rsambo Posted October 20, 2015 Posted October 20, 2015 I've taken Pete up on one of his boards with the load cell and will do my own casing which saves a lot of cash. I have rigged various arduino tests stands up but as Pete points out the resolution just isn't there. By the time you've bought all the bits and put it together I reckon all the software and hard work of Pete's design makes complete sense..... I'll let you know.
Col Posted October 20, 2015 Posted October 20, 2015 I will likely do the same, better to spend cash on tried and tested than on trial and error Being able to directly compare motors made on different continents (tested on Pete`s stand) is reason enough to buy one.
jessoman Posted October 20, 2015 Posted October 20, 2015 (edited) That's very true, comparing apple with apples is the right way to go. Because in the end we are all, on a global scale, comparing each other's curves. Using the same measurement equipment does make sense. That is the biggest thing I found in my search, the high hz/resolution data logging. No use data logging once per second . Same as Col, those terrible postage costs! (Australia) Edited October 20, 2015 by jessoman
stix Posted October 21, 2015 Posted October 21, 2015 ...I have rigged various arduino tests stands up but as Pete points out the resolution just isn't there. By the time you've bought all the bits and put it together I reckon all the software and hard work of Pete's design makes complete sense..... I'll let you know. I wouldn't say that the Arduino Uno (or equiv.) resolution is inadequate for testing most of the rocket motors (bp or sugar) that are made by members of this forum. I would however agree that by the time you put all the bits together including the software, an "out of the box - stand alone solution" is very appealing and likely cost effective. Here's some appox. and basic specs for two of the Arduino development boards that may be of interest.This is based on a load cell with a maximum 15 kilograms (33 pound capacity) and best case scenario. Arduino UnoCost: Around $30USAnalog Resolution: 10bitThis unit is capable of recording 1000+ and beyond samples per second.For our purposes we'll stick with 100 samples per sec (more than enough) with a respectable resolution of 15grams (.5 ounces) Arduino DueCost: Around $60USAnalog Resolution: 12bitThis unit is capable of recording 1000+ and beyond samples per second.For our purposes we'll stick with 100 samples per sec (more than enough) with a respectable resolution of 3.7grams (.13 ounces) Overall I think the capabilities of these boards are excellent. Also you can modify the software to suit. But, then you need to have a load cell, load cell amplifier and put it all together with reliable software to record and analyse the results. A lot of time and effort - which I particularly enjoy because I like to make things myself - if only for the journey I'd be interested to know what the specs are for the Acme test stand, ie. sample rate and resolution. Cheers.
Oinikis Posted October 21, 2015 Posted October 21, 2015 First of, buy an arduino UNO clone, will cost couple of bucks. Then you need INA125P opamp, which will cost about 5 bucks, and the loadcell itself, you can get from a bathroom scale. Then you'll need a resistor for the opamp, around 30-50 ohms (I have a switch, so I can change the value and have different resolutions). Then you'll need to calibrate it by changin the loadcells resiatance, I did it by making a small coils and connecting it to one of the signals. the resistance if that was enough. Then about resolution being too small, it's not too small, plus there seems to be some weird noise, which forms a sine wave. This actually increases the accuracy. Let's say we change the load a tiny bit, so the values doesn't change, but the timing between them will, and when you average it, you get more precise answer, because the sine cancels out. sorry for mistakes and lack of formatting, I'm on a phone.
stix Posted October 21, 2015 Posted October 21, 2015 . . . the loadcell itself, you can get from a bathroom scale . . . Yep - that's the way my brother has got his load cells, off the side of the road from someone's junk collection. I was talking to him about it just this last weekend. He told me that the main point is that there is a "smoothing" capacitor that has to be removed to make the load cell more responsive - this step is very important to make the load cell "raw" and be able to record sudden dynamic changes.
stix Posted October 21, 2015 Posted October 21, 2015 (edited) . . . Using the same measurement equipment does make sense. . . Nothing personal jesso, but no, it doesn't make sense. The same measuring equipment is not a requirement. To compare apples with apples, all you need is an agreed set of standards and methods of measuring to arrive of what constitutes an apple. The apple standard (definition) has already been agreed upon no matter which side of the globe you are on. Edited October 21, 2015 by stix
jessoman Posted October 21, 2015 Posted October 21, 2015 I mean just calibration, yes I agree the standard may be the same, and should be. However, variances in manufacture MAY lead to slight different results. Minimising that would be ideal :-). As long as the standard is the same is all I desire.
Peret Posted October 22, 2015 Posted October 22, 2015 The Acme sample rate is 80/second and the resolution is 16 bits. Actually the ADC is 24 bits but only about 19 bits are meaningful - the rest are lost in the noise. That's processed down to 16 bits and scaled so that load cell full scale load = 65535. The rig has been calibrated with a known load and when it presents the data it does so in engineering units (grams). With a 100kg load cell that gives a resolution in the order of 1.5 grams and a full scale of 100,000 grams, so it can count nickels and test a 6 pound whistle rocket. I've always sold the electronics board separately for people who want to build their own rig. Ned Gorsky did it that way. I can provide a load cell too if you can't source one locally. I'm out of boards at the moment but I'll be making a batch by the end of the month. You can make your own with an Arduino, and I kind of encourage you to try, but with only 10 bits of resolution, you really have to have a set of load cells to swap out so that whichever one you're using is near full scale for the motor. You also have the far from trivial problem of amplifying and scaling the millivolt load cell output. You can spend more money on the front end than on the whole of the rest of it. I cheated by using an ADC with a built in differential amplifier, and yep, the ADC costs more than all the other parts on the board put together. I believe I'm going to put the schematic diagram up in the file area since there seem to be some experimenters here. 1
rsambo Posted October 22, 2015 Posted October 22, 2015 Have to say I agree with Pete. I have 3 load cells an INA125p and an arduino Uno - I have it setup nicely for a 15mm bp rocket but you can't go to it, switch it out for whistle / different sizes etc without hassle and i like to test up to 3lb rockets and even gerbs. I guess in my case I've just decided to concentrate on a different set of experiments so I can't wait to get my hands on one of those boards...
stix Posted October 27, 2015 Posted October 27, 2015 (edited) Agreed with what rsambo posted - it all depends on what type of motors you are measuring, although I'm interested in why you would want to measure the thrust of a gerb ??. Pete, thanks for posting the info including the circuit diagram. My interests lie more in the software - perhaps we can talk about some mods to suit my purposes - which I would pay for, as others would pay for hardware. I do have to question some things though. In my view, a sample rate of 80 per second is bordering on not good enough. Especially considering a resolution of 1.5grams over 100 kilos - it seems a bit counter-intuitive. 100 at the least, 500 would be more than enough, 1000 just for simplicity. Obviously there are limitations with the ADC chip that can't be changed. I don't think the video is a positive endorsement of the device. The motor is moving around at random and unsecured - nullifying any apparent accuracy in resolution, and therefore test accuracy. I certainly wouldn't be mounting a motor capable of pushing 20 kilos+ thrust on that device. It is not supported enough. It seems you've made a device that wants to be too many things, but as a result suffers in some important areas. I'd forget about trying to make an "all-purpose mount". A simple "V" mount with strong springs will suffice for most motors. If it's too small, users can simply wrap some thin card around to make up the difference - if it's too large then you need to be able to easily exchange the mount to suit - 2 mounts should cover most situations. Also, I don't think there's an issue with a device that enables you to change the load cell to suit, providing it's made easy to do - it should be a "feature" of the device. If you can't make a determination, as a rough guess between a possible peak thrust of 5 kilos or 25 kilos, then it's time to go back to basics - no thrust meter that I know of (yet) is going to help. This post isn't meant as an attack (my thrust meter is better than yours ) but as scientific scrutiny based upon the information already posted. Let's take this back to where it all began with the original post:http://www.amateurpyro.com/forums/topic/11189-how-to-measure-thrust-of-my-rocket/ I'm needing help figuring out how powerful my rocket is and i can't seem to figure out how to get it. If tried using a scale and the