Help Please! Arduino LVDT for Model Railroad

Other Hardware Sensors
1

Hello,

This is my first post here. I have been building robots in clubs using self-designed circuitry, but lately I have been trying to build a track scale for my model railroad.

A track scale is used to weigh railroad cars so that the shipper can be billed accordingly.

I am building this 1:160th proportion scale from plans in a magazine from the 2000's that uses an LVDT to accomplish weighing the cars and displaying the scale weight on a multimeter.

The magazine has a circuit using op-amps and some other stuff, but I was hoping to use an arduino for this.

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Any ideas on what to use here?

-David

2

I should also say that there are four wires, two for the primary and two for the secondaries. The ferrite core is sticking out of the bottom in the pics. The green wires are for the switch, used to calibrate the scale in the original circuitry. There should be about 1mm of deflection when the scale is loaded.

-David

3

Please supply the circuit that you want to replace. A picture of a couple of coils does not give much information!

Take a clear picture of the circuit and attach it to a post.

Weedpharma

4

You can use an Arduino to do the calculations, but you will need a sensor to measure the weight, and then feed it into the Arduino.

You may want to first find a suitable weight sensor, then we can help you connect it to the Arduino.

5

The photo is of the sensor... basically a spring with a ferrite core at the end.

6

"The photo is of the sensor... basically a spring with a ferrite core at the end."

What kind of output does that provide? .05 V per ton (to scale), or what?

7

weedpharma:
Please supply the circuit that you want to replace. A picture of a couple of coils does not give much information!

Take a clear picture of the circuit and attach it to a post.

Weedpharma

8

No clue at all. The article doesn't give that info. Hopefully the schematic helped.

9

"No clue at all. The article doesn't give that info"

Ok, I can think of two options then.

  1. Use a VOM and read the voltage and or resistance, as the weight changes. Does it read anything?
  2. Replace that sensor with a newer weight sensor.
10

Across which points should I read it? Do I need to energize it? There are four wires coming off the primary and secondary coils.

-David

11

@David,
It seems that we don't have enough specs on this device, to know how/if it works.
If I were to take a guess, I would guess, that you need to energize it by applying a signal (frequency) into one coil, and then detect the output from the second coil. As the rod moves in and out, the inductance would change.

Unless you are in love with this device, I would suggest looking for more modern weight sensors.

12

dcutting2:
I should also say that there are four wires, two for the primary and two for the secondaries. The ferrite core is sticking out of the bottom in the pics. The green wires are for the switch, used to calibrate the scale in the original circuitry. There should be about 1mm of deflection when the scale is loaded.

-David

This is just a variable transformer with a summing amplifier. You will have build the specified circuit and replace the meter (M) with a resistor and measure the voltage across this resistor using one of the Analog inputs.

The circuit just oscillates the primary of the 'transformer' Using the U1a opamp, then the movable ferrite core couples the magnetic field to the secondary coil of the transformer, the voltage transferred is turn into a PWM signal by U1b. the PWM is summed by U2a. U2b is an offset amplifier/nullifier the rest of the opAmp chain just create a fixed amplifer value. None of these 'function' can be easily replaced with digital circuits and software.

If you want to use this hardware you will have to build the Circuit.

Chuck.

13

Thanks guys! I will build the circuit and let you know how it goes. Do you know what the output voltage would be on this circuit?

-David

14

dcutting2:
Thanks guys! I will build the circuit and let you know how it goes. Do you know what the output voltage would be on this circuit?

-David

based on the powersupply, the max would be 5V, but the last stage opamp is configured as a current driver. I cannot guess what the voltage range would be. You will have to replace the meter (M) with a fixed value resistor, read the article, it should specify the meter specs.

Chuck.