This unit described a number of different lifting mechanisms, but which one will work best on a given robot?
There are a few key considerations, discussed below, which designers should take into account when designing a lifting mechanism.
The first and most important consideration a designer needs to take into account is the elevation required. At what height does the robot pick up the object? At what height does the robot need to lift the object? Are there multiple heights the object will need to be scored at? What mechanism will accomplish these changes in elevation?
Another important consideration for a designer is the change of orientation the object will go through, if any. What orientation(s) will the objects be in when picked up? What orientation(s) will the object be in when scored? Are these orientations different at different elevations? Often, the lifting mechanism can be used to accomplish these orientation changes. Some lifting mechanisms will be better suited to an application than others due to required orientation changes. For instance, if the object needs to stay in the same orientation from when it is picked up until it is scored, a rotating joint may not be as good a choice as a linear elevator.
Starting Configuration & Other Size Limitations:
VEX Robotics Competition robots must start each match within a limited size, as described in the Game Rules released each year. This starting size may restrict which lifting mechanisms can be used. For instance, a single jointed arm which starts within an 18” x 18” x 18” box cannot reach up four feet – though a multiple jointed arm might be able to do it. There are often other size limitations involved in the creation of a competition robot. What if there is a bar on the field that the robot must drive under? In this situation a designer may choose to design so his or her robot’s lifting mechanism can fold down under the height of this bar. Size limitations play a large role in the design of lifting mechanisms, especially when combined with elevation requirements as discussed above. It is easy to make a robot reach up four feet; it is more difficult to make a robot start in an 18” cube and then reach up four feet.
Some lifting mechanisms may take up more space than others. If the robot has a large hopper-style object manipulator, and it takes up almost all of the space in the robot’s starting configuration, the designer will need to find a lifting mechanism that will package into the space available. Some lifting mechanisms may “fit” better than others.
In some situations, multiple lifting mechanisms may all work. However, these mechanisms can vary greatly in complexity. It is usually a good idea to choose the simplest mechanism possible which accomplishes the design goals. Sometimes, it is even beneficial to make tradeoffs and reduce the design goals. For example, if a simple mechanism is “almost good enough” to achieve the design goals, and if fully accomplishing the goals would require a VERY complex mechanism, it is probably better to choose the simple mechanism. Simple mechanisms have fewer moving parts, are more robust, and are less likely to fail; in these ways, they are usually better.
Competition robots are limited in the number of motors and other actuators that they are allowed to use. Designers should be cognizant of how many motors are required for each lifting mechanism option. A 2-jointed arm requires two motors, one for each joint, whereas a single jointed arm only requires one motor. In this case, the designer could still use two motors (as described in Units 7 & 8) and the mechanism would be able to handle its load twice as fast!