A capstan drive includes a first capstan that moves a device on a carriage and a second capstan that receives a second rotary motion to move the carriage. A first power transmission has a driven shaft coupled to the first capstan, a first drive shaft coaxial with the driven shaft to receive a first rotary motion for driving the device, and a second drive shaft coaxial with the driven shaft. A second power transmission couples the second rotary motion to the second drive shaft. The second power transmission has a drive ratio such that if there is no first rotary motion, a first cable is drawn in by the first capstan at the same rate as a second cable is drawn in by the second capstan in response to the second rotary motion.