Self Built RC DYNO

[Iceyankee-Tsi]

RC Damo:

Here are some pictures of my build. Hope you and everybody else enjoy.

Regards, Andres

[billygreen1973]

All these builds are looking great, I'm getting V. jelous.

I am having trouble finding suitable material for the rollers (weight and diamter) so I was wondering Damo,

Is it possible using your software to calculate the 'aparrant' weight of a roller.

What I'm thinking is this, If i add some form of resistance or load to the roller in the form of the roller driving an electric motor ( used as a generator) and then loading that motor with a variable resistor, and hence slowing the roller.

If the roller is spun up to a given RPM (eg, 3000 RPM) then allowed to 'free wheel' and run down, the time taken to run down should be proportoinal to the apparant weight of the roller. Only thing is, I don't know how to work this out and even if my theory is correct.

Any info or comments are very welcome!

Thanks

[DamoRC]

@billygreen

The short answer to the "can I use a motor" question is yes. However, I am not sure how this would work exactly. If we understood the various attributes of the motor then we could convert this to an apparent mass / diameter combination (representing a certain load) and input this into the software. Your "freewheeling to a stop" solution might work to provide this info. I would need to think about this a little more.

I would like to know a little more about your car to determine why it is you think you can't find rollers to suit. So what kind of car do you have and what weight is it?

[Haywiremav]

I like this idea, I dont need the figures but a roling road would definately help me get my hpi mt2 running right as I can adjust the fueling whilst its running.

[billygreen1973]

Damo

I am using my sons car, its a 4 wheel drive Kyosho GP3, it weighs 1.667kg.

I am having problems finding tubing with a wide enough diamter that is heavy enough to use for the rollers

The reason I'm looking to use a motor as a brake or load on the roller is so I can adjust the apparent weight of the rollers so as to be as accurate as possible with a number of different cars at our RC club.

I know I could just add weighted disks to the roller shaft to adjust the weight, but I was just thinking it could be more accurate and quicker to just change the load on the motor.

This method is used in full-size Dyno's, but I cant find any maths to work out a reliable example.

Dyno's that use a motor in this way are called 'absorbtion' or 'universal' dynos

Thanks

Edited September 29, 2010 by billygreen1973

[DamoRC]

@billygreen

Okay - I see what you are trying to do and I will help out in any way I can, but be warned, this approach is likely to be very complicated and probably not as accurate (close to the true value) or precise (same value for a given car time after time) as the simple roller approach. Let me explain.

Choosing a motor. It could either be a brushed or a brushless affair. Advantages to the brushless are that it should not change its' performance over time due to brush wear. The disadvantage is that you are going to have to come up with some circuitry to feed the power from this type of motor through a resistor (a rectifier of some sort). This would not be needed for the brushed motor, but as the brushes on this type of motor wear, the performance (and therfore the results from the dyno) might change and may lead to the need for frequent calibration.

Choosing a resistor or load. A simple POT is not going to do the job here. 1 HP is 745 Watts. Lets say you have a 0.5 HP car that was somehow connected directly to your motor / generator and the motor is about 50% efficient. Thats 0.25 HP or 190 Watts (ish). Your load resistor will have to be able to dissapate this power as heat so if you want a variable transistor, you are going to need a rheostat. A quick search on Digikey puts a 50 Watt rheostat at $30 - $40. So even if your rollers were taking up another 50% of the power, you are going to need 2 rheostats (or 1 with a 50 watt resistor in series). I have seen other discussions where folks have suggested using incandescent light bulbs (a couple of 100 watt ones would work) or even the rings from an electric cooker as the load for the motor to drive, but you are not likely to be able to control this electrical load with any granularity using these approaches.

Building the dyno. I am assuming that you can build some sort of roller setup and only want to augment the apparent roller mass with the motor/generator. As you are using a 4WD car, you are going to somehow have to connect the motor to both rollers (a belt drive between the rollers perhaps).

Linearity. I am assuming that the load produced by the motor over an rpm range is linear. If not, then simply modifying the apparent roller mass in the software won't be quite right and your results will be less accurate.

