1. The George W. Woodruff School of Mechanical Engineering, Georgia Tech, Atlanta, GA 30332-0405e-mail:
2. Department of Mechanical Engineering, Indian Institute of Technology, Kharagpur, India 721302 e-mail:
3. The Systems Realization Laboratory, The University of Oklahoma, Norman, OK 73019e-mail:
4. The Collective Systems Laboratory, Washington State University, Pullman, Washington 99164 e-mail: