Small Dynomometer Questions

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MobyDick
 
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Small Dynomometer Questions

Post by MobyDick »

Bear in mind I know very little about the actual programming of Arduinos or similar microcontrollers, so I'm mostly just looking for some "high level" advice. I race mopeds (yes, that is a thing and yes, it is exactly as awesome as it sounds), and I'm looking for an economical way to graph power output and torque as a function of RPM. I want to start experimenting with pipe resonance, port and ignition timing, etc. and I need to be able to quantify my results in the real world. Basically, I need to build a chassis dyno.

I picked up a very cheap, quite large 24V DC motor (I don't recall the exact specs, but the thing is mondo) from the scrap pile at work, and I would love to use it as a load cell. So here's the question:

If I input voltage drop across the circuit, circuit amperage (probably using a shunt) and engine RPM, is there any reason I couldn't get an Arduino to output power and torque curves as a function of RPM?

Basically, I want to calculate watts from volts and amps, and the horsepower from watts. Torque can be calculated using the hosepower output at a specific RPM. It's a bit weird, since normally a dyno starts with torque and works the other way, but is there any reason that wouldn't work? Am I fundamentally misunderstanding something here?

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Re: Small Dynomometer Questions

Post by adafruit_support_bill »

That sounds very do-able. You will probably want to use one of these: https://www.adafruit.com/products/904
The actual acceleration might be another measurement of interest: https://www.adafruit.com/products/2019

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Re: Small Dynomometer Questions

Post by MobyDick »

Unfortunately, I will need to measure a lot more amperage than that little guy will be capable of. Like in the 175amp to 200amp range. I'll probably use one of these bad boys: http://www.rc-electronics-usa.com/current-shunt.html

In theory though, I should be able to calculate the power output of the engine by measuring the circuit voltage and amperage and engine RPM? Assuming I can prevent everything from melting into a little puddle of molten copper, that is?

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Re: Small Dynomometer Questions

Post by adafruit_support_bill »

The INA219 works with external shunts as well. Just remove the on-board shunt and run wires from the VIN- and VIN+ breakout holes.
Advanced hackers can remove the 0.1 ohm current sense resistor and replace it with their own to change the range

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MobyDick
 
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Re: Small Dynomometer Questions

Post by MobyDick »

Oh, neat. Yeah, I'll definitely give that a shot. In fact, it looks like I could base the circuit on this setup: https://learn.adafruit.com/pro-trinket- ... r/overview

Are there any tricks to isolating the Arduino circuit from the high voltage/current circuit? While I'm curious to see what happens if you pump 200amps through an Arduino, I'd rather not find out while I'm trying to do something productive with it.

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Re: Small Dynomometer Questions

Post by adafruit_support_bill »

You could use something like a Ruggeduino. It has over-current protection on all the pins.

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Re: Small Dynomometer Questions

Post by MobyDick »

Alrighty, I got an Arduino and an Adafruit INA219 (neat little chip, btw) and I've started playing around and familiarizing myself with how it all works. Blew up a few LED's, made some stuff blink and move around, that kind of thing. I have a very, very preliminary idea of how I want to lay out the circuit, and I wanted to run it by you guys to see if I'm going to electrocute myself/vaporize something expensive/set my garage on fire/etc.

Also, will this do what I want it to do?. I'm an ME, not an EE: my expertise in electrical circuit design is basically "don't let the blue smoke out."

Note: this only shows the load circuit. I am going to need a circuit to measure RPM too (I'll probably use a Hall effect sensor or an optical tach or something). I should be able to figure that part out without too much trouble though.

Image

I wanted to use a relay to switch the main load circuit, but I can't find a relay capable of handling 5 or 6kW at 500A with a coil small enough to be run directly off of the Arduino board. I have seen a ton of smaller relay breakout boards specifically designed for the Arduino though. Basically, the Arduino will power the coil on the small relay which will close the 12V circuit, powering the coil on the big relay which will close the high power circuit. It also has the added benefit of another layer of separation between the Arduino and the computer attached to it and fiery death.

