My friend and I are building a small direct drive helicopter. We both have a background in mechanical engineering and we found it very difficult to obtain good specs on brushless motors and propellers. Some websites provide a results database, but these databases are frequently for a combination of both motor and airplane propeller, which makes building something different very difficult. When we started rewinding motors, we built a system to measure torque, current, rpm, voltage and temperature. With torque, we can actually completely decouple the motor and propeller in the results, which really speeds up the development and allows calculating ideal gear ratios or find the correct propeller, in order to maximize efficiency.
The code is based on a modified MultiWii system with a series of connected sensors. Currently, the tests are entirely automated to test different pitch and torque combinations. There are safeguards in the code to stop the tests based on measured temperature, rpm and current. The code automatically generates graphs of the results.
That got us thinking. Would people be interested in a community-based website with motor and propeller specs? Our code is already open source and we could make our test rig available for purchase. The rig could be sold on kickstarter for approximately 150-250$ if we sell 40-100 copies. We don't expect this project to generate money really, but the goal is to help the community obtain better motor and prop tests. The system would include torque, thrust, current, voltage, rpm and temperature sensors. The instrument would include a mount for most standard motors. People with test rigs could upload the motor and propeller results on the website and specify the brand, spec or custom winding, etc.
Below are a few more images of our tests. The helicopter is a modified FBL100 and we tested quite a few motors and propellers. Our tests demonstrated that it is very difficult to make a small and efficient direct drive helicopter. It flies though.
If you are interested, please reply to this survey!
Comments
@Greg, glad you agree to the idea :)
@Marin, yes you are correct, that it must be accessible, and design decisions will have to be made. But we do not want it to be cheap and a bad quality device. The hobbyking one has +-10g accuracy, which is very low. Of course we won't get 1/100g precision, but that is not necessary. In any case, we will clearly state the accuracy we end up with.
I agree. Build yours for small motors as they are more common. I'd be more than happy to make my own for larger setups if you provide code and build specs.
I like your project, and i´ll be waiting for more details about sensors and everithing.
It is important to keep an accesible price, because the ideal is too much people uploading information and tests. If you make an expensive rig then only na few can access it.
@ Greg, you raise an important question about the rig's design. I'd like to ask everyone's input on this: what is the most common motor/prop size you work with in your projects?
If we support powerful motors (>5kg trust) then the whole rig will be bigger, heavier, and more expensive. Also, it would reduce the ability to work with tiny motors/props due to decreased sensitivity of the sensors. If our system has open-source code, and if 1% of people actually work with larger machinery, they could mod the rig themselves to support it. We may provide instructions on our site to do so. What do you think?
Hi, I am Charles's parner in this project, I will pitch in from time to time to the conversation. @Martin, thanks for sharing the trust rig. Yes it is a good deal, keep in mind it has no torque sensor and cannot decouple the motor and the propeller into two separate entities. As Brian said, we can consider the ESC, motor, and propeller, as three separate control volumes, and that given the proper sensors, can be completely separated in the results. Isolating the motor from the propeller allows one to predict what would be the best motor (from all the community data) for a specif propeller, or specific trust, even if this specific combination has not been tested.
Our plan is to allow our device to upload data to our website, which very quickly will provide usable community data. We will have quality plots of the results and a method to search based on your criteria.
In terms of the test rig we are developing, we want to use high precision 24bit analog-to-digital converters, and design a quality circuit giving consistent and calibrated results. I worked in a research institute where my duties where to design and calibrate high precision sensors.
Of course, all our source code will be open-source so anyone will be able to hack the rig for their own purposes if they want to add additional sensors, etc...
In the next weeks, we will post more details on the whole project, where you will be able to give valuable feedback on how YOU want it to be.
I see this the other day and O put it on my wish list.
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idproduct=...
But having it conected to a pc and graph everithing is much much better.
@Greg thanks!
@Brian Martin That is interesting. Testing the motor without load was mostly useful to obtain the kV now. The motor speed is function of the input and the load torque from the propeller. We could change the load on the motor by changing the pitch of the helicopter head. It would also be possible to test different load with fixed pitch propellers by just changing them. We are definitely working on an improved version now, and we will post an update in a few weeks hopefully. We want to build this database you are talking about.
@Charles Blouin Thanks for the response. Yes i have some experience with these load cells. I think i understand your set up so you are measuring the torque at the motor shaft but you also have the propeller attached. Is this correct?
If so then what you have is a chassis dyno because you are measuring the torque that the motor delivers to the propeller, but you are still measuring the motor propeller system. But i thought the goal would be to characterize the motor, by generating a proper torque curve over its entire speed range. Then reattach the propeller and conduct the same test (the dyno now becomes a chassis dyno not an engine dyno). Then since power = torque x angular speed you can simply multiply torque and speed at every discrete measurement point to obtain the power curve.
You do this for both test that you ran
then we would have 2 power curves as a function of angular speed.
Subtract the 2 and that should give you the propellers power curve as function of angular speed. If you add a load cell for measuring thrust for the second test (or you have a corresponding thrust vs angular speed curve for the test prop.) then you can create a thrust vs engine power curve. Then using the motor curve and prop curve (assuming there was a database for all of these) then you could select the best motor for a specific propeller or vice verse. Its worth noting that that propeller curve would only be valid at the test altitude temperature and pressure, and for zero free stream air velocity (assuming that's what you conducted your test at). Like you guys i am a mechanical engineer too but it has been since 2009 since i dealt with this stuff so my logic could be flawed. Time to dust off the old books ;-)
Anyway i feel that this form of testing is necessary to further advance the efficiency of multirotors (trial and error only gets you so far). Keep us all updated on your progress with this and if their is anyway that i can help just let me know.
-Brian
I'd buy one on Kickstarter. Just make sure it can handle big motors and props (think U10, 30" props).
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