Designing Encoder/Tach Circuit for Digital Display and CPU Interface 3

[DanBo1221]

Mechanical guy tackling electronics here. I'm looking to design and fab a consumer grade personal-use wind turbine with portability and cost being main design constraints. My main goal is to have a charge controller box that displays voltage from the DC motor, as well as the motor shaft RPM, on the top of the controller box. I've also toyed around with the idea of having USB interface with the controller to record data onto a laptop for efficiency measurements for various blade designs. What's the most effective means of monitoring motor shaft speed and displaying it on a box some far distance away from the motor in the wind turbine nacelle, and having the option open to record that data onto a PC? Colleagues have suggested using resistors to calibrate the voltage to correspond with RPM, but I'm not sure that's the most effective/accurate means. Would an encoder be better for this task? Outside my territory here, so any and all advice is appreciated!

Deisng criteria:
Perm DC Brushless motor, .38" shaft diameter, 5" diametrical area behind the motor for monitoring equipment/cabling
Calculated RPM speeds 0-300 RPM (furling device will be used as braking method)

 

[IRstuff]

Seems like that's gross overkill. Do you actually need to know the instantaneous angular position to 1 degree? I think not. A simple Hall sensor ought to be sufficient for what you've stated.

TTFN
faq731-376

 

[IRstuff]

I just noticed the brushless motor as I clicked the submit button. Brushless motors have nice commutation signals that can be used as a tach output, which is how many controllers figure out how and when to commutate.

TTFN
faq731-376

 

[Skogsgurra]

A very simple speed pick-up is a little magnet plus a pick-up coil. Small, low-cost, robust and easy to integrate into your control system. Use reciprocal measurement instead of pulse counting.
Next step up is the SKF sensor bearing. Google Sensor Bearing GKE for a presentation that has been on the net for twelve years now. But still covers the basics.
Putting a full-blown encoder up a wind turbine that size is asking for trouble. But, it can be done - of course. But it will probably ruin the project cost-wise.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LiteYear]

Agree with IRstuff - since it's a BLDC it will already be detecting rotation. Look to tap into that signal before duplicating the hardware. If that's really not feasible (hard to imagine) then my experience doing the same on a induction motor based small wind turbine suggests that the cheapest way to go is to mount a magnet or two on the rotor and use a hall effect sensor. You can build one yourself, which is what we did originally, but because of labour costs and reliability concerns, we actually ended up going commercial. Unfortunately, they're not that cheap. Here's the one we selected:

http://www.electro-sensors.com/product_info.php/cPath/60/products_id/155

 

[VEBill]

If you're trying to untangle blade efficiency as a parameter, then you'd *also* need to measure the wind speed.

You probably already knew that.

 

[DanBo1221]

Great suggestions, I would like the ability to monitor through the motor itself. Now the question is how one picks up on those signals when not providing power to the motor, and how to display/monitor it. Any recommended sources for parts or information that could guide me? Again appreciate it.

 

[DanBo1221]

VE1BLL yes we've selected and tested a small anemometer aft of the nacelle.

 

[waross]

When you spin a permanent magnet brushless DC motor the raw output is AC. The frequency is directly proportional to the speed.

Another option that is quite cheap is to buy a $2.00 toy motor, a cheap milliammeter, a fixed resistor and a potentiometer. Arrange the toy motor to be driven by the main shaft. It will generate a voltage proportional to the RPM. Select a resistor value that limits the voltage to the meter to about 125% at full speed. Calibrate the reading with the potentiometer. You may use a digital milliammeter if you want digital displays.
We tried this solution on a printing press for a customer. It worked well but the customer didn't like the $2.00 toy motor, because it looked like a $2.00 toy motor. We bought a replacement $14.00 toy motor and he was happy.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

...when not providing power to the motor...

Isn't the 'motor' being used as a generator?

 

[DanBo1221]

the raw output is AC. The frequency is directly proportional to the speed.

Right, should have thought of that before! I have the pole pair number of the motor, and I understand it's just calibrating to count the number of sync cycles per rotation. I've seen programmable rate meters through some prelim searches. Is this a good street to go down, and if so any product recommendations?

