# Loco-Detection of Car-Load?



## RT_Coker (Dec 6, 2012)

I am exploring ways that a locomotive can detect the load that it is pulling. I want to use this in OS-DBTC as a safety check for the success/failure of automated coupling/uncoupling. It could also be achieved by detecting coupler movement/non-movement.

The first attempt to achieve this feature in OS-DBTC will be by using the difference (load minus no-load) in Bemf values. This has the advantage of not adding additional hardware.

Pertinent discussions welcomed and appreciated. Thank You!
Bob


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## gregc (Apr 25, 2015)

you've made me curious.

i guess you've considered measuring current, but saw that additional circuitry is needed.

but i also wonder how much extra force (current, volt) is needed to move another car with metal wheels and properly tuned trucks. My layout is flat, but cars roll on a section with an imperceptible grade.

and by coupler movement, do you mean longitudinal, actually pulling the coupler out of the pocket? mounting the coupler box in a box, where it can easily slide, withind limits might make it possible to easily detect that the coupler is attached. Wasn't there a thread on this?


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## RT_Coker (Dec 6, 2012)

The Bemf I am talking about is a voltage that is proportional to the motor-current. 
The Bemf measurement would be made as the locomotive is starting out, basically measuring the force needed to move the overall load for a known acceleration on a level track.
The “by coupler movement” is just me wondering what might be in use or possible.
Thank You!
Bob


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## Shdwdrgn (Dec 23, 2014)

One of the things I was considering was some kind of strain sensor in the loco's coupler, which could then be used to get a rough idea of the weight being pulled. My goal in this was to detect when cars had been successfully coupled/decoupled, and also to detect lost cars while moving.

Unfortunately, I'm thinking the goal may be unrealistic on a budget. Most strain sensors I found were too large for HO scale, and then there's the consideration of having enough range in sensitivity to work through the range we might see, from pulling a single unloaded flat all the way up to long trains fully loaded with coal... and I just don't see that happening in something small enough to be embedded with the coupler. Maybe you'll have better luck than I did though?


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## RT_Coker (Dec 6, 2012)

The approach with Bemf will use a locomotive command to set the check-threshold for the particular coupling/uncoupling event that is about to take place. Because it will be just a safety check it will be optional. 

You make an interesting point in “lost cars while moving”. I will have to look into the possibility of detecting this with Bemf. It would probably just work for lost of a long set of cars, derailments, and ... Most of the “lost cars while moving” that I have experienced were an uncoupling of a large group of cars and/or derailments. There were others that were due to my not checking out the cars before I started using them.

Thank You!
Bob


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## gregc (Apr 25, 2015)

RT_Coker said:


> The Bemf I am talking about is a voltage that is proportional to the motor-current.


isn't BEMF proportional to motor speed?

... the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor.​
trying to think how it can be used to measure load. Maybe its relationship to the current required to obtain that speed.


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## RT_Coker (Dec 6, 2012)

gregc said:


> isn't BEMF proportional to motor speed?
> 
> ... the voltage is proportional to the magnetic field, length of wire in the armature, and the speed of the motor.​
> trying to think how it can be used to measure load. Maybe its relationship to the current required to obtain that speed.


I am not a BEMF expert. Just going by the documentation on the DRV8801 motor control board that I am using.
Bob


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## gregc (Apr 25, 2015)

not sure how much of this you understand already

in a conventional throttle, the output voltage is proportional to the speed setting. But speed depends both on the voltage and load on the motor. 

The spinning motor, like a generator, produces BEMF, a voltage that opposes the voltage applied to the motor and limiting the current. A motor under load spins slower, decreasing the BEMF and increasing the current. The load will be vary as a train goes up or down hill.

It's desirable for the speed setting on the throttle to actually control the speed by
adjusting the voltage to achieve the desired speed. And BEMF is proportional to speed.

My understanding is that the BEMF is measured while momentarily (? 10-100s msec) setting the output voltage to zero. The motor continues to spin, operating like a generator producing BEMF on its terminals.

The load on the motor can be measured by comparing the BEMF to the voltage applied to the motor.

it's not obvious to me that a car increases the load on the motor enough to measure.


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## RT_Coker (Dec 6, 2012)

In my BEMF ignorance I am probably misusing the term. What I will be using is definitely a motor-current measurement. My firmware already has the ability to control of the sample/hold relative to the PWM waveform. I will not know how well motor-current measurement works until I get the unit off my breadboard and installed in the test locomotive. I do not expect to detect car gain/loss in all scenarios, just the more typical and more dangerous ones. I am not against adding weight to car(s) used in switching scenarios.
Bob


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