rocket upside down on it but it didn't read the thrust, I think there is an issue with the scale. Please tell me how you guys test your rockets newtons per second. I'm needing to figure this out in order to use other calculations for height and speed. Douglas, it seems to me that you're not quite there yet and need to concentrate on your methods of making motors. Looking at the video, my experience tells me that there was very little thrust to be measured. There are many members of this forum that would be more than willing to help you make some usable motors - leave the precise measuring for later, a simple set of kitchen scales and video will get you started. It's a very rewarding hobby and a thrill when you get it right. Cheers. Edited October 27, 2015 by stix
dagabu Posted October 27, 2015 Posted October 27, 2015 Agreed with what rsambo posted - it all depends on what type of motors you are measuring, although I'm interested in why you would want to measure the thrust of a gerb ??. Pete, thanks for posting the info including the circuit diagram. My interests lie more in the software - perhaps we can talk about some mods to suit my purposes - which I would pay for, as others would pay for hardware. I do have to question some things though. In my view, a sample rate of 80 per second is bordering on not good enough. Especially considering a resolution of 1.5grams over 100 kilos - it seems a bit counter-intuitive. 100 at the least, 500 would be more than enough, 1000 just for simplicity. Obviously there are limitations with the ADC chip that can't be changed. I don't think the video is a positive endorsement of the device. The motor is moving around at random and unsecured - nullifying any apparent accuracy in resolution, and therefore test accuracy. I certainly wouldn't be mounting a motor capable of pushing 20 kilos+ thrust on that device. It is not supported enough. It seems you've made a device that wants to be too many things, but as a result suffers in some important areas. I'd forget about trying to make an "all-purpose mount". A simple "V" mount with strong springs will suffice for most motors. If it's too small, users can simply wrap some thin card around to make up the difference - if it's too large then you need to be able to easily exchange the mount to suit - 2 mounts should cover most situations. Also, I don't think there's an issue with a device that enables you to change the load cell to suit, providing it's made easy to do - it should be a "feature" of the device. If you can't make a determination, as a rough guess between a possible peak thrust of 5 kilos or 25 kilos, then it's time to go back to basics - no thrust meter that I know of (yet) is going to help. This post isn't meant as an attack (my thrust meter is better than yours ) but as scientific scrutiny based upon the information already posted. Let's take this back to where it all began with the original post:http://www.amateurpyro.com/forums/topic/11189-how-to-measure-thrust-of-my-rocket/ Douglas, it seems to me that you're not quite there yet and need to concentrate on your methods of making motors. Looking at the video, my experience tells me that there was very little thrust to be measured. There are many members of this forum that would be more than willing to help you make some usable motors - leave the precise measuring for later, a simple set of kitchen scales and video will get you started. It's a very rewarding hobby and a thrill when you get it right. Cheers. STIX, I am not sure why you are attempting to decry the ACME rig, the sample rate is fine for all HPR and amateur pyro as long as the load cell is matched to the motor but this is the same for any thrust measuring rig. The motor holder is a simple device and is a starting point for motor testing but was never meant to be the "one fits all" that you seem to indicate in your post. The ACME rig is solid, extremely transportable and easy to use, it is an all-in-one device and very unlike anything out there. It works and works well all the time with rock solid output and is in my opinion, the standard for other test stands to match. I really dont think you know enough about it to cast an opinion about it. The idea of a V-mount is a good idea and I would love to see a drawing of your idea on how to make it self-centering so that the pressure is centered above the load cell.
