Capacitive sensor for gas/plasma cnc ...


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    Default Capacitive sensor for gas/plasma cnc ...

    Hi,

    Iam looking for an off-the-shelf capacitive ring sensor, that I can use for gap/height control in my gas/plasma CNC machine.
    The only site I found so far is : http://www.agelkom.com.tr/ in Turkey.
    Are there any products available in US?
    Looking for a economical solution.

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    Capacitive sensor torch height controls are primarily used for oxy-fuel cutting applications only, however if the majority of your cutting is on materials thinner than about 5mm, then you can use capacitive height control with plasma. With plasma, the capacitive sensor seems to get affected more by splatter, and most of the capacitive THC systems do not have the ability to control a separate pierce and cutting height. Hypertherm produces a capacitive sensor that could be used with plasma.. it is designed for industrial high production use, and is probably not the lowest cost solution, but would likely be the most reliable. Here is a link:

    http://www.hypertherm.com/en/Product...sensor_ohc.jsp

    Jim Colt



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    Hi All,

    i also have some similar issues. I have 2 IHT's M4000 CAP, on which 2 oxyfuel torches are installed.
    I am thinking to replace one Oxyfuel torch with Plasma system. I know what all things are required, buti dont want spend 7000Euros on IHT's M4000PCS. Can someone guide me if i can use THC with may be a bigger CAP ring.

    Gagan



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    Default Re: Capacitive sensor for gas/plasma cnc ...

    Hello Gentlemen,

    I am new to this forum and I am building my own CNC Plasma and Oxy torch.
    I want to know why ohmic contact sensor is used for plasma and why capacitive sensor is used for Oxy torch.
    I want to the technical reasons behind this. Both of them, are used for sensing the distance then why not use one sensor for both applications.
    I will be cutting 16 mm thick plate using plasma and 25 mm using Oxy torch.

    Cheers.



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    Default Re: Capacitive sensor for gas/plasma cnc ...

    Ohmic sensing is simply a method for sensing the surface of the material when probing where the torch shield and the material form a switch that is turned on on contact. There are other methods like a floating probe sensor and even stall sensing on the Z axis motor. Torch height control (THC) systems rely on the fact that there is a linear (eg. straight line) relationship between the arc voltage and torch height. So that means if you know the torch voltage, you can calculate the torch height very accurately. This is not possible with an oxy torch so the capacitive sensor senses the proximity of the ring to the surface of the material. I've no experience with it but I suspect its not as accurate as a THC and requires additional hardware which is not required with a THC system.

    Rod Webster
    www.vmn.com.au


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    Default Re: Capacitive sensor for gas/plasma cnc ...

    1. Ohmic contact (Plasma) uses the front most metal component of a shielded plasma torch to locate the surface of the material being cut. This process puts very little downward pressure on the material....so even very thin metal sheets can be sensed accurately and without deflection. It is very important to locate the surface accurately before the cutting starts so that the torch can then retract to the proper height for piercing the material. If a plasma torch is too close when the arc starts and the piercing begins expect immediate torch nozzle damage, and subsequent poor cut quality. If the torch pierce height is too high, expect that the process may not transfer to the material, and the cutting process will not start. Please note that ohmic is not used for the real time height control process.....only for locating the surface of the material, and on some machines ohmic is used as a plate collision device that can send a signal to stop motion in the event of a collision with material when traversing or cutting.
    2. Arc voltage sensing takes over as the real time height control once the plasma has pierced the material and the x and y cutting motion gets to at least 90% of the programmed cutting speed. Arc voltage control monitors the voltage between the negative torch electrode and the positive workpiece (material being cut) and compares that voltage to a known voltage that maintains the correct physical height (the known voltage varies with each thickness, type of material, consumable set and amperage as well as cut speed....you will find the known voltage in the plasma torch manufacturers cut charts.) There are some good arc voltage based height control technologys and some not so good!
    3. Capacitive sensing is used primarily with oxy-fuel cutting....it is a bit sluggish for plasma, but for oxy-fuel speeds it works adequately and is rather simple. The electro-magnetic field developed with the high arc voltage on a plasma torch affects the accuracy of cpacitive sensing...as does material thickness and various types of material.

    Feel free to ask more questions directly if you have the need. Jim.colt@hypertherm.com



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