I am currently working on a project that involves using a stepper motor (and or servo) to control a metering rod to meter fuel into an engine, it then reads the exhaust temperature to adjust the amount of fuel being metered into the engine. The metering rod is attached to a "crankshaft" that only spins from 0' to 180'. I'm Trying to figure out what would be best for this because it needs to react fast, .5 seconds is the slowest speed it can safely go. So which should I use, a stepper motor or a servo? Another question on servo versus stepper motor is what size do I need, or do I need to weigh my components to decide that?
Also I was wondering if I should use the Adafruit Universal Thermocouple Amplifier MAX31856 Breakout with a type K thermocoupler (not their fiber glass one or stainless coated because they melt past 500C, so A different companies.) since exhaust temperatures can go up to 2400F (usually runs about 1200*F).
Any Help would be awesome.
Thanks, Dread Quill (not my real name but I prefer you guys to call me it )
If you have the detailed specs you can check-out the maximum speed of the motors but it get's tricky under load. But big unknown is the load (force & inertia) so you'll probably just have to guess & experiment.
This seems like a "servo application."
Servos are geared-down, so that gives you more torque at the cost of speed. But, steppers aren't that speedy either.
Are "steps" OK? Standard stepper-motors have 200 steps per revolution, so is 100 steps enough resolution for your application? You'll also need at least one position/home sensor for a stepper.
If it's responding to temperature, does it really have to respond in 1/2 second? Your temperature measurement probably wont be that fast.
I'm not a thermocouple expert, but a thermocouple or an IR sensor is probably the best way to handle these higher temperatures. With a higher temperature you'll get higher voltage and that's a good thing because you get a better signal-to-noise ratio. Is the Adafruit board calibrated? Or is the chip pre-calibrated? Does it go to 2400 degrees? We build some thermocouple amplifiers where I work and they are calibrated (digitally in flash memory) before shipment. We have a thermocouple simulator so we don't use actual temperature to calibrate.
(not their fiber glass one or stainless coated because they melt past 500*C
The stainless one will have some kind of insulation inside, so check the specs. ...And, quickly searching, 2400 degrees may be too hot for the thermocouple itself.
I do have to have fast speeds on it because if I don't it can get screwy with the metering.
Lets put it this way, you give it gas fast, it gives a high vacuum pressure and you go lean for a minute, If you give it gas and let off very fast it causes a flood of fuel and becomes overly rich. If you have a slow adjusting speed you'll throw the metering way off doing that a few times fast (which is what you do with that particular motor anyways) so slower metering speeds in not a good idea...
As for the steps versus servo. I honestly don't know, I know either would work but I don't know which is better in this application. Correct me if I'm wrong but you cant go in between steps on a stepper motor. And although in 360' the 100 step maybe fine, I'm worried that where the metering rod needs to adjust could potentially be right in a spot the stepper motor can't adjust to... Also I don't see the assembly (Crank, little connecting rod, and the actual metering rod and its seat) weighing more then 2 Lbs (maybe 6 Lbs to give a range).
As for the adafruit amplifier. I believe it can go to that temperature, I believe its the probes that hold it back, But I have found probes that can go to those higher temperatures. As for the question of if type-K thermocouples work or not, it's what everyone with dynos in the ATV industry use to dyno and read exhaust temps on their ATV's. My dads channel uses type-K couplers. And some of those engines run methanol that's harder to light, slower to burn and generates more heat. So I'd assume it could handle the job. Since its job is going to be metering the fuel to that engine...
Also how would you even code something like this? I'm such a newbie with coding. I barely got a Drag race Christmas tree to work using coding via dramatically changing the blink file and adding a bunch of ifs and stuff. So I'm very much new to this hole programming scheme.
I have began working on the design of my carburetor/throttle body. Still don't know which, stepper motor or servo, to choose. My father has pointed me towards a site that could make a stepper motor or servo to the specs I need, so I'm choosing to get the carburetor designed first before I add the self tuning (arduino) part. Any advice is greatly appreciated.
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