The Key to Accurate Vibration Measurement: Accelerometer Installation and Usage

In the realm of vibration testing, the accelerometer acts as the crucial sensory organ of your vibration test system. It's a transducer that converts the physical quantity of vibration (acceleration, and indirectly related to sound) into a measurable electrical signal, providing the essential input for analysis and control. At Dongguan Precision Test Equipment Co., Ltd., we emphasize the importance of proper accelerometer installation and usage for obtaining reliable and meaningful vibration data.

Accelerometers are the preferred choice for vibration measurement due to their advantageous characteristics, including:

• Wide Dynamic Range: Capable of measuring both very small and very large vibration amplitudes.
• Broad Frequency Range: Can accurately capture vibrations across a wide spectrum of frequencies.
• Excellent Linearity: Provides a proportional electrical output to the input acceleration.
• High Stability: Offers consistent and dependable measurements over time.
• Relatively Convenient Installation: Can be mounted using various methods depending on the application.

To harness the full potential of your accelerometer and ensure accurate vibration measurements, adhere to the following installation principles and methods:

I. Installation Principles: Setting the Foundation for Accuracy

For optimal performance, observe these fundamental rules when installing your accelerometer:

a) Strategic Location: Position the accelerometer as close as possible to the specific test point of interest on the structure to ensure it experiences the same motion as the area being analyzed.

b) Secure and Firm Mounting: The accelerometer and its mounting surface must be as rigid and firmly connected as possible. The mounting surface should be clean and flat to ensure maximum contact and the most direct or shortest transmission path of the vibration. For uniaxial accelerometers, carefully align the sensing direction (main axis) with the direction of interest.

c) Minimizing Mass Loading Effects: The introduction of the accelerometer should cause minimal alteration to the test structure's motion. Employ symmetrical mounting techniques where possible to minimize motion distortion. The mass of the accelerometer and any mounting hardware should be significantly smaller than the dynamic mass of the measured structure (ideally a mass ratio of less than 1/10 for small, light objects).

d) Avoiding Resonance Interference: The maximum operating frequency of your test should be significantly lower than the mounting resonance frequency of the chosen accelerometer. Operating near the mounting resonance can lead to amplified and inaccurate readings.

e) Cable Management: When using axially connected accelerometers, stiff cables can induce strain on the housing, potentially affecting measurements. Securely clamp the cable close to the accelerometer to prevent this. For piezoelectric accelerometers, loose cables can generate triboelectric noise (frictional static electricity).

(Diagram showing Axial Lead Accelerometer and Side Lead Accelerometer) (Caption for diagram: 1 - Do not subject to force, 2 - Connection surface of the vibrating body, 3 - Secure the cable to the vibrating surface)

f) Electrical Isolation: Accelerometers have varying electrical insulation properties. Some have built-in insulated bases, while others require insulated mounting screws and mica washers to prevent ground loops in the measurement system. Using insulated screws with mica washers at the contact points is an effective way to resolve ground loop issues.

II. Specific Installation Methods: Practical Application

Here's a breakdown of common accelerometer installation methods:

a. Screw Mounting:

(Diagram showing a typical frequency response curve for a screw-mounted accelerometer with grease) (Caption for diagram: Typical frequency response curve of an accelerometer screw-mounted with grease (relative to the absolute acceleration of the structure at the connection point))

• Surface Preparation: The mounting surface on both the accelerometer and the test structure must be clean, flat, and smoothly machined, meeting the manufacturer's recommended specifications. The mounting screw hole should be perpendicular to the mounting surface.
• Torque Application: Tighten the mounting screw to the manufacturer's recommended torque to achieve a secure connection without damaging the accelerometer.
• Coupling Medium: Apply a thin layer of oil or grease between the mating surfaces to enhance contact and maximize stiffness, improving high-frequency response.
• Screw Length: Ensure the screw does not bottom out in the tapped hole, as this can create a small gap between the mounting surfaces, reducing stiffness.

b. Adhesive Bonding:

This method is suitable when drilling holes in the test structure is not feasible, electrical isolation is required, or the mounting surface has insufficient flatness. Adhesive mounting screws (studs with threads on one end and a bonding platform on the other) are also commonly used.