I am planning on removing the shunt resistor from the INA219 board and running leads from the Vin+ and Vin- terminals to the mondo shunt resistor installed on the main circuit (which I already have). I'll have to do some math to convert the reading from the board to the correct amperage, but that shouldn't be terribly difficult. Now that I'm looking at it, I should definitely have a fuse between the shunt and the INA219 too; if for some reason the shunt lost continuity, the entire current would flow through the board.

The basic test procedure I'm thinking of goes as follows:
1: Start and warm the engine with the main power circuit disconnected.
2: Rev the engine to the max RPM I want to test.
3: Hit a button or something to start the test. The Arduino will trigger the main circuit and start recording voltage, amperage and RPM data.
4: With the throttle pegged, dial in the resistance on the carbon pile resistor until the engine bogs down to the low end of where I want to test (or until I am out of resistor, one or the other).
5: Hit another button to end the test, which opens the relays and breaks the circuit. Let off the throttle.
6: Go crunch some data while everything cools down. I'm thinking of using GoBetwino or something similar to graph the results in Excel.

Ideally, the entire sweep should take less than 10 seconds, so I'm hoping I'll be able to get away with running most of that high power circuit pretty close to the limit.

I am a little concerned because the carbon pile is only designed to run around 16V or so, but I hope I won't run into arcing issues with 24V. Worst comes to worst I'll blow up a garbage Harbor Freight resistor and have to come up with something else (liquid rheostat maybe?). I'm also a little concerned because I genuinely don't know very much about the motor. It is designed to power a hydraulic system on a mobile platform, but I have no idea what the voltage and amperage curves will look like when I use it as a generator (assuming it works for that purpose at all).

So what do you think? Will everything blow up the first time I try this? Am I going to kill myself? Am I barking up the wrong tree altogether?

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Re: Small Dynomometer Questions

Post by adafruit_support_bill »

I wanted to use a relay to switch the main load circuit, but I can't find a relay capable of handling 5 or 6kW at 500A with a coil small enough to be run directly off of the Arduino board.
An N-channel power MOSFET would do the job. You will want a diode as well for kickback protection.
https://www.adafruit.com/products/355
https://www.adafruit.com/products/755
Image
I am a little concerned because the carbon pile is only designed to run around 16V or so
You could run two in series. That way each one would only see half the voltage.

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Re: Small Dynomometer Questions

Post by MobyDick »

I don't have much experience with transistors, but 500A seems like an insane amount of current passing through something so small, right? Wouldn't it just melt? I mean, the wires in the high current circuit are probably going to be 2ga, and those legs look about the size of breadboard pins.

Or were you talking about using it to switch the 12V intermediate circuit, and then still using a big relay for the load circuit? Is there some clever circuitry I could use to keep the full current from flowing through the MOSFET?

If I do run the tandem relay setup, would I have to use a diodes on both the Ardunio circuit and the 12V circuit? That's a good idea to protect the Arduino, but am I wrong in thinking that a kickback in the 12V wouldn't really matter?

As for running a second carbon pile in series, that's a good idea. I would have to figure out a way to adjust both of them simultaneously, but that wouldn't be terribly difficult to figure out. That would potentially keep the temperature of each of them down too, right? Having one catch fire would be awfully irritating.

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Re: Small Dynomometer Questions

Post by adafruit_support_bill »

don't have much experience with transistors, but 500A seems like an insane amount of current passing through something so small, right?
Sorry, what I meant was to use the MOSFET on the output of the arduino to switch the main relay - instead of the cascaded relays you have in your diagram. If you drive it from a PWM pin, you can use full power to actuate the relay, then back off the duty cycle to the minimum needed to hold the relay closed to save power.

In any case, you need a diode wherever you have a relay coil switched by a semiconductor to prevent damage from the reverse voltage inductive kickback.

Running the carbon piles in series, each one would only need to dissipate half the power, so they would run cooler to.

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