Okay - so it sounds like I am bashing the idea, but I am not. Typically, in the 1:1 scale world, these types of dynos (using a generator load) are very complicated and expensive. If they are expensive and yet folks still buy them, they must be good for something. That something though, is a different approach to power determination and tuning. These types of dynos typically are designed so that by holding the car a a set speed / rpm, adjusting the load on the dyno and then tuning the car so that it performs optimally under that load, you can map the various fuel and ignition settings over a range of rpm so you are at optimal settings all the way through the RPM range. One big advantage to this approach is the speed with which you can tune in comparison to making a tweak and then running a full power curve to see if the tweak worked.

All in all, I think that building the best set of rollers that you can, with the ability to add extra end weight disks will provide you with the best results. However, if you want to try the motor generator approach, I am more than happy to look at the data and see if we can get it to work.

Good luck,

DamoRC

[billygreen1973]

Thanks for the input!

I was thinking something along the lines of an electric cooker ring as the variable load (using control circuit and ring direct from an old hob).

Your probably right though, using extra disks on the roller shafts would be alot easier for now.

I think what I'll do is get the setup working this way first and maybe try and add somthing later.

Thanks again for the advice..

[DamoRC]

Just an FYI. Version 4_2b of the software is available at the site. It "should" be Vista and Win7 compatible.

DamoRC

[DMOne]

Ok forgot the cam again so took it home. still not finished yet still got to box and paint it

[DMOne]

Hey Dash ive been looking for a dyno for about five or six months now with no luck until today, I have just modded out two of my mills and nothing to test them out on im in the middle of timing and shiming them but want to build the dyno you have here can you tell me what type of rollers you used here? they dont look like pvc pipe i would like to get started building right away. thanks for any help you can provide. great job by the way.

[DMOne]

Hey sorry aboutthe dummy move as i read more carefully i saw that you listed all of the parts used again sorry but can you tell me anything more about the reed switch? or should i stick to the magnet?

[-dash-]

Hi sorry lost touch with this topic been so busy with family things not been able to play

As for the reed switch its very hit and miss some will top out 2000rpm others 4000rpm

only real plus side was would pickup the pulse at very low rpm 10rpm rather than a coil that would start at 200rpm

(from my memory might be bit of)

halleffect sensor would be better option as that would easly go upto 9000+rpm never got around to messing with that tho.

or other option is optical sensor from scroll wheel on old pc mouse.

if really honest is alot messing around just to smooth out the signal magnet and coil was allways very clear once got all everything in just right spot.

had better results with non magnetic core on the coil. only 2 waves high and low vs metal core get 3 waves high low high.

hope that helps

@ damo hows things going with you matey

[joeqsmith49]

Hello everyone. I found this site on Yahoo after searching for small dynamometers. It looks like some very nice work has been done with the software and people building their own setups.

Is anyone still working with theirs? Were you able to better tune your engines?

[joeqsmith49]

This is a small direct drive dynamometer I am working on. The following shows the couplers, sense magnets and bearing supports. Setscrews are stainless.

Torque arm and absorber.

Base.

Test motors.

Load

Initial checkout using low torque motor.

Data was collected by hand to start. 0.2 in oz of torque is the drag of the absorber. Horz is in RPM. This is the lower limit for this dynamometer. The green motor can create 100mnm of torque which the absorber handles just fine. I can't run sweeps on this motor without damage to it until I automate it. The problem is the these motors will over heat quickly under high loads.

The jig will allows reading of input power to motor as well.

[joeqsmith49]

A terminal block was added and some small gauge wires for the output of the absorber. The wires are looped and light as possible to attempt to reduce the amount of error they will cause.

[DamoRC]

@Dash

Good to hear from you. Been kind of in the same boat, lots going on not enough time to mess with the dyno stuff. There was a lot of activity on the SimpleDyno forum Dec/Jan as I tried to upgrade the software to run without problems on Vista, Win7 etc. Worked for the most part although there was one installation that seemed to refuse to work for the longest time. Interesting mix of folks on the forum from guys running IC RCs to mopeds and dirt bikes. There is a link to the froum at the SimpleDyno website if you get back into the Dyno "thang".