(Diagram showing a typical frequency response curve for an adhesively bonded accelerometer) (Caption for diagram: Typical frequency response curve of an adhesively bonded accelerometer (relative to the absolute acceleration of the structure at the connection point))

• Surface Cleaning: Clean the bonding surfaces according to the adhesive manufacturer's recommendations.
• Thin Adhesive Layer: Apply the adhesive to form a thin film, which should ideally act as a rigid spring for optimal frequency response.
• Adhesive Selection: Acrylic or thermosetting adhesives are often used. Avoid soft adhesives or those that retain significant flexibility after solvent evaporation, as they can lower the resonant frequency. Cyanoacrylate adhesives (super glue like 502) offer a wide frequency response but are not suitable for all applications and can contaminate screw threads. Before application, clean the mounting surface with a hydrocarbon solvent, keeping the solvent away from cables and connectors. Press the sensor firmly into the adhesive quickly to achieve a thin bond line. Temperature limitations of the adhesive must also be considered.

c. Mounting Devices:

Mounting devices, including electrically insulated screws, should be stiff, lightweight, have a small moment of inertia, and be structurally symmetrical about the sensing axis. Avoid using brackets whenever possible. If necessary, opt for small, rigid metal cubes securely mounted to the structure with machined surfaces and tapped holes for screw connection.

d. Other Mounting Methods:

Alternative mounting techniques include using a thin layer of solidified beeswax, double-sided adhesive tape, magnetic bases, quick-mount clamps, and vacuum mounting bases.

(Diagram showing a typical frequency response curve for a beeswax-mounted accelerometer) (Caption for diagram: Typical frequency response curve of an accelerometer mounted with a thin layer of beeswax (relative to the absolute acceleration of the structure at the connection point))

• Beeswax Mounting: Suitable for room temperature applications with sensors weighing less than 100 grams. It's convenient but limits the operating temperature to below 40°C and is suitable for lower acceleration levels.

(Diagram showing a typical frequency response curve for a double-sided tape-mounted accelerometer) (Caption for diagram: Typical frequency response curve of an accelerometer mounted with double-sided tape)

• Double-Sided Tape Mounting: Various types of double-sided tapes offer different operating temperatures and thicknesses. Choose a tape appropriate for your application.

(Diagram showing a typical frequency response curve for a magnetic base-mounted accelerometer) (Caption for diagram: Typical frequency response curve of an accelerometer mounted with a magnetic base)

• Magnetic Base Mounting: Convenient for quick measurements on ferromagnetic surfaces but limits the maximum vibration level and measurement frequency. This method typically lowers the mounting resonance frequency to around 7 kHz, reducing the usable frequency range to about 2 kHz (around 1/3 of the mounting resonance). Magnetic bases also add significant mass and have limited holding force, typically suitable for accelerations below 200g.

III. Important Precautions: Ensuring Longevity and Data Integrity

Keep these crucial points in mind when handling and using accelerometers:

a) Gentle Removal: When dismounting sensors, gently cut any adhesive or beeswax from the side rather than pulling directly from the mounting surface, which can damage the sensor.

b) Direct Bonding Caution: Direct bonding of most accelerometers without proper consideration for removal and potential damage is generally not recommended.

c) Cable Security for Charge-Type Accelerometers: Ensure charge-type accelerometer cables are securely fixed. Movement, bending, or stretching of these cables during measurement can cause changes in local capacitance and charge between the conductor and shield, introducing significant noise. IEPE (Integrated Electronic Piezoelectric) accelerometers with built-in amplifiers are much less susceptible to cable noise.

d) Connector Integrity: When using multiple extension cables, ensure connectors are kept clean and free from dust, water, or conductive contaminants.

e) Mass Loading on Light Objects: For small and lightweight objects (e.g., small blades), carefully consider the mass loading effect of the accelerometer. Aim for a mass ratio of the accelerometer to the test object of less than 1/10.

f) Avoid Dropping: Never drop the sensor onto hard surfaces, as this can cause irreparable damage.

g) Temperature Limits: Always operate sensors within their specified temperature range to prevent damage and ensure accurate measurements.

By adhering to these installation guidelines and usage precautions, you can maximize the accuracy, reliability, and lifespan of your accelerometers, ensuring high-quality