"Sane" error margins for emotor torque measurement

[imavirgo]

TL;DR Summary: Between design simulation, prototyping and measurement, what are sane error margins for emotor torque?

We're going to design and prototype some small emotors. There are essentially 3 "stages";

-design software

-prototype

-measurement system

Measurement system will be either a lever arm on a scale or a chain or string connected to a load cell. Now, the basic question is, what is acceptable error margin at each stage? And, I apologize in advance for any lack of clarity but it strikes me that this problem will get very messy very quickly. For instance, say I input identical designs into 3 different software packages. How much disagreement should I expect between packages? Do I have any way of deciding which software is most accurate before building the prototype? How much disagreement should I expect between the software and experimental results? How can I calibrate the dyno? Is it simply a matter of hanging a known weight off the strain gauge?

As I hopefully made somewhat clear with the preceding questions, I'm trying to figure out which of the 3 stages is the most "trustworthy" and how to use the most "trustworthy" stage to improve the accuracy of the other 2. I have some _very_ basic reading in metrology but I have no idea how much of it is applicable.

Many thanks in advance for any suggestions

Joe

 

[berkeman]

For instance, say I input identical designs into 3 different software packages. How much disagreement should I expect between packages? Do I have any way of deciding which software is most accurate before building the prototype? How much disagreement should I expect between the software and experimental results?

What have your instructors said about this question? What are the 3 software simulation packages that you will be using? Have your instructors used all 3 packages? If so, they may have noticed some differences.

 

[jrmichler]

Some general rules:
1) All prototypes differ from what was designed due to some combination of manufacturing tolerances, differing material properties, unrealistic designs, and mistakes. Figuring this out is part of your job.

2) All simulation packages give results that are only as accurate as the model AND the algorithms in the simulation package. It's your job to correctly model your motor. It's your job to figure out if the simulation package is giving correct results for your model. You can learn a lot by modelling and simulating an existing motor that has good published performance data.

3) It's your job to figure out the error bounds on your experimental setup. You do this by calculating the entire error budget for all possible sources of error.

4) A typical project of this type will make several passes through a cycle of Design >> Simulate >> Prototype >> Test >> Redesign >> and repeat until the motor meets the original design specification.

Sorry, but there are no simple easy answers to your questions. Just a lot of work.