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  1. #1
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    Thảo luận về cảm biến Loadcell

    Tùy chỉnh font chữ: Xem với cỡ chữ lớn hơn Xem với cỡ chữ nhỏ hơn



    cân xe tải

    Giả sử tôi có một cái cân xe ô tô, được bố trí 6 load cell như hình vẽ. Tôi không hiểu nguyên lý đo của load cell khi có một chiếc xe đứng trên đó. ai có thể nói rõ nguyên tắc cân của load cell và với hệ thống gồm 6 load cell như trên thì cách thức tính tổng từ 6 load cell để hiển thị ra kết quả cần cân của chiếc xe là như thế nào?

    Xem chủ đề mới nhất cùng loại:

    ►Chia Sẻ Cho Bạn Bè:


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    Loadcell là 1 chuyển đổi đo áp lực, gồm 1 cảm biến đàn hồi kết hợp với 1 chuyển đổi đo, chuyển đổi đo thường là cảm biến tenzo, cảm biến đàn hồi thường cấu tạo từ 4 cảm biến đo biến dạng (4 điện trở tenzo mắc theo hình cầu), trong đó có 2 cảm biến chịu biến dạng kéo và 2 cảm biến chịu biến dạng nén. Trong giới hạn làm việc, đặc tính của loadcell được xem như là tuyến tính. Cần xem kĩ các thông tin về độ nhạy, giới hạn đo ghi trên loadcell khi sử dụng.

    Với 1 hệ thống nhiều loadcell như trên, người ta thường dùng 1 hộp nối dây (gọi là Junction Box), tùy từng loại mà có thể kết nối được 4,6,8... loadcell lại với nhau. Nguyên tắc của Junction Box là cộng tất cả các tín hiệu thu được từ các loadcell nối vào nó rồi chia trung bình để tìm ra khối lượng chính xác của vật cần cân.

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  3. #3
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    khái niệm stain gauge



    THE STRAIN GAUGE
    The strain gauge has been in use for many years and is the fundamental
    sensing element for many types of sensors, including pressure sensors,
    load cells, torque sensors, position sensors, etc.

    The majority of strain gauges are foil types, available in a wide choice
    of shapes and sizes to suit a variety of applications. They consist of a
    pattern of resistive foil which is mounted on a backing material. They
    operate on the principle that as the foil is subjected to stress, the
    resistance of the foil changes in a defined way.



    The strain gauge is connected into a Wheatstone Bridge circuit with a combination of four active gauges (full bridge), two gauges (half bridge),
    or, less commonly, a single gauge (quarter bridge). In the half and
    quarter circuits, the bridge is completed with precision resistors.


    The complete Wheatstone Bridge is excited with a stabilised DC supply
    and with additional conditioning electronics, can be zeroed at the null
    point of measurement. As stress is applied to the bonded strain gauge,
    a resistive changes takes place and unbalances the Wheatstone Bridge.
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]
    This results in a signal output, related to the stress value.
    As the signal
    value is small, (typically a few millivolts) the signal conditioning
    electronics provides amplification to increase the signal level to 5 to 10
    volts, a suitable level for application to external data collection systems
    such as recorders or PC Data Acquistion and Analysis Systems.


    Some of the many Gauge Patterns available

    Most manufacturers of strain gauges offer extensive ranges of differing
    patterns to suit a wide variety of applications in research and industrial
    projects.
    They also supply all the necessary accessories including preparation
    materials, bonding adhesives, connections tags, cable, etc. The bonding
    of strain gauges is a skill and training courses are offered by some suppliers.
    There are also companies which offer bonding and calibration services,
    either as an in-house or on-site service.
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]
    More about the Strain Gauge...
    If a strip of conductive metal is stretched, it will become skinnier and
    longer, both changes resulting in an increase of electrical resistance
    end-to-end. Conversely, if a strip of conductive metal is placed under
    compressive force (without buckling), it will broaden and shorten. If
    these stresses are kept within the elastic limit of the metal strip (so
    that the strip does not permanently deform), the strip can be used as
    a measuring element for physical force, the amount of applied force
    inferred from measuring its resistance.

