Motorcycle temperature gauge help

Discussion in 'Automotive Electronics' started by Autobike, Aug 10, 2019.

  1. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    Hello :) i have something to get clarified about my analog temperature gauge in my Honda CBR.
    this gauge works using a NTC (Negative Temperature Coefficient) thermal sensor which measures the coolant temperature of my cooling system. since it's a NTC sensor, when the coolant temperature rises the resistance of the sensor decreases. as the resistance decreases the temperature gauge grounding path becomes a low resistance path, which makes the temperature gauge indicator to show higher temperature readings.

    below is the type of NTC thermal sensor i have.

    [​IMG]

    this is the circuit of my bike. i have highlighted the temperature gauge grounding path in red.

    [​IMG]

    so i can understand how above NTC sensor works. but the question is how my temperature gauge works. basically how the indicator rotates? it has two blue color resistors and a fine copper winding just below the gauge. i'm attaching the pictures below.

    [​IMG][​IMG]

    so i just wonder how the temperature indicator rotates. is that something to do with a magnetic field? if you can explain how it rotates according to the resistance which is controlled by the thermal sensor, it would be much appreciated. i have a fair knowledge in basic automotive electrical systems so hopefully i will understand it. thank you very much.
     
  2. Alec_t

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    Sep 17, 2013
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    Google 'moving iron meter' for an explanation.
     
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  3. shortbus

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  4. MrChips

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    Since the temperature sensor is an NTC resistor, as temperature rises, the resistance decreases. This behaviour can be used to create an increasing current as the temperature rises.

    There are different types of current/volt meters, some electromagnetic and others purely mechanical.

    In an electromagnetic meter there is an electrical coil of wire and a permanent magnet. One is fixed while the other is allowed to move or rotate. Hence you have a moving coil meter vs a moving iron (or magnet) meter.

    [​IMG]

    In a moving coil meter, the coil is suspended in a fixed magnetic field. Current flowing in the coil creates a magnetic field which wants to align itself with the fixed magnetic field. This creates a rotational torque and is indicated by a pointer (or needle) mounted on the frame of the coil. A spiral spring attached to the coil opposes the rotation of the coil and returns the pointer to its neutral position when there is no current in the coil. This type of meter is also known as a D'Arsonval movement and is the most common design used in analog meters. These meters are sensitive to current, in the milli-amp or micro-amp ranges. In order to respond to higher currents or voltages, external resistors are used to reduce the current flowing through the meter movement.


    [​IMG]

    In a moving iron meter, the electrical coil is fixed while an iron core (or magnetic material) is movable.



    [​IMG]

    In a hot wire meter, heat from an electrical current causes a metal wire to expand. The expansion of the hot wire is translated to movement in the pointer, entirely through a mechanical arrangement.
     
  5. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thank you very much for your explanation :)

    thank you :)
     
  6. debe

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    Its what shortbus said its an aircore type meter where 2 coils are wound 90 deg to each other.
     
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  7. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thx a lot. i think this is the exact answer i was looking for :) did a search and had a good understanding about this air core gauge method :)
    below is a clear picture of a temp gauge from a different Japanese bike (same year and same Japanese gauge manufacturer). mine is the same but unfortunately i don't have a clear picture of the winding at the moment.

    [​IMG]

    this is another picture i took from google and this article clearly explains how it works :) this guy disassembles his air core gauge while explaining its parts. it gives a good idea too.

    i have a few questions i would like to ask if you don't mind.

    1) we know that the thermal sensor changes the resistance of this circuit. so according to that article above, when the magnitude of the current changes, it also changes the direction of the electromagnetic field . the magnetic rotor in the middle of the winding tries to align itself with the field and this rotation happens.
    so actually if we change the magnitude of a current can we change the electromagnetic field direction ? i thought we can only change the strength of the electromagnetic field if we change the current. so is it the winding pattern which does the rotation ? i mean does the rotation entirely rely on how the winding winded around the magnet ?

    2) the article says the steel cup around this winding is an annealed steel cup which reduces external magnetic influences. if we anneal a steel do the magnetic properties of the steel disappear or weaken ?

    thank you very much :)
     
    Last edited: Aug 11, 2019
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  8. Autobike

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    Feb 23, 2018
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    yea he is correct :) have two questions which i asked above. thank you :)
     
  9. debe

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    This is a better picture of the coils. 2.JPG 1.JPG
     
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  10. Autobike

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    Feb 23, 2018
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    that's it. yea this is what i have in my bike. unfortunately i don't have a clear picture of the winding at the moment. but it's exactly the same 90 degree winding.
    is there a special reason behind winding these two coils 90 degree to each other? almost all the temp gauges i saw in google are in this 90 degree pattern. thank you.
     
