Added by J. Simmons almost 12 years ago

There are two projects that I think might help us sort out how to get started with data collection.

Replies (53)

RE: Resources - Added by Jeremy Wright about 11 years ago

Yet another inexpensive load sensor.

RE: Resources - Added by Jeremy Wright about 11 years ago

I came across this information on Estes motors that said if their designation ends in a 'P' for "Plugged" (i.e. B6-P), that there is no ejection charge. It seems like those motors would be ideal for using in a test stand.

Here's an example of plugged engines:

The following page had some information that was helpful to me as a model rocket neophyte.

RE: Resources - Added by J. Simmons about 11 years ago

More good finds. The Estes PDF file is especially good. I think I somehow missed that the same class motors could come in different sizes (A's come in standard and mini, C's come in different subclasses, C6 vs C11, which are different sizes). This will need to be accounted for in the motor mount for the test stand, or we should update the requirements to use a particular size of motor for the first version. I think I would lean toward the latter, and Choose the D/E size since they are the same diameter, and there are plugs to make a D or C11 fit in an E motor mount. This would simplify the design considerably.

As for the plug nozzles, that would be another opportunity for simplification. Of course, I say that and it looks like plugged motors only come in A class motors (at least that is all I see on the website under plugged motors).

RE: Resources - Added by Greg Moran about 11 years ago

@Jeremy, This .pdf handout is also a great tool for Rockets 101. Good research! On your second point (RE: plugged motors), I think that is backwards thinking to limit our test stand in that manner. I'd like to build a test stand to test a variety of motors, and not install artificial limitations. I expect it will be no more complicated to incorporate motors with ejections charges as it will be for motors without them.

RE: Resources - Added by J. Simmons about 11 years ago


In the end I think we will have to go with supporting standard motors just because there seems to be few (maybe only one right now) motors that are available in a plugged variant. But, I would like to remind us all that we should always be thinking "this is just the first version, we can, and will, make more versions with more capabilities." So, I think limiting things at this stage of the game is a good thing. It lets us focus on the essentials and ensures we will produce something in time for the conferences. I also want to honor Jeremy's concern that protecting the sensor(s) from the ejection charge will take some amount of extra work, so plugged motors would have been convenient for this version.

Again, considering just how few motors appear to be available in a plugged variant, this is probably a moot point.

RE: Resources - Added by Jeremy Wright about 11 years ago

I do personally feel that using plugged motors would simplify the design. I quickly worked through the 0.33% LSB requirement and unless I'm mistaken (and I easily could be since it was early in the morning when I did it), we can't use anything over a 6 lb load cell with the 10 bit ADC on the Arduino to get the resolution we need. Depending on how we handle the ejection charge I am worried (as J mentioned) that it might be possible to damage the load cell. With that said, it sounds like standard motors are the way to go, and that will just have to be one of the design challenges/features of the stand. Hopefully Greg is right and it won't end up being any more complicated.

BTW, if someone could run through math for the 0.33% LSB requirement for Shepard's measurement system, that would be great. I think it would be good to have an idea of how that requirement limits us before we gel the requirements doc, and I would like someone to either back up or challenge my 6 lb number. Either way I think it will be a helpful discussion.

STSR 4.7 In accordance with (IAW) NAR testing manual requirements specified in Section 8.5, the STS shall use a data collection system “where the least significant bit (LSB) has a value that is” 0.006 lbs (“less than 0.33% percent of the average thrust of the motor under test”).

RE: Resources - Added by J. Simmons about 11 years ago


I'd be happy to run the numbers myself for comparison. Alternatively, I'd also be happy to review your numbers if you posted them. I assume you just worked out the numbers by hand? If so, we could create a folder for the data collection sub-system in DMSF and you could upload a scan of your numbers for me to review. This would also have the advantage of starting to document this aspect of the design, and we could reference the math in the requirements wiki page (since the .006 lbs is a derived value from the specified percentage in the NAR requirements).


RE: Resources - Added by Jeremy Wright about 11 years ago

I was kind of hoping for someone to run through it independently so that we could compare logic as well as numbers. If you want to do that, stop reading here and go run your own numbers.

1. Assuming linear load cell output (depends on load cell we use):
  • R = W / 2^B Where R is the ADC resolution, W is the weight range, and B is the ADC's bit resolution.
  • For a 10-bit ADC (like the one in the Arduino Uno), 2^10 = 1024 bits
  • If we take the 0.006 lb derived value (see J's post above) and spread it over 1024 bits we get (0.006 x 1024 = 6.144) about 6.1 lbs max for our load cell.
2. You can also do this based on the LSB voltage:
  • We don't know what voltage range the ADC we will select will use, so I'm assuming 10 volts for this example.
  • R = V / 2^B Where R is the ADC resolution, V is the voltage range, and B is the ADC's bit resolution.
  • So, R = 10 volts / 2^10, or R = 0.00977 (0.010 volts).