    Such a device is called a strain gauge. Strain gauges are frequently used
    in mechanical engineering research and development to measure the
    stresses generated by machinery. Aircraft component testing is one area
    of application, tiny strain-gauge strips glued to structural members,
    linkages, and any other critical component of an airframe to measure
    stress. Most strain gauges are smaller than a postage stamp, and they
    look something like this:


    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]A strain gauge's conductors are very thin: if made of round wire, about
    1/1000 inch in diameter. Alternatively, strain gauge conductors may be
    thin strips of metallic film deposited on a nonconducting substrate
    material called the carrier. The latter form of strain gauge is represented
    in the previous illustration. The name "bonded gauge" is given to strain gauges that are glued to a larger structure under stress (called the test specimen) The task of bonding strain gauges to test specimens may
    appear to be very simple, but it is not. "Gauging" is a craft in its own
    right, absolutely essential for obtaining accurate, stable strain measurements. It is also possible to use an unmounted gauge wire
    stretched between two mechanical points to measure tension, but this technique has its limitations.

    Typical strain gauge resistances range from 30 Ohms to 3 kOhms (unstressed). This resistance may change only a fraction of a percent
    for the full force range of the gauge, given the limitations imposed by
    the elastic limits of the gauge material and of the test specimen. Forces
    great enough to induce greater resistance changes would permanently
    deform the test specimen and/or the gauge conductors themselves, thus
    ruining the gauge as a measurement device. Thus, in order to use the
    train gauge as a practical instrument, we must measure extremely small
    changes in resistance with high accuracy.

    Such demanding precision calls for a bridge measurement circuit. Unlike
    the Wheatstone bridge shown in the last chapter using a null-balance
    detector and a human operator to maintain a state of balance, a strain
    gauge bridge circuit indicates measured strain by the degree of
    imbalance, and uses a precision voltmeter in the center of the bridge
    to provide an accurate measurement of that imbalance:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]

    Typically, the rheostat arm of the bridge (R2 in the diagram) is set
    at a value equal to the strain gauge resistance with no force applied.
    The two ratio arms of the bridge (R1 and R3) are set equal to each
    other. Thus, with no force applied to the strain gauge, the bridge
    will be symmetrically balanced and the voltmeter will indicate zero
    volts, representing zero force on the strain gauge. As the strain
    gauge is either compressed or tensed, its resistance will decrease
    or increase, respectively, thus unbalancing the bridge and producing
    an indication at the voltmeter. This arrangement, with a single element
    of the bridge changing resistance in response to the measured variable (mechanical force), is known as a quarter-bridge circuit.

    As the distance between the strain gauge and the three other
    resistances in the bridge circuit may be substantial, wire resistance
    has a significant impact on the operation of the circuit. To illustrate
    the effects of wire resistance, I'll show the same schematic diagram,
    but add two resistor symbols in series with the strain gauge to
    represent the wires:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]

    The strain gauge's resistance (Rgauge) is not the only resistance being
    measured: the wire resistances Rwire1 and Rwire2, being in series with
    Rgauge, also contribute to the resistance of the lower half of the
    rheostat arm of the bridge, and consequently contribute to the
    voltmeter's indication. This, of course, will be falsely interpreted by
    the meter as physical strain on the gauge.

    While this effect cannot be completely eliminated in this configuration,
    it can be minimized with the addition of a third wire, connecting the right
    side of the voltmeter directly to the upper wire of the strain gauge:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]

    Because the third wire carries practically no current (due to the
    voltmeter's extremely high internal resistance), its resistance will not
    drop any substantial amount of voltage. Notice how the resistance
    of the top wire (Rwire1) has been "bypassed" now that the voltmeter
    connects directly to the top terminal of the strain gauge, leaving only
    the lower wire's resistance (Rwire2) to contribute any stray resistance
    in series with the gauge. Not a perfect solution, of course, but twice
    as good as the last circuit!

    There is a way, however, to reduce wire resistance error far beyond
    the method just described, and also help mitigate another kind of
    measurement error due to temperature. An unfortunate characteristic
    of strain gauges is that of resistance change with changes in
    temperature. This is a property common to all conductors, some more
    than others. Thus, our quarter-bridge circuit as shown (either with
    two or with three wires connecting the gauge to the bridge) works
    as a thermometer just as well as it does a strain indicator.
    If all we want to do is measure strain, this is not good. We can
    transcend this problem, however, by using a "dummy" strain gauge
    in place of R2, so that both elements of the rheostat arm will change
    resistance in the same proportion when temperature changes, thus
    canceling the effects of temperature change:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]



