After watching the excellent performance of Team 7357’s robot tape arm, we decided to make one of our own. Since our access to 3-D design and printing expertise was lacking, we sought a way to make it using more conventional tools and parts.
Flexible steel measuring tapes are designed to be manually pulled out and then retracted by a spring. Here is a drawing of a typical unit from its patent application:
We wished to use the same steel tape, but use a motor to push the tape out and pull it back. Mounting a drum on a motor shaft and attaching the tape to it works well for pulling tape, but the tape will unwrap from the drum when the motor is reversed to push tape out. We added a system of rollers around the tape spool to limit this unwrapping. Because of material and part availability as well as the tool set on hand, we designed the unit to have a hardwood frame with polycarbonate sides, a turned hardwood drum, and nylon rollers turning on long machine screw axles:
Through the course of the month and a half between mid-December and the end of January, this tape arm was designed and re-designed several times.
Our first hub was turned from greenheart, a very hard and dense wood, but unfortunately too prone to splitting to keep together. Our second was turned from walnut, but again proved too fragile under stress. We then tried a recycled plastic composite (deck planking), but this material was too soft. Our final and workable hub was made from a 1.5″ diameter delrin rod, bought as a 1 foot length from Amazon and sliced to the proper width. This extremely tough plastic worked quite well. From left to right in the picture below, the composite hub, a broken walnut hub, and a delrin hub:
Our first version used a 3/4″ wide tape. We tested the “standout,” which is the length of tape that can be extended without breaking over. We needed about 3 feet, and this tape would reach about 5 feet. However in practice, robot motion and acceleration caused more break overs than we liked. We moved to a 1″ wide tape and in hindsight, should have gone to a 1.25″ wide “Fatmax” tape.
Except for the hub fragility, solved by the delrin version, the worst problem we encountered was a tendency for the tape to wrap back on itself when pushing out. This would occur for certain if the tape was over-extended, but also would sometimes occur if the tape encountered any obstacle. The result was for the tape to immediately jam and become unusable. Here are a couple of examples:
Our team became very proficient at changing tapes when this happened!
In the end, the tape arm did function as desired most of the time. We had planned to provide a servo-operated way to raise and lower the angle of the tape, but discovered that a fixed angle worked fine. At this angle, the tape could be aimed at the churro bar just above the high zone by moving the robot through the angle between the floor and the mountain low zone. Once hooked, the tape and treads were used together to climb the mountain. Upon reaching the high zone, the tape could be extended a bit to unhook. Because of the robot tilt at that point, the aiming point was high enough to reach the chin-up bar and hang the robot. The arm had more than adequate strength to do this, with the Andymark NeveRest 40 motor providing up to 350 oz-in of torque, or more than 16 pounds with our 0.75″ radius hub.
Here is a photo of the tape arm in situ on the robot:
With ample time to think it through now that the competition season is over, we would make some changes:
- Use a wider tape with a greater standout length (11 feet for the “Fatmax).
- Instead of rollers to confine the tape roll, just use a circular housing with teflon tape inside to minimize friction. We will probably design and 3-D print this part. The inside radius should be kept to a minimum and this would do a much better job of preventing tape wrap-back.
- Provide servo-operated levers to allow the tape to be aimed up and down, left and right, with a joystick.
- Use a higher gear ratio on the motor to achieve more torque and pulling power.