 

[OperaHouse]

.38" shaft diameter....... This is a toy already. But seriously you should investigate the Arduino UNO. These are cheap. Last one I picked up was $14 shipped from China (about $30 locally for one that is not a knockoff). The programmer is a free download so you can try before you buy and it is a nice easy higher level language that any neophyte can pick up. If you have a laptop with USB port then you have everything needed to program. I have one at my camp controlling the regulation of various voltage solar panels, a fridge, water heater, and pumps. This can monitor voltage, speed, log data and do PWM so you can control cut in and set power point no matter what the motor ends up being.

 

[DanBo1221]

Thanks OperaHouse. I'll definitely look into this, the Uno looks like everything we need. Still would like to put a voltage, power, and speed meter(s) on the controller box itself, for instantaneous readings.

 

[DanBo1221]

Well the volt/amp meter is easy by all means; showing RPM is still not getting through my skull.

 

[IRstuff]

You would have to do a conversion, obviously. 0V is clearly 0 rpm, and there must be a voltage that corresponds to 300 rpm.

TTFN
faq731-376

 

[DanBo1221]

I'm moving towards an LCD 16x2 shield for the UNO. Already stock to fit and easily programmable. It seems as simple writing an Anduino program counting the sync cycles from the motor, or converting volts to RPM itself, then displaying it on the LCD. Adding a scrolling line to the program with the voltage and amperage displayed, and we're in business.

 

[waross]

Volts from the motor will not be a reliable indication of the speed unless the motor is completely unloaded.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VEBill]

Also... Motor (blade) speed will not be a reliable indicator of anything unless the load is held constant. The OP did mention that he planned to also measure "voltage, power". The untangling of blade efficiency from all the other uncontroller parameters is going to be near impossible.

Beware the end result where the control system requires more watt-hours per day than is generated by the renewable energy supply itself. It's easy to avoid, so long as you're a tiny bit careful.

 

[OperaHouse]

Anytime you are working with displays it can be a little tricky for a novice. It just takes a little time to get used to coding. The development system has no qualms about telling you that you are a moron over the smallest thing. Just start with simple things and keep adding on functions. The TOOLS section has a nice serial monitor. I build in multiple diagnostic screens and this really helps with understanding what happens. One downside is that plugging into the USB port resets the uno so stored data is lost. I use the PWM function and read out stored data as an analog signal to DVM. Not on my computer now, but I believe this has a pulsein function that will read out in time. Tie that into the handy MAP function and you get RPM. Google Arduino and any function you need and lots will come up. Visit fieldlines.com forum for a lot of blade info though they are slow to adapt any technology you can't use a hammer on. The following is some sample code from an art I wrote to give you a start.

Any type sensor may be used like a thermisistor. The conversion is very easy using the MAP function. Through a diagnostic debug screen, just get the reading at room temperature (82deg with 7090 count) and in an ice bath (32deg with 7930 count). The linear interpolation mapping actually extends beyond those two points. Now you have a temperature in understandable terms. Note how this works even though increasing temperatur creates a decreasing voltage.

TEMP = map(Ctemp, 7090, 7930, 82, 32);
// For five small signal diodes with 2.2K pullup to +5V

Battery voltage works in much the same way. A voltage divider of 300K from the battery with a 10K pot in series is tapped with a 87K resistor. This gives about 25mv per count for a 4:1 divider. A . 22uF capacitor at the input pin adds a little filtering and gives a stable reading. The multiplyinjg effect creates a number that approximates the battery voltage. Adjusting the pot finishes the calibration. Make sure in normal operation that the voltahe at the analog pin does not exceed 2.5V for safe operation when monitoring other voltages.


analogRead (AI0);
battery = battery - battery / 26;
// Sum the readings by subtracting one average reading

battery = battery + AI0; // Add latest A/D reading

 

[DanBo1221]

Thanks OperaHouse again. I have started writing some baseline sketches and look forward to using the Uno when it comes in, even though I'm starting at a crawl. VE1BLL, testing will be done in a homemade turbulent flow wind tunnel and in real-world environments, measuring the wind speed, voltage, etc. We'll be getting enough power too.