, an omni-directional mobility platform can be constructed.

An advantage of this invention over the existing omni-directional mobility design concepts is that with its conventional wheels, it is simple, has a high loading capacity and is less sensitive to floor conditions. The dual wheel design effectively alleviates the problem of scrubbing during steering. A patent application has been filed with the US Patent Office for this design.

Omni-directional mobility platform

 

B.    Control System Design

The control system ensures the smart-cane follows the trajectories from the planner. It also responds to user inputs and environment changes to provide smooth and safe motion. The controller therefore consists of two important functional parts, the user interaction control and the motion controller. Depends on the functional requirements, PAMM operates on different control modes.

 

The novel technique of PAMM control system design is its admittance-based user interaction control. The idea of the admittance-based control is to design a dynamic model and make the system behave like the dynamic system specified by the model. By varying the model we can let the user of PAMM feels as if he is interacting with a virtual dynamic system.

Admittance based control

 

While mechanically simple, the motion planning and control of such a system is complex due to its non-holonomic constraints. A posture tracking control algorithm based on the non-linear feedback developed by Samson is adopted for the trajectory control of the smart-cane. One advantage of this algorithm over many other non-holonomic control methods is that is has no control action when the desired speed is zero, even when position errors exist. It is thus most suitable for this application as it allows the user to stop and will not force the user to the trajectory.

 

Papers

Yu, H., Dubowsky, S., and Skwersky, A., "Omni-directional Mobility Using Active Split Offset Castors", Proceedings  ASME Design Engineering Technical Conferences, , September 2000  [166]