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Sensor needed to measure speed of small dc motor - underwater.

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Charlie J

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I am looking for an appropriate sensor to create a signal pulse, from which a fairly accurate speed measurement can be obtained (via LabVIEW). I have no experience using such sensors, so any advice/suggestions welcome. I am aware of optical, proximity and magnetic sensors, but never used any. The sensor is to be used to measure the (shaft/propellor) speed
of a 12V (1100GPH) dc motor,which will be operating UNDERWATER.

Ideally the sensor needs to be small (to effect the operation of the motor as little as possibe), relatively inexpensive and easily acquired.
 
Charlie J,

First off, I hate putting electronics in water. Especially sea water.

That said, how accurate does the speed data need to be?

Since the pump no doubt has a data sheet describing the flow rate at a prescribed DC current rate, you can monitor the current (Amps) the pump is using to extrapolate, quite accurately with the appropriate math, the flow volume.

If the power source for the pump is external (above water), then this could be done without submerging the sensor. It can be done with a extremely simple circuit (a resistor) that senses the current flow to the motor which could then be supplied, through an A/D converter, to LabVIEW. I can provide a schematic, if you like.

Even if the power source is at the pump, it could still be done: just need to provide a connection to the current sensor.

Had great success with this concept monitoring the current usage (and, therefore, general effectiveness and flow rates) of pumps providing aeration and water exchange for a commercial shrimp aquaculture operation.

AC was, of course, the power source (and we used a BUNCH), and the sensors were current transformers, but the concept is the same.

It also had the secondary advantage of providing really basic information like, "is the thing running at all" or, "has the impeller gotten loose", for alarm purposes.
 
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Charlie J,

First off, I hate putting electronics in water. Especially sea water.

That said, how accurate does the speed data need to be?

Since the pump no doubt has a data sheet describing the flow rate at a prescribed DC current rate, you can monitor the current (Amps) the pump is using to extrapolate, quite accurately with the appropriate algorithm, the flow volume.

If the power source for the pump is external (above water), then this could be done without submerging the sensor. It can be done with a extremely simple circuit (a simple resistor) that senses the current flow to the motor which could then be supplied, through an A/D converter, to LabVIEW. I can provide a schematic, if you like.

Even if the power source is at the pump, it could still be done: just need to provide a connection to the current sensor.

Had great success with this concept monitoring the current usage (and, therefore, general effectiveness and flow rates) of pumps providing aeration and water exchange for a commercial shrimp aquaculture operation.

AC was, of course, the power source (and we used a BUNCH), and the sensors were current transformers, but the concept is the same.

It also had the secondary advantage of providing really basic information like, "is the thing running at all" or, "has the impeller gotten loose", for alarm purposes.

And how would this give the OP the speed of the pump?
 
Only reason I can think of for knowing the speed of the pump alluded to is to assess the volume of water being moved.

Most liquid handling pump's data sheets list the the current draw at a given flow (GPH, GPM, etc.) and head, giving us the constants against which varying performance parameters can be determined.

The speed could also, if needed, be assessed in a similar fashion (within limits). By knowing the current draw at full pumping capacity, any other speed would show up as a variation in the current being drawn. It might take some testing of volume flow/pump performance to establish curve equations, but it could be done.

ALL THIS ASSUMES a constant voltage source, but even if that were to vary, this too could be included in the math determining everything else.

Even a varying head pressure could be accounted for, although a bit more difficult to pick out of the current data.

The power being used by a motor can give you volumes of performance information if the data is appropriately manipulated.
 
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colin55 said:
The electronics doesn't go in the water.

True

The resistor is placed in series with one of the wires feeding the pump. It can be placed at any point in the line. It's just my preference that it not be underwater.

It could be if the OP chose to put it underwater. There's just no particular reason to do that.
 
Thanks for all the replies so far!

Just to be clear the power source for the pump will be out of water.

The project is to obtain a measurement of force created by small dc motor used
in the design of small ROV. I am still just in the planning stage.

A motor will be attached to a specimen (probably metal) clamped at the top
above a water tank. The lower end of the specimen will be submerged in the tank,
with a motor attached. Strain guages will be attached to the specimen in a bridge
formation, with the output being fed in to LabVIEW to calculate a force measurement
which will be displayed/recorded.

For the testing of the motor, I need the motor to run at a constant speed for a
fixed period of time. The experiment will be repeated at a number of fixed speeds
(and also with different propellors on the motor) to gather sufficient data in order to identify the
the relationship between the motor speed and the force created.

I was planning to set the motor speed by creating PWM in LabVIEW (which will
be connected to the power source for the motor via darlington pair), whereby
the user would set the duty cycle using a front panel control. When started
the motor will get up to speed and upon reaching the a constant speed, a timer will
begin and strain and speed measurement data will be recorded.

I had planned to keep the strain measurement, motor control, and speed measurement
seperate - the reason for this being that it may be desired for different motors to
be used so i would like the system to be easily adaptable.
 
Motor speed and current draw don't necessarily have a simple linear relationship. For an absolute RPM measurement my preference would be to use a small magnet attached to the motor shaft, with a pick-up coil adjacent.
 
I'm pretty sure I understand what you're trying to do. If I've missed something, please don't hesitate to set me straight.