Does my logic track?

Ben, you're an analog hardware guy, do you have any thoughts on this?

RE: Resources - Added by Ben Barnett about 11 years ago

The ADC resolution calc's look fine to me.

Please remind me what the plan is regarding signal-conditioning and budget constraints. Both thermocouples and force-sensors (strain-gauge bridges) have outputs of a few millivolts, that will require amplification to "fill" the ADC range of the Arduino board. Load cells are available with amplifiers included, but they are far more expensive than "bare" force sensors.

(I have been watching the e-mail notices, but have not participated in the discussion due to an overload of stalled projects and excessive time spent dealing with medical issues involving both myself and my wife.)


RE: Resources - Added by J. Simmons about 11 years ago

Thanks, Ben for the extra set of eyes on the math. The budget is still at $200 (not including consumables like motors and igniters, etc). I do not think we have discussed signal conditioning yet. I imagine that will come up during or after the Block Diagram discussion, which is coming up very soon.

RE: Resources - Added by Jeremy Wright about 11 years ago

J beat me to the post, so please forgive me for repeating things he said.

@Ben - Thanks for checking those calcs, I appreciate it.

I'm sorry to hear about the medical issues. I completely understand if you don't have time to participate, but we value any input you have time to give. We're getting very close to finishing the requirements document, and I anticipate that we'll be able to make fairly short work of the block diagram. That will put us at step 4 of the engineering process, which if I remember right is where you said your strengths and interest would start kicking in more.

We haven't gotten to the point where we can discuss signal conditioning in depth yet, but I think you make a good point about the increase in cost that comes with built-in conditioning. The project budget is only $200, so we'll have to give this some serious thought. Our deadline for starting the preliminary design is the 29th of this month, and I think signal conditioning needs to be a hot topic right off the bat.

RE: Resources - Added by Jeremy Wright almost 11 years ago

I was going back through these resources collecting information for a post to the DAQ preliminary design discussion, and I couldn't find a way to make any of the load cells that we've talked about here fit into the budget. I looked around a little more and found an interesting product line, and a possible less expensive force sensor that has a range closer to what we need.

There's the Phidgets product line which are proprietary designs (as far as I can tell) that connect to a USB adapter board. The thing that caught my attention was that the load sensor comes out of the box measuring a max of a little over 6 pounds.

The same company (and others) also have Force Sensing Resistors:

The FSRs have a range that goes up to about 22.5 pounds. I'm an amateur when it comes to electronics, but couldn't we crop/window/scale that range to work with our 6 lb range limit for the NAR resolution requirements? What would that require in the signal conditioning department?

RE: Resources - Added by Jeremy Wright almost 11 years ago

Burn Sim software for "Solid Propellant Internal Ballistics Simulation".

RE: Resources - Added by Jeremy Wright almost 11 years ago

I just noticed this link that was posted on Facebook awhile ago. I wanted to make sure we captured it here.

RE: Resources - Added by J. Simmons over 10 years ago

DAQ board for Netduinos - might be worth looking into in terms of features and implementation.

RE: Resources - Added by J. Simmons over 10 years ago

32-bit Arduino Due - might be helpful for improved DAQ (especially in terms of sampling rate)

RE: Resources - Added by J. Simmons over 9 years ago

Link to a PDF about model rocketry - included flame temps, useful info for spec'ing sensors.

RE: Resources - Added by J. Simmons over 9 years ago - looks like a possible place to promote Shepard after we get the kits going.

RE: Resources - Added by J. Simmons over 9 years ago - Someone working on larger test stands and with his own engines (he even releases most, if not all, of his documentation)

RE: Resources - Added by J. Simmons about 9 years ago

Some more links (note to self, need to come back and review these resources and make a wiki page from them).

RE: Resources - Added by J. Simmons about 8 years ago

From Aaron's G+ feed:

Design considerations for things which use linear bearings or bushings such as some of the designs for Gen1 Shepard, CNC gantries, and 3D printer mechanisms.

RE: Resources - Added by J. Simmons almost 8 years ago

Sparkfun's newsletter this month is full of load cell goodness.

RE: Resources - Added by Jeremy Wright almost 8 years ago

This looks like an interesting option for the successors to Shepard.

SparkFun Load Cell Amplifier - HX711

RE: Resources - Added by Jeremy Wright almost 8 years ago

There's some other good stuff in the newsletter that amplifier came from.

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