Friday, March 14, 2008

Safety factors in robotic automation

Safety Factor

If you are unsure of various perhaps uncalculatable factors, always add what is called a factor of safety. For example, suppose you guess a beer can weighs between 1 to 2 pounds. A factor of safety would say, 'design the system to handle 2.5 pounds, just in case.'

So what should your factor be? Guess. I would recommend 1.2, but its really up to you. What does this number mean? Suppose your calculations say you need a motor rated at least 100Nm, then multiple that by 1.2 to get 120Nm as your minimum motor force. The factor of safety is not an exact science, obviously. If you expect to have high fatigue from shock or overuse, high friction, or bending, make the factor of safety higher.

So why not make my safety factor really high? Well, you can, but motors with higher torques are also more expensive Thicker robot materials can cost you more too. So why not a small safety factor? Well, if friction is much higher in your robot than you expected, your robot just won't work very well.

There is a more scientific method to the safety factor, called statistical analysis. This involves building then actually testing your robot part under various circumstances until it breaks. Then statistically (through a histogram) you can determine the optimal properties so it will NEVER break. However this involves building and breaking a part many times - too much effort for a single robot. This method is common for car and cell phone manufacturers. Did you know they statistically determine how many times you can drop a cell phone at any particular angle to make it user proof?



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