@Joeqsmith49

That's a really nice looking setup.Is the absorber a slave motor connected to a resistor bank? When you say the data are collected by hand but you don't do motor sweeps are you setting the test motor up to run at different speeds with different voltages then recording the rpm and weight readings from the scales, then repeat the process with a new voltage?

DamoRC

[joeqsmith49]

That's a really nice looking setup.Is the absorber a slave motor connected to a resistor bank? When you say the data are collected by hand but you don't do motor sweeps are you setting the test motor up to run at different speeds with different voltages then recording the rpm and weight readings from the scales, then repeat the process with a new voltage?

DamoRC

Thanks. The absorber is a motor that acts as a generator. The energy is dissipated in the resistor bank (now one resistor). A FET is used to couple the absorber to the resistor. The more that is coupled, the more load placed on the test motor. The absorber is mounted on bearings, so the case is free to rotate. As load is placed on on the motor, force is increased on the arm. This force is directly measured with a balance. The absorber is capable of putting enough load on the green motor to over heat it. It can stall the smaller motor. If not clear, the more load on the test motor, the slower it will turn. This is how the torque/power curve is created.

The voltage to the motor can be swept as well.

I didn't have the PC tied in yet, so I controlled the drag by hand then recorded the motor current, force and RPM by hand and plotted it in Excel. This was just a sanity test to make sure the numbers were working out.

The two wires connecting to the absorber can effect the readings and so I changed them out for some high strand. These wires are lighter and VERY flexible. Like holding a string.

[joeqsmith49]

I started to work on the software.

Here is a sweep showing torque (in oz), output power (Watts) and rotational speed during a pull.

[DamoRC]

@joeqsmith

Thanks for the explanation.

What are you using to write the software - it looks very good.

DamoRC

[joeqsmith49]

@joeqsmith

Thanks for the explanation.

What are you using to write the software - it looks very good.

DamoRC

No problem.

Software is wrote in National Instruments Labview. The graphs are very simple, just click and drag. Software is then wrote in a graphical form. You 'wire' things together. You can create an EXE with an installer package. It's very good for rapid prototype work. Execution rate depends on how the program is implimented. If speed becomes an issue, I will write the functions in C, then create a DLL. This is then called from Labview.

It's not a real eye popping GUI. Labview is very old and they really have not done a good job in this area (IMO).

[joeqsmith49]

Here pulls are made from 5 to 12 volts in increments of 1 volt. The verticle axis is motor output power in Watts.

Here is a 3-D view of torque in mNm.

[henzy190]

nice software there

looks great, tut now please?

[joeqsmith49]

Thanks.

What's "tut"?

[joeqsmith49]

It looks like I will need to remake the shafts. The bearings are not good enough and it has a little vibration. I ran a few sweeps on the Gold colored motor.

I started out at 4 Volts, then stepped it to 14. In normal mode, you can see all the pulls as they are ran but it's not a very good way to look at the data. Shown is RPM.

Here is the same sweeps but torque is displayed. Notice that the system is still running. The graph is like an old strip chart recorder.

To sort out this data, we want to stack the pulls up into a single graph. Here are all 11 pulls overlaid. This motor is rated for 6VDC operation. So while the absorber can pull more out of it, we really want to not damage the motor.

The 3D effects just provide another way to look at the data. Shown here is the power output from the motor.

[joeqsmith49]

Can the absorber really pull more power? Sure it can....

This is what happens when we run a motor rated for 6V at 18V. RPM is over 5000 with no load and I doubt they ever thought someone would do this to the motor. Here is a zoom in of the max load. Now about 370 RPM difference is all.

Now let's compare the torque. The cursors are placed on peak output power.

I did a little search and came across this interesting link. They talk about the basic torque and power curves for a DC motor.

Motor torque power curves basics