Choosing, Prototyping and Testing Mechanisms

By now your team probably has more mechanism ideas than time to build them. Strong teams do not pick the idea that sounds coolest or the one suggested by the loudest person. They compare options honestly, build quick prototypes, and test them with numbers. This lesson gives you a process you can reuse all season.

Mechanical trade-offs

• Speed: how quickly the mechanism completes its job. Faster mechanisms shorten each scoring cycle.
• Reliability: how often it works on the first try. A mechanism that fails during a match scores nothing.
• Complexity: how many parts, motors, servos and lines of code it needs. More parts means more things to break and to fix.
• Weight: heavy mechanisms, especially high up, slow the robot, make it tippy and strain motors.

These goals pull against each other. A faster intake might need more compression and a stronger motor; a lighter arm might flex more. For a new team, reliability usually matters most. Think about it this way: a mechanism that works 9 times out of 10 at a moderate speed will usually outscore a faster one that works half the time, because every failure costs a whole cycle, and sometimes the rest of the match.

The weighted decision matrix

1. 1
   List the options

   Write down three or more mechanism ideas for the same job.
2. 2
   Pick criteria

   Choose 4 to 6 things that matter, such as reliability, speed, simplicity, weight and how well it fits your tools and time.
3. 3
   Assign weights

   Give each criterion a weight from 1 to 5 based on how important it is to your team. Agree on these before scoring.
4. 4
   Score each option

   Rate every option from 1 (poor) to 5 (excellent) on each criterion. For weight, a lighter design gets a higher score.
5. 5
   Multiply and add

   Multiply each score by its weight and add the results to get a total for each option.

In this example the roller intake wins, but only narrowly over the passive scoop. When totals are this close, the matrix is telling you that you do not have enough information yet. The best next step is to prototype the top two and let testing decide. A decision matrix does not replace testing; it organises your team's discussion and records why you chose what you chose, which is also excellent material for your engineering portfolio.

Prototyping fast

A prototype is a quick, rough version built to answer a question, such as 'Will tubing grip this element?' or 'Is this arm long enough?'. Cardboard, zip ties, hot glue, tape, wooden dowels and spare kit parts are perfect materials. A cardboard intake spun by hand can show in 20 minutes whether an idea works at all. Only once the basic idea is proven should you spend hours on a metal version. Keep each prototype focused on one question and change one thing at a time.

Reliability testing

A cycle test means running the mechanism through its full job many times in a row, the same way it will be used in a match, and recording every result. Pick a fixed number of attempts, N, before you start, for example 20. Count the successes and calculate the success rate: successes divided by attempts, times 100%. Write down how each failure happened, because the failure notes tell you what to fix next. Also time each attempt to find the average cycle time.