  11. Alec_t

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    Yes. Think of one coil as pulling the rotor in the Noth-South direction and the other one as pulling in the East-West direction. The rotor ends up in a compromise direction determined by the ratio of the currents in the two coils.
     
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  12. MrChips

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    @Autobike - You are correct. The electric current controls the strength of the magnetic field, not the direction.
    In order to change the direction of the pointer you need two magnetic fields.

    In the D'Arsonval movement I described above, one magnetic field is supplied by a permanent magnet. There is no reason why this permanent field couldn't be supplied by another electrical coil. This is the concept behind the cross-field meter movement.

    You will notice that the D'Arsonval movement has only two terminals, i.e. connections to the two ends of the coil. If your meter has three or four terminals, then you are looking at the connections to two coils aligned at 90° to each other. To understand how this arrangement moves the pointer you may look at the design of something called a tangent galvanometer.

    [​IMG]

    In a tangent galvanometer, two magnetic fields, one produced by the coil and the second being that of the earth's magnetic field are aligned at 90°. In the two-coil meter, the reference field is created by the second coil. A compass needle, or any ferro-magnetic material will align itself with the combined effect of the two cross-coupled fields.

    If we were to name one field B1 and the second field (the reference field) B2, then the resulting field will lie in the direction α-degrees from zero using the formula:

    tan(α) = B1 / B2

    In other words, the needle will align to α-degrees. The direction of the magnetic fields B1 and B2 do not change. They are 90° apart. What changes is the combined effect or resultant field of the two fields.The direction of the resultant field is easily determined using trigonometry or something called vector analysis. With zero current in the coil producing B1, the needle aligns with B2. As the current responsible for B1 increases, the needle will move increasingly towards the direction of B1. Note that to get the needle to point exactly along B1, the current would have to be infinitely large since the tangent of 90° is infinity.

    The D'Arsonval meter movement operates on a similar principle except that the reference magnetic field is applied using a permanent magnet, very much in the same way a permanent magnet motor works.
     
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  13. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thank you :)
     
  14. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thank you very much :) i'll try to understand this in two steps.
    as you said in my bike the temp gauge has three terminals. we can see that in our circuit diagram. so what you explained above is pretty clear.
    but before coming in to it if we talk about the D’Arsonval movement, can we decide the indicator movement using Fleming's left hand rule?
    in a simple DC motor the rotation can be decided using that law. in a D’Arsonval movement we have a same permanent magnetic field and a current flow direction. so if we apply it to the below image

    [​IMG]

    the moving force aims at us ( thumb aims at us ). but apparently the indicator rotates from left to right. not sure i applied it correctly. thank you.
     
  15. Alec_t

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    If conventional current flows as shown the pointer will move towards zero.
     
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  16. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    ops sorry. i think it was a mistake i did when i visualized it in 3 dimension. tried it again and worked fine. yea assuming that it's conventional current, the pointer moves towards zero. thank you :)
     
  17. MrChips

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    You are correct. Fleming's Left Hand Rule (LHR) is used for motors while Fleming's Right Hand Rule (RHR) is used for generators.

    It is uncertain on the accuracy of that drawing and the correct direction of the current.

    Here is a drawing that works correctly according to Fleming's LHR.

    [​IMG]





    You can also apply Fleming's RHR to determine the direction of the magnetic field created by the current carrying coil of wire.

    In the mathematics of the physics behind the force on the current carrying wire in a magnetic field, we use the cross-product of the two orthogonal vectors, current and field.


    [​IMG]


    Z = X x Y

    (Note that these are all orthogonal vectors. x stands for cross-product, not multiplication.)

    Force = Current x Magnetic Field

    or

    Motion = Current x Field

    [​IMG]
     
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  18. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thx a lot :) it's very useful.

    i read the second part of your explanation and it's pretty clear and got the idea :) if we apply it to my bike,
    mine has just 3 terminals like in this picture. one is the battery power, another one grounds to the chassis via thermal sensor and the last one grounds directly to the chassis.
    so according to your explanation the one which makes the reference field could be the one which grounds directly to the chassis. the coil which grounds through the thermal sensor should be the one which controls current hence changing the resultant field. am i correct ? thank you.
     
    Last edited: Aug 13, 2019
  19. MrChips

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    I cannot elaborate on the construction and wiring of the actual temperature indicator.

    Suffice to say that one coil produces a reference field and the other coil produces a magnetic field that is dependent on the current flowing through that coil.

    The angle of deflection of the pointer depends on the balance between the motive force and the torsion on the spring.
     
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  20. Autobike

    Thread Starter New Member

    Feb 23, 2018
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    thank you very much for your support :) i understood the basic principle. i just assumed the above since apparently the only thing which controls current is the thermal sensor and the other one grounds directly to the chassis. thx again :)
     
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