Simulation Overview
Biodyanamic Systems
In human subjects testing data collected is often in the form of a time series. These time series include parameters such as position, angle, force, or EMG recorded at each instance in time. Time series data is often noisey and must be filtered prior to analyzing. In addition, biodynamic systems have highly complex geometry and a sophisticated controller (the brain). Real systems are usually highly nonlinear and have many degrees of freedom. All of these factors make analyzing biomechanical systems difficult.
Why use a Simulation?
When developing new tools for analyzing dynamic systems, it is often easier to analyze simulations first. These simulations can be designed such that they are low order, conservative, ideal, and use simple controllers. These simulations allow the developer of analysis tool to test and "debug" the new tool prior to analyzing a complex real system. Often the expected results are intuative so errors in the code can more easily be found. Because the systems are often lower order, analysis time is usually faster. However, one of the most important reasons for using simulations is to gain a fundamental understanding of the system behavior that may be obscured within the details of a more complex system.
Forward Dynamic Modeling
One method to generate simulations is by using forward dynamic modeling. In this method, motion is predicted using the equations of motion and a set of initial conditions. For deterministic systems, the future state of the system can be predicted based on the inital conditions, and the results will always be the same (except for the case of deterministic chaos). Random noise may also be added to the system resulting in a stocastic system. In this case, the different results are obtained each time the simulation is run. Some examples are shown below.
Planar Inverted Pendulum
The planar inverted pendulum is one of the simplest simulations to generate. It has one degree of freedom (angle) and a two dimensional state space (angle and angular velocity). Because of these characteristics, it is useful in testing a new analysis technique because proper behavior is easily predicted. It also can be used to develop surrogate data for an anterior-posterior standing postural sway test (using ankle strategy).
Double Planar Inverted Pendulum (Wobble Chair)
The double planar inverted pendulum was developed to simulate the motion of a person sitting on an ustable sitting apparatus. This system has two degrees of freedom and a four dimensional state space. In addition, the two degrees of freedom are highly coupled leading to interesting dynamic behavior.