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  4. #4
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    Resistors R1 and R3 are of equal resistance value, and the strain
    gauges are identical to one another. With no applied force, the bridge
    should be in a perfectly balanced condition and the voltmeter should
    register 0 volts. Both gauges are bonded to the same test specimen,
    but only one is placed in a position and orientation so as to be exposed
    to physical strain (the active gauge). The other gauge is isolated
    from all mechanical stress, and acts merely as a temperature
    compensation device (the "dummy" gauge). If the temperature
    changes, both gauge resistances will change by the same percentage,
    and the bridge's state of balance will remain unaffected. Only a
    differential resistance (difference of resistance between the two strain
    gauges) produced by physical force on the test specimen can alter the
    balance of the bridge.
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]
    Wire resistance doesn't impact the accuracy of the circuit as much as
    before, because the wires connecting both strain gauges to the bridge
    are approximately equal length. Therefore, the upper and lower sections
    of the bridge's rheostat arm contain approximately the same amount of
    stray resistance, and their effects tend to cancel:


    Even though there are now two strain gauges in the bridge circuit, only
    one is responsive to mechanical strain, and thus we would still refer to
    this arrangement as a quarter-bridge. However, if we were to take the
    upper strain gauge and position it so that it is exposed to the opposite
    force as the lower gauge (i.e. when the upper gauge is compressed, the
    lower gauge will be stretched, and visa-versa), we will have both gauges
    responding to strain, and the bridge will be more responsive to applied
    force. This utilization is known as a half-bridge. Since both strain gauges
    will either increase or decrease resistance by the same proportion in
    response to changes in temperature, the effects of temperature change
    remain canceled and the circuit will suffer minimal temperature-induced
    measurement error:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]

    An example of how a pair of strain gauges may be bonded to a test
    specimen so as to yield this effect is illustrated here:
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]

    With no force applied to the test specimen, both strain gauges have
    equal resistance and the bridge circuit is balanced. However, when a
    downward force is applied to the free end of the specimen, it will bend
    downward, stretching gauge #1 and compressing gauge #2 at the
    same time:


    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]In applications where such complementary pairs of strain gauges can
    be bonded to the test specimen, it may be advantageous to make
    all four elements of the bridge "active" for even greater sensitivity.
    This is called a full-bridge circuit:


    Both half-bridge and full-bridge configurations grant greater sensitivity
    over the quarter-bridge circuit, but often it is not possible to bond
    complementary pairs of strain gauges to the test specimen. Thus,
    the quarter-bridge circuit is frequently used in strain measurement
    systems.

    When possible, the full-bridge configuration is the best to use. This
    is true not only because it is more sensitive than the others, but
    because it is linear while the others are not. Quarter-bridge and
    half-bridge circuits provide an output (imbalance) signal that is
    only approximately proportional to applied strain gauge force.
    Linearity, or proportionality, of these bridge circuits is best when
    the amount of resistance change due to applied force is very
    small compared to the nominal resistance of the gauge(s). With
    a full-bridge, however, the output voltage is directly proportional
    to applied force, with no approximation (provided that the change
    in resistance caused by the applied force is equal for all four
    strain gauges!).
    [Chỉ có thành viên mới có thể nhìn thấy đường links. ]
    Unlike the Wheatstone and Kelvin bridges, which provide measurement
    at a condition of perfect balance and therefore function irrespective
    of source voltage, the amount of source (or "excitation") voltage
    matters in an unbalanced bridge like this. Therefore, strain gauge
    bridges are rated in millivolts of imbalance produced per volt of
    excitation, per unit measure of force. A typical example for a
    strain gauge of the type used for measuring force in industrial
    environments is 15 mV/V at 1000 pounds. That is, at exactly
    1000 pounds applied force (either compressive or tensile), the
    bridge will be unbalanced by 15 millivolts for every volt of
    excitation voltage. Again, such a figure is precise if the bridge
    circuit is full-active (four active strain gauges, one in each arm
    of the bridge), but only approximate for half-bridge and quarter
    -bridge arrangements.

    Strain gauges may be purchased as complete units, with both strain
    gauge elements and bridge resistors in one housing, sealed and
    encapsulated for protection from the elements, and equipped with
    mechanical fastening points for attachment to a machine or structure.
    Such a package is typically called a load cell.

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  5. #5
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    cho em hỏi về mạch lưcj cho hệ thống cân oto với ạ??



    EM đang tìm hiểu làm btap về hệ thống cân oto cho nhà máy sx thức ăn gia súc. cái mạch lực cho hệ thống khó quá. ai giúp em với ạ??

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