stix Posted October 28, 2015 Posted October 28, 2015 (edited) STIX, I am not sure why you are attempting to decry the ACME rig, the sample rate is fine for all HPR and amateur pyro as long as the load cell is matched to the motor but this is the same for any thrust measuring rig. The motor holder is a simple device and is a starting point for motor testing but was never meant to be the "one fits all" that you seem to indicate in your post. The ACME rig is solid, extremely transportable and easy to use, it is an all-in-one device and very unlike anything out there. It works and works well all the time with rock solid output and is in my opinion, the standard for other test stands to match. I really dont think you know enough about it to cast an opinion about it. The idea of a V-mount is a good idea and I would love to see a drawing of your idea on how to make it self-centering so that the pressure is centered above the load cell. I wouldn't go so far as saying I'm "decrying or condemning" the ACME test rig. I'm simply questioning what has already been posted by yourself and pete. Yes, perhaps I am being a bit critical and picking up on things that others wouldn't, but criticisms can always be refuted and discussed openly. I would look at this as more of a "peer review". I accept that there may be a tinge of "thrust meter envy" on my part, but only for the fact of creating a "stand-alone" device, not for the application and design of the device itself. However that does not, and nor should it cloud my view. "I really dont think you know enough about it to cast an opinion about it." Ok, perhaps I don't know about any current improvements - I'm going off what has been stated here, and in the original thread: http://www.amateurpyro.com/forums/topic/7184-rocket-test-rig/?hl=%2Btest+%2Bstand#entry94500 If there have been changes that I'm unaware of, apart from pete's answer from a earlier post: "Some minor changes to the rig. It's USB now, no serial adapter needed. The analysis software has improved a lot." then fine - I don't know. Happy to hear about it though. "The idea of a V-mount is a good idea and I would love to see a drawing of your idea on how to make it self-centering so that the pressure is centered above the load cell." Yes, the V-mount is a good idea - images are attached. In regards to your concern: "how to make it self-centering so that the pressure is centered above the load cell." Well, that's like saying I should take a ruler to the supermarket to ensure that my apple that I bought is exactly centered on the scales, lest I be over or undercharged - think about it. Anyway, just to lighten things up and to show the "V-mount": May I introduce Stephanie - always ready - certainly not perfect, but suits small desires. Midrange: Always reliable and accurate but has some limitations. And finally, Bruno. Bruno is always right, has a large capacity for understanding reality, and definitely not to be messed with. If you can see it, the 9v battery should give a sense of scale. Cheers. Edited October 28, 2015 by stix
edbrown Posted October 28, 2015 Posted October 28, 2015 Hi all, This will be my inaugural post to (I Hope) fulfill my final requirements in order to see attachments. I'm retired from 40+ years at Estes and have had a little experience with rocket testing and related problems. Thought this would be a good thread to add my 2 cents worth.The ACME stand by Peret is more than adequate for almost any hobby purpose that I can think of. I've only played with it with weights, but it is fun to use. Although I'm retired from Estes, I'm able to access thrust stands that cost several thousands of dollars. Some of which I developed and did the simple QuickBasic programming that was required. My replacement has developed and built a next generation stand based on National Instruments hardware and LabView programming. Very expensive and definite overkill for any hobby use for most of us. My electronic knowledge is much, much less than Peret's, but, hopefully sufficient to appreciate the ACME's qualities. Regards to all,Ed 1
Maserface Posted October 28, 2015 Posted October 28, 2015 Good to see you, Ed, and welcome! "Yes, the V-mount is a good idea - images are attached. In regards to your concern: "how to make it self-centering so that the pressure is centered above the load cell." Well, that's like saying I should take a ruler to the supermarket to ensure that my apple that I bought is exactly centered on the scales, lest I be over or undercharged - think about it." So, then, how much discrepancy do you think is caused by the small amount of wiggling that occurred in the video you cited, and likewise for the off center V block assembly that you claim isnt a big deal?