Hope this helps.


The concept behind current monitoring is simple. Motors, (bless their hearts) produce mechanical energy in a relatively direct proportion to energy used. And if you can control the heating effects, you can rely on a power monitoring system to correlate to mechanical effort (thrust, for instance).

Putting a precision (>1% accuracy) 1 ohm resistor (of sufficient carrying capacity) in series with one lead to the motor will generate a value that can be read directly by a voltmeter (or A/D input configured for volts) as amps and, as a result, watts, which can be easily converted to horsepower (HP).

For example, if 1.00 amp (of straight DC) were passing through the 1 ohm resistor, then 1.00 volt would be generated, thus indicating 1 watt of power being used: 1.02 amp would yield 1.02 watts, etc.. You would, of course, have to subtract an inefficiency constant(s), but that's for you to determine. In other words, sensitivity(s) of 1 part per 100 (or even better, actually) can be achieved.

In your case, using PWM, the total current present at at any given moment would, obviously, be a function of the PW. Increasing the PW (i.e., speed) will, perforce, increase the power level and the HP generated. Decreasing PW the obverse.

With your strain gauge data, you should be able to the generate a sufficiently accurate force value. And you'd have power usage data to help determine battery requirements.

And this would adapt, with no changes other than the base line constants, to whatever DC motor you chose to test.
 
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Given the options available and your application I would likely think about the suggestion Alec made. However, I would modify it a little. I would embed two small magnets into the shaft, using two spaced 180 degrees apart helps maintain the rotating shaft balance. Then for the pickup I would use a tiny hall effect sensor. With some minor work the sensor can easily be made water proof. This will give you two pulses per revolution. Doing the math to derive RPM in Lab View should not be a problem. Look for hall effect sensors with a digital output. Matter of fact, along the lines of what you are doing hack apart a 3 wire or 4 wire 12 VDC computer fan with a tach output to get the idea of what I am talking about. Such fans are cheap and abundant.

While current sensing does work I would just lean towards using a hall sensor for your particular application.
 
Incorrect:
For example, if 1.00 amp (of straight DC) were passing through the 1 ohm resistor, then 1.00 volt would be generated, thus indicating 1 watt of power being used:


The same pulses from the motor can be detected from reading the lines to the motor at the battery.
 
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I agree with Reloadron,

no complicated math and accurate rpm measurement.

I suggest to use one or two TLE4905L (unipolar Hall sensor) and two supermagnets.

It has to "see" the Southpole of the passing magnet for valid information.

Boncuk
 
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...
The same pulses from the motor can be detected from reading the lines to the motor at the battery.

Colin, he said he was going to use PWM to drive the motor. Reading armature current pulses will be practically impossible if he is using PWM.

Since the motor is outside the water, why not use a variable DC supply to set the motor speed, and a $30 handheld optical tacho from ebay to make sure the speed is right?

The load (water) is constant so nothing should vary the speed, and that should work fine for you to test the motor thrust at some different speeds.
 
Thanks for the help!

The magnetic sensors seem like the simplest solution for the basic requirements of the system, so I may initially try to implement that.

Although, what cowboybob suggested whereby power usage data can be obtained (which could be used to help determine battery requirements) sounds like it would be beneficial also, so I will try to include this as well.

Thanks again.
 
Your Welcome.

The project sounds like a lot of fun.
 
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Hi Charlie

Several years ago I did a small project involving fan speed. What's left of that little science experiment can be found here. While it does not have a direct bearing on what you are doing, it does have a table on the bottom of the page where I include the small fan motor current at assorted voltages, the fan current and also the fan speed. While I don't think that monitoring current is the best way to get rotational speed in your case the correlation shows up in the table and does show what CowboyBob was getting at. Looking at that table with for example 12.078 VDC applied to the fan motor the RPM was 2739 and the current draw was 157.4 mA. With 6.025 VDC applied to the fan motor the RPM was 1460 and the current dropped to 75.0 mA so it can be seen there is a pretty close relationship between rotational speed and current for this type of brush less DC motor. So CowboyBob's suggestion was reasonable.

Something to consider is you plan to drive the motor and control the speed using PWM. With PWM we turn the voltage on and off for periods of time and so goes the volatge, so goes the current. That adds a few problems as it would not be quite as simple as my table shows using a constant DC voltage applied to the fan motor. That led to suggesting the use of hall effect sensors for measuring the rotational speed. While I don't know the shaft speeds you expect, I would still consider using two magnets (as a minimum) equally spaced on the shaft.

Since you mention a battery, monitoring voltage and current may be a good way to go so you can figure your battery life remaining give or take. You can also plot your load current against load as measured with the strain gauges.

When you get things done and start getting data I would be curious as to how things go so as time allows please post back.

Ron
 
Thanks for that Reloadron, interesting stuff!
If I obtain current/voltage readings I may be able to monitor and record that data also in LabVIEW and perhaps arrange (maybe using formula nodes or statistical VI) to obtain average values for these also. Although accuracy wise, I'll have to wait and see - I'll probably use external monitoring units in addition to LabVIEW to identify the validity of the results.

It may be a while until I get this up and running (if I can get it up and running..) but I'll try to post results/progress as and when.
 
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