Interesting Project

[hartparr]

I am looking to build a device to measure RPM and torque then do some simple math and output horsepower produced.

Can someone point me in the right direction for a loadcell and tachometer that a picaxe controller can take accurate readings from that won't break the bank?

Thanks,

JTB

 

[fernando_g]

Magtrol makes torque transducers.
Don't know how expensive they are, though.

It will certainly depend on the actual horsepower capacity.

 

[westaust55]

I am looking to build a device to measure RPM and torque then do some simple math and output horsepower produced.

Can someone point me in the right direction for a loadcell and tachometer that a picaxe controller can take accurate readings from that won't break the bank?

Thanks,

JTB

For a load cell it may help if you indicate the required working range and accuracy that you desire. Then folks can narrow down what advise they give you to match your requirements.

For the speed, others here have used sensors of various types (non contact includes optical, hall effect etc). Again indicating what access you have to rotating parts, diameters, speed ranges can help folks identify items that may help you.

Which PICAXE are look considering. Some will only have analogue and pulse inputs. M2 and the X/X1/X2 parts also have inbuilt i2c comms (and even hspi) that will easily interface to external modules that can expand the range of sensors you can look at.

What value is deemed "not to break the bank"? US$10, US$100, US$1000 ?

 

[gengis]

For stationary engines it's fairly easy - a brake with the brake arm pushing against a load cell - or electronic scale or mechanical scale with something to read the scale electronically.

Or heat water through friction and measure the caloric rise and calculate power directly.

For a moving engine it gets tricky. You end up doing something like introducing a spring in the drive train and measure the angular offset of the engine working against the spring - or use the drive train itself if there's a propeller shaft and you can measure the tiny offset the torque will produce.

You are right about interesting...

 

[hartparr]

Thanks for the response guys.

I've already purchased and have been experimenting with a 20X2.

The engines will be stationary during testing.

For load cell accuracy, I'd be happy with plus or minus 3 lbs.

I believe I will need an S type load cell with a 0 - 5000 lb capacity.

I also have full access to all rotating parts for the tach sensor. I can attach anything to the shaft and mount the associated sensor to count RPM. Whatever is best for picaxe works for me. Max RPM I will be running is 500.

I would rather not spend more than $1000 US if possible.

Thanks again for any suggestions.

 

[westaust55]

5,000 lb rating/range and 3 lb accuracy. That is 0.06% accuracy at full scale.

Load cells I am used to installing for conveyors carrying 10,000 tonnes per hour are usually rated at 0.5%.

Those load cells see a total of something like 6,000 lbs per second and guessing but maybe rated like 10,000 lb or a bit higher.

But lets see what can be found . . . . .

EDIT:
a very quick search finds:
http://www.omega.com/pptst/LCHD.html

LCHD-5K In Stock $765.00 5,000 lb capacity load cell

but accuracy around 0.25%

another:
http://www.americanweigh.com/product_info.php?cPath=52&products_id=192&osCsid=418d3bef97162e1d08d070c92a57810f
S-type Load Cell 5,000lb Capacity for $175
Non-linearity...........................................0.03%FSO
Hysteresis.............................................0.03%FSO
Non-repeatability.....................................0.02%FSO
Creep in 20minutes.................................0.03%FSO

 

[hartparr]

Right on. I think actually getting a load cell is going to be the easy part.

What about its associated amplifier so the picaxe can take readings?

The amps I've found are in the $300 to $500 dollar range. There has got to be a cheaper way to do it....Bathroom scales only cost 20 bucks.

 

[westaust55]

What is the commercial side of your project. That is, can you construct your own circuit, or do you need calibrated and certified equipment in terms of accuracy.

Those alternatives will make a significant different in the price.

 

[hartparr]

No, this whole project is for personal use...I'd like it to be accurate, but there is a lot of fudge space if needed to keep it cheap.

 

[gengis]

Load cells are nice and have a lot to recommend them, but if you want to measure weight cheaply an off the shelf digital scale is a good prospect. Load cells measure relatively low changes in resistance in a balanced bridge circuit. It takes a good clean low voltage power supply for the bridge and probably another for the op amp signal massage circuitry.

You've doubtless seen spring "balances?" Cheap linear and accurate, and easy to check or calibrate. Usually some compression spring with limited amount of travel and scale to measure displacement. A linear slide style potentiometer can be used to measure the compression of a die hard spring for measuring torque against a moment arm.

We cobbled together some accurate and reliable instruments for measuring the breaking strength of tablets using this method in the 80's. Our first potentiometer of choice was an "optical" style. It had no contact slider, but it was susceptible to dust and aging of the light source. An inductive LVDT took care of that limitation - basically a solenoid style transformer that measures the displacement of its core between an "exciter" coil and one of two signal coils.

 

[boriz]

Cheap DIY solution/idea.

Interrupt the power train with two similar interlocking wheels that lock together a bit like a dog clutch. Machine them with a small gap for some play. Place inside the gaps some of this QTC stuff: http://www.peratech.com/qtcmaterial.php

A Picaxe can then read the QTC resistance directly. If the QTC is overloaded at higher torque, place some stiffer material alongside the QTC in the gaps. A little calibration and you should have a fairly robust and repeatable system. And cheap