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Post date: Nov 25, 2018 11:11:50 PM
In FTC, using linear lifts are very important as they are much more reliable compared to the traditional rotation of a motor. A linear lift is a linear motion system that converts the rotational motion of a motor into a linear motion. There are several types of linear motions systems including rack and pinions, lead screws, linear actuators, and more. However, in this blog, I want to focus on linear slides and the common difficulty: stringing. A linear slide is basically beams that slide on top of each other when pulled by a string. Linear slides are a very versatile form of lifts as compared to the rack and pinions, as an infinite number of stages can be added to create the desired extension length. In relation to stringing of the lifts, there are two types, continuous stringing and cascading stringing. Continuous stringing is easier to do, and requires less force to pull. However, it is slower as it moves each stage of the lift individually. Cascading stringing, on the other hand, is a more difficult but is a faster system that requires a lot more force to pull, but is very useful for quickly extending or retracting a large lift. Continuous stringing, the more basic style, is strung with the string being wrapped around pulleys at each stage of extrusion so that each stage of the lift moves up.
As shown in the diagram above, the string is routed through the multiple stages of the lift, with one side being attached to the last extrusion and the other string being attached to a spool. Then, when the spool turns, the string will get pulled, forcing the stages of the lift to move outwards. When adding more stages, the string would continue in that pattern and will always be tied to the last stage of the lift. For a retraction string, a very similar arrangement in the opposite direction would pull the lift back. However, with two strings, a pull up and down string, attachment to the spool must be correct as otherwise both strings will try to pull at the same time, causing damage to the motor, lift, or strings.
This is an example of a retract string, routed in a similar way except pulling in the opposite direction.
By attaching the string on opposite sides of the spools, one will release while the other tightens. In order to accurately do this without causing looseness in the string, the extend string should be tied while the lift is in the retracted position, and before adding the retract string, the lift should be in the extended position. This ensures that one string will be completely wound so that it can loosen. Otherwise, if neither are completely wound, both strings would simultaneously pull again causing damage onto the parts. This form of stringing is well suited to smaller, slower lifts, as it moves each stage individually, which takes more time.
On the other hand, cascading stringing is the much faster approach, more suited for larger lifts with a lot more stages. Cascading stringing, unlike continuous stringing is not one large string that is routed through all stages of the lift and instead several short strings that all pull each other.
With multiple small strings, the cascading stringing has a unique pattern, with each string connected to individual stages. In a cascading lift, a string ties to the bottom of the second stage, over a pulley on the first stage, then tied to the spool. Now with just this string, the second stage would move up. To add on, a second string is attached to the bottoms of both the first and third stage, going over a pulley connected to the second stage. Now, when the spool turns, the second stage still lifts up, which also pushes up the pulley connecting the first and third stage. Since the second string is now also getting pulled, the second and third stage move up simultaneously creating a much faster lift. To add more stages to the lift, as cascading works better with more stages, the same pattern would continue. A third string ties to the bottom of the second stage, over a pulley on the third stage, and ties to the bottom of the fourth stage. Then to add a fifth stage, a string would be tied to the bottom of the third stage, go over a pulley on the fourth stage, and tie to the bottom of the fifth stage.
Now, by the same principle, with each stage that moves up pushes the pulley upwards and pulls the neighboring stage continues to more stages and now pulls all five stages at the same time. With this method of stringing, the speed at which the extension length is reached increases, but at the same time, the force required to pull the lift increases as well. Just like continuous stringing where a reverse string can be used in the opposite direction to pull the lift back, a similar method can be used to pull a cascading lift back. Again, in relation to spool directions, the same principle applies as with continuous string, one spool must be fully wound before attaching a string in the opposite direction to avoid damage to the materials. With this, the two main forms of stringing are complete, however there are still improvements and adjustments that can be made to fit the exact application needed. For the linear slides themselves, there are multiple choices for what to use. Commonly used in FTC are the REV extrusions, paired with the REV sliders. This arrangement is very versatile as it allows for an infinite number of stages, and also can be cut to the right size according to constraints. A similar extrusion system is found in Servocity X rail slide kit. Another form of linear slides are ordinary drawer slides, which can be found at any hardware store, such as Lowes or HomeDepot. The drawer slides are much more compact compared to the extrusions as each lift stage is within the previous stage however the amount of lift is limited and they are overall harder to use.
Additionally, for the actual string itself, there are also multiple options.The most common is heavy duty fishing line, which is thin and strong, perfect for a linear lift. However, in some occasions, especially with cascading stringing which requires lot’s of force, a good options is braided steel cable which is much more durable and is tighter as the use of crimps holds to the slides much stronger than tying fishing line. These are the main types of strings our team used for the multiple linear lifts present within our robot.
Overall, the use of linear motion is very important in FTC competitions, and knowing how to accurately string a linear lift can help for FTC or any robotic project. Whether a short or long lift, either cascading or continuous stringing can be used to create the perfect lift. With that, I hope everyone has a great FTC season, go ROVER RUCKUS!
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