Not sure why I was downvoted. A 2 minute search yields just these patents:

http://www.freepatentsonline.com/7069755.html

http://www.freepatentsonline.com/5701828.html

http://www.freepatentsonline.com/5974367.html

http://www.freepatentsonline.com/5999095.html

http://www.freepatentsonline.com/6325429.html

http://www.freepatentsonline.com/6867685.html

http://www.freepatentsonline.com/6483424.html

http://www.freepatentsonline.com/6609738.html

http://www.freepatentsonline.com/5886644.html