Peret Posted October 29, 2015 Posted October 29, 2015 (edited) Yes, the sampling rate seems slow. That's the trouble with delta-sigma converters. My original prototype used a regular 16 bit successive approximation register with a differential amplifier front end, but I was lucky to get 11 or 12 bits out of it. The least significant bit was around fifty microvolts. If you think you can make an amplifier with a gain of 500 and output drift and noise below 50 microvolts, using components that you can buy off the shelf from distributors, to work in all ambient conditions, all I can say is you've never tried it. The best is the enemy of the good enough. The ACME is full of trade-offs, but fortunately the people who have actually used it find it to be good enough. As for the video, the first motor CATOd and wiped out the test cup. It had a replacement cup that was too big and the user didn't secure it properly. He also didn't use the donut sandbag that normally secures it. <edit> I don't know why I'm defending it to you, anyway. Please point out the inaccuracies in the thrust curve, which was included in the video. You must be able to see the effect of that wobble, right? Edited October 29, 2015 by Peret 1
Col Posted October 29, 2015 Posted October 29, 2015 Rockets can have a bit of wobble on the way up, so it may even be a bonus 1
stix Posted October 29, 2015 Posted October 29, 2015 (edited) Yes, the sampling rate seems slow. That's the trouble with delta-sigma converters. My original prototype used a regular 16 bit successive approximation register with a differential amplifier front end, but I was lucky to get 11 or 12 bits out of it. The least significant bit was around fifty microvolts. If you think you can make an amplifier with a gain of 500 and output drift and noise below 50 microvolts, using components that you can buy off the shelf from distributors, to work in all ambient conditions, all I can say is you've never tried it. The best is the enemy of the good enough. The ACME is full of trade-offs, but fortunately the people who have actually used it find it to be good enough. As for the video, the first motor CATOd and wiped out the test cup. It had a replacement cup that was too big and the user didn't secure it properly. He also didn't use the donut sandbag that normally secures it. <edit> I don't know why I'm defending it to you, anyway. Please point out the inaccuracies in the thrust curve, which was included in the video. You must be able to see the effect of that wobble, right? graph.jpg Thanks for posting the facts Pete. Now that you have explained why the motor wobbled, and also that there is usually a donut sandbag that surrounds it - it makes more sense. It may be a good idea to remove that video and replace it with one that shows the device in a more favourable light. As you have gathered, I found it rather disturbing. The sample rate of 80 is the limit of the chip as I thought. Given a choice, I'm sure you would prefer higher, but 80 is good enough as pointed out by people who have actually used the device. The 100 samples is stuck in my head from early experiments where lines of data had to be counted to determine thrust time without needing a calculator. I don't know much about electronics and accept what you've said is correct - my strength is more with software and analysis. Regarding any correlation between the video and the graph, perhaps there may well be some detection of the wobble on the graph if only the sample rate was higher! - hang on, maybe that's a good thing anyway - hmmmmm. The wobble wouldn't make much difference to the overall results - it just didn't look good in the video, especially given the positive claims about the device - that was my main bitch, but now all has been explained - thanks. I've ordered some parts for making my own new device. I'll be using the Arduino Due with a voltage amplifier and parallel beam load cell, all sourced very cheaply from China with free shipping. Unfortunately it's too cost prohibitive to source the parts here. It won't be anywhere near as accurate as the ACME, but much better than Stephanie - it'll take me some time to get it together and come up with a new pet name. btw. what type of load cell are you using, ie. "S", button or parallel beam or other? [EDIT] Don't worry about defending your device, I would too. It's like a being a parent and someone tells you that your kids are ugly - you have to somehow justify it. I usually just point to my wife. Cheers. Edited October 29, 2015 by stix
Peret Posted October 29, 2015 Posted October 29, 2015 It's a Chinese parallel beam load cell, 2mV/V and of generous size. The ADC will actually run a little faster if you clock it externally. It will take a clock up to 8MHz, which I can get from the MPU. However, it takes 61,440 clock cycles to make a conversion, so at 8MHz you still only get 130 samples/sec and now you have an inconvenient scale factor. Bear in mind that the motor, the holder and the load cell all have mass and inertia, so it isn't like the system as a whole can actually respond at 100Hz. 1
