Acceleration measurement optimization: Mounting ... 4. Sensor and mounting accessory mass effect 5

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    Acceleration measurement optimization: Mounting considerations and sensor mass effect

    Authors

    Marine Dumont Acceleration Product Manager

    Andy Cook Mechanical Engineer

    Norton Kinsley Engineering Lab Supervisor, Kistler Instrument Corp. 75 John Glenn Drive, Amherst, NY 14228-2171 USA

    Abstract In the world of acceleration, a common topic is that of modal analysis. Within this topic, applications can range from the study of bridges as vehicles roll across them, to the qualification of very delicate space and aviation equipment. One can see that modal analysis contains a very large spectrum of applications. However, there is a small detail that is commonly overlooked, and that is the proper mounting of very sensitive accelerometers. Mounting options include direct stud mounting, wax mounting, magnetic mounting and a variety of options in-between. Although these options are diverse, they come with varying stiffness and sometimes with the cost of addition mass, termed the mass loading effect. The first part of this paper will take an in-depth look into some of the more common mistakes made during mounting, as well as a look into what can be done to optimize the mounting to avoid unwanted results. In the second part of this paper, there will be an exploration into the results of poorly mounted accelerometers; then a look in more detail at what the mass loading effect and stiffness are and how these can drastically change the measurement results.

    Key Words: Accelerometers, Resonance, Mounting, Sensor, Frequency

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    Content

    1. Introduction – sensor mounting considerations 2 2. Sensor mounting and handling rules 3 3. Hsu-Nielsen source test – simple method for

    resonance frequency analysis 6 4. Sensor and mounting accessory mass effect 7 5. Experimental analysis of mounting method

    influence on frequency response 8 6. Conclusions 12

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    1. Introduction – sensor mounting considerations 2. Sensor mounting and handling rules 3. Hsu-Nielsen source test – simple method for resonance frequency analysis 4. Sensor and mounting accessory mass effect 5. Experimental analysis of mounting method influence on frequency response 6. Conclusions

    To obtain useful measurement information, an accelerometer must be coupled so that complete event information is transferred. Mounting methods may vary, with some transferring event information more effectively than others. A high performance accelerometer will behave like a low performance accelerometer if the mounting method is inadequate. The transfer function behavior between the mechanical input properties and electrical output properties can be characterized by a “Single Degree of Freedom” (SDOF) system with a mounted resonance frequency, which will decrease if the mounting method becomes less stiff. This is illustrated in Figure 1 below.

    Fig. 1: Oscillating Single Degree of Freedom system and its corresponding frequency spectrum

    If we simplify the sensor and its mounting method to be a similar oscillating system with a period Tn in seconds and a natural frequency fn = 1/Tn in Hz, where Hz = 1/seconds, the natural frequency of the system will be dependent on the mass m of the system and the stiffness of the system through the spring constant k according to the equation (1) below:

    m

    k fn

    π2

    1 = (1)

    As a rule, the most rigid and lightest available mounting method option should be used at all times. The easiest mountings,

    such as magnetic mounting and wax mounting, affect the high frequency event information reaching the accelerometer. The

    reason for this is that it adds mass m to the system, as in the case of the magnet mounting base, and also reduces the spring

    constant k as well. The application and the type of data desired should ultimately drive the mounting approach. If low

    frequency events are being measured, a secure, easy mounting method is suitable.

    Fig. 2 Sensor mounting configurations from lowest mounted resonance to highest mounted resonance

    Figure 2 lists numerous accelerometer mounting options. To achieve the most accurate frequency response with the highest

    stiffness k, the stud mounting method should be used. Unfortunately, this often leads to a more demanding preparation, such

    as drilling and tapping a mounting hole, creating a high surface quality and incorporating the use of a torque wrench.

    Although stud mounting an accelerometer produces the most accurate results, drilling and tapping a hole can cause changes

    detrimental to the structure under study. Adhesive mounting using glue or wax is easier to handle but will restrict the

    measurement temperature range and may also require solvent or heat to remove the sensor. Magnetic mounting bases allow

    for a wider range of mounting positions, but with this flexibility, however, the magnetic mounting bases will restrict the

    acceleration amplitude due to the higher mass m of the sensor and mounting base with respect to the magnetic force holding

    the sensor to the structure. Magnetic mounting also requires a ferromagnetic surface as well as the additional weight of the

    magnetic mounting base itself.

    B- Sensor Mounting and Handling Rules

    Once a mounting method is chosen, there are basic rules to follow in order to ensure the best measurement accuracy. First of

    all, we are going to be looking into the stud mount method, which is the most commonly used mounting method. We can

    start be looking at an application such as calibration.

    Many accelerometers are specifically designed for stud mounting. Most mounting studs are machined from beryllium copper

    which is known for high strength, low modulus of elasticity and high elastic limits. The studs on many types are removable

    allowing for both stud and adhesive mounting. The following guidelines should be followed when stud mounting

    accelerometers. Drill and tap an adequate hole to allow flush mounting of the accelerometer. Make sure the stud does not

    bottom out and firmly secures the accelerometer. A chamfer should be machined at the top of the mounting hole to ensure

    that the base of the accelerometer makes full contact with the mounting surface, as seen in figure 3. Completely clean the

    surface prior to mounting and apply a thin coat of silicon grease to both the accelerometer and mounting surface. The

    influence of this will be discussed later in this paper. Always use the proper sockets and a torque wrench when installing

    accelerometers. Tighten the accelerometer to the torque specified on the individually supplied calibration certificate. Do not

    overtighten.

    As a rule, the most rigid and lightest available mounting method option should be used at all times. The easiest mountings, such as magnetic mounting and wax mounting, affect the high frequency event information reaching the accelerometer. The reason for this is that it adds mass m to the system, as in the case of the magnet mounting base, and also reduces the spring constant k as well. The application and the type of data desired should ultimately drive the mounting approach. If low frequency events are being measured, a secure, easy mounting method is suitable.

    1. Introduction – sensor mounting considerations

    Fig. 2: Sensor mounting configurations from lowest mounted resonance to highest mounted resonance

    Figure 2 lists numerous accelerometer mounting options. To achieve the most accurate frequency response with the highest stiffness k, the stud mounting method should be used. Unfortunately, this often leads to a more demanding preparation, such as drilling and tapping a mounting hole, creating a high surface quality and incorporating the use of a torque wrench. Although stud mounting an accelerometer produces the most accurate results, drilling and tapping a hole can cause changes detrimental to the structure under study. Adhesive mounting using glue or wax is easier to handle but will restrict the measurement temperature range and may also require solvent or heat to remove the sensor. Magnetic mounting bases allow for a wider range of mounting positions, but with this flexibility, however, the magnetic mounting bases will restrict the acceleration amplitude due to the higher mass m of the sensor and mounting base with respect to the magnetic force holding the sensor to the structure. Magnetic mounting also requires a ferromagnetic surface as well as the additional weight of the magnetic mounting base itself.

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    2. Sensor mounting and handling rules

    Once a mounting method is chosen, there are basic rules to follow in order to ensure the best measurement accuracy. First of all, we are going to be looking into the stud mount method, which is the most commonly used mounting method. We can start be looking at an application such as calibration.

    Many accelerometers are specifically designed for stud mounting. Most mounting studs are machined from beryllium copper which is known for high strength, low modulus of elasticity and high elastic limits. The studs on many types are removable allowing for both stud and adhesive mounting. The following guidelines should be followed when stud mounting accelerometers. Drill and tap an adequate hole to allow flush mounting of the accelerometer. Make sure the s

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