Entropy is an approach to measure the irregularity of a set of data and one of rare quantitative analysis indicators for chaos.By using the method of entropy the edge of chaos for a mechanism is found and the threshold for the calculation of approximate entropy is selected.The chaotic motion arising from a planar closed-chain underactuated five-bar mechanism is studied.SimMechanics is employed to model the mechanism.By the time series extracted from the results of simulation

the approximate entropy is calculated and the variation chart of entropy is plotted

the range of rod length corresponding to the system changing from periodic motion to chaotic motion is found.Based on the Lyapunov exponents and the phase diagrams

the range of appropriate threshold value is fixed.A new approach to determine threshold value for the calculation of approximate entropy is proposed.