stix Posted October 31, 2015 Posted October 31, 2015 (edited) ... Bear in mind that the motor, the holder and the load cell all have mass and inertia, so it isn't like the system as a whole can actually respond at 100Hz. I suppose that does make sense, although it would depend on how large the motor was and there must be a point of "diminishing returns". I'm still not entirely convinced that 100hz is too much - I believe it should be higher but maybe that depends on the intended thrust profile of the motor, ie. ported end burner or core burner. Look at the sample data and graph attached - this 1lb motor was recorded on the AD7730 (mid-range test stand) 1gram res @ 100Hz. You can clearly see at peak thrust - if there was an opportunity for a "double-up" of the recorded data due to "over-sampling", we would see it, as in a repeat of the numbers. It was nice to see a recording of 1gm resolution between 3797 & 3796. My view is that once you start to see a double-up of sample data, then the sample rate is high enough - this is not. This motor IS actually responding at 100hz. - or have I lost the plot??... Although, it probably doesn't make that much difference to the overall final measured results. Arriving at peak thrust and heading down (skint-31):3746377037883797379637833756 One important thing to take into consideration is the configuration of the motor. The motors that I make are KNSU internal/core burners - bates grains with inhibited ends. The thrust profile is deliberate, ie. progressive over time (skint-31). Whereas your posted test (run number 8) is a bp rocket, ported end burner (I think), showing an initial high peak thrust (over approx. 0.1sec) with approx. 8secs of sustained and relatively even thrust. Therefore a higher sample rate may well have recorded an even higher peak thrust?. With mine the sample rate actually could be less, because there is no short pulse, and it's progressive. With a progressive profile it's not as important to have high resolution. 5 grams would be fine because with a progressive profile, it tends to average it out. Food for thought. Cheers. Edited October 31, 2015 by stix
stix Posted November 4, 2015 Posted November 4, 2015 . . . the motor, the holder and the load cell all have mass and inertia . . . Yes, upon reflection, that point should have been taken into consideration more seriously - sorry, my error. The skint-31 graph may well have recorded the force accurately enough, but how much was "overthrow" due to its mass?... Smaller motors not so much, bigger ones would have more of a bearing on the overall result I would think. Cheers.
stix Posted March 8, 2016 Posted March 8, 2016 .... I've ordered some parts for making my own new device. I'll be using the Arduino Due with a voltage amplifier and parallel beam load cell, all sourced very cheaply from China with free shipping. Unfortunately it's too cost prohibitive to source the parts here. It won't be anywhere near as accurate as the ACME, but much better than Stephanie - it'll take me some time to get it together and come up with a new pet name. btw. what type of load cell are you using, ie. "S", button or parallel beam or other? [EDIT] Don't worry about defending your device, I would too. It's like a being a parent and someone tells you that your kids are ugly - you have to somehow justify it. I usually just point to my wife. Cheers. Sorry If I quoted myself and dragged up an old thread. If nothing else, I've been true to my word - it's taken about 5 months though. I'm certainly not saying "my" device is better - it's just a alternate way to approach it. I've started a new thread here: http://www.amateurpyro.com/forums/topic/11545-thrust-metertest-rig-prototype-ver10/ It would be good to discuss and read others opinions. Cheers.
Redrocketman Posted June 25, 2016 Posted June 25, 2016 I have to say from my experience with what looks like an "E" motor, that is not putting out sufficient thrust, way too long burn time, & nozzle dia too big. To be honest I have videos of nozzle-less "E" class motors with much faster aggressive burn times. What percentage of the total grain dia was the nozzle? How far was it cored? If done correctly you should get amazing results from your motor. At present, I'm trying to tame mine back as their too "hot", though I used a fuel with addition of 1% red iron oxide which I use in my "H" class motor and above. Not ever to be used again in small motors. A simple 65 / 35 mix will be fine if u ensure to boil out ALL the water. As also stated, for your first thrust meter, Spring scales are the go. I have almost ( will 2day ) finished mine, thanks to the advice for people here. Good luck & stay safe
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