1. Department of Electrical and Computer Engineering, Virginia Tech, 1075 Life Science Circle, Blacksburg, Virginia 24061, USA
2. Shandong University–Virginia Tech International Laboratory, Shandong University, Shanda South Road 27, Jinan, Shandong 250100, China
3. Department of Mechanical Engineering, Virginia Tech, 1075 Life Science Circle, Blacksburg, Virginia 24061, USA