One of the major design challenges in a cable-suspended haptic interface is the creation of the spool. The spool must be designed in a way to prevent the line from piling up as demonstrated by this video at 34 seconds.
[youtube]http://www.youtube.com/watch?v=uwgFkACxgWc[/youtube]
Not only does the piling up cause the line to get snagged, but allowing the line to spool in an uncontrolled manner will create a large and undefined variance in the amount of line spooled out per revolution of the drum. This variance makes the control problem more difficult. Three of the solutions below are variations on the same idea, build a mechanism that guides the line to lay in the space created by the spooled line beneath it. The last one solves the problem with direct measurement of the line spooled out.
Solution 1: Self-reversing screw
[youtube]http://www.youtube.com/watch?v=2_Mlk2jDUok[/youtube]
Typically this problem is solved with a self reversing screw. This has a great benefit over the other solutions since no additional motors are added and it is mechanically tied to the spool avoiding differences in speed between the two. It also can function in any tension condition. The problem is the device is that it is extremely hard to build. The cart needs to be able to float in the diamond track to self reverse as demonstrated in the following video. This is also a large amount of trial and error in designing these systems. The custom made self-reversing screw will need to be refabricated if the first design fails.
[youtube]http://www.youtube.com/watch?v=zv7cXR1UkWQ[/youtube]
Solution 2: Linear Actuator
This is exactly the same as the one above, but instead of a self reversing screw, either a linear actuator or a standard lead screw is used. A second motor handles the reversing motion. Using two motors allows for the software to be adjusted instead of remaking a self-reversing screw as in the above solution. Errors come in due to the delay in measuring the speed for the spool drum and the adjusting the speed of the cart.The idea would be the best, however it doubles the number of motors and increases the number of calculations that the computer needs to make. It is limited by the speed of our computation device.
[youtube]http://www.youtube.com/watch?v=EdTBpW8l1t4[/youtube]
Solution 3: Tension Driven
Fleet angle compensators or a system developed by Allied Power Products could be used. They work by freely floating to allow the forces generated by the line falling into the grove to advance the cart. While they are automatic and a fully mechanical solution, the tension in a cable-suspended habitic mechanism might not be high enough to support this option.
[vimeo]http://vimeo.com/6871038[/vimeo]
Solution 4: Take-up/Pay-out design
This design is a bit unconventional. It is typically used when cutting wire stock on a bigger spool into smaller spools for field use. While more design work and fabrication effort, it has a massive advantage in the creation of a refined haptic feedback system since the length of the line is directly measured. The spool would not be motorized but instead a constant force spring (McMaster-Carr #9293K14) would be attached, much like that in a tape measure. The motor would then pull the line off the spool, fighting the spring. An encoder also directly measures the amount of line pulled off. This can fail if the line slips, but this should be expected and time set aside for trail and error to solve the slipping problem. It also means the motors will be stalled if the system is not moving, cooling will have to be provided to the motors.