Devices, systems and methods are provided for facilitating knee balancing during a knee replacement surgery. The zones are capable of being separately driven in order to accumulate or transport cartons. The enable period is set to be shorter than a predetermined reference period when the current drive capability of the semiconductor device is higher than a predetermined reference current drive capability, and the enable period is set to be longer than the reference period when the current drive capability of the semiconductor device is lower than the reference current drive capability. The retractor blades may have features to cooperatively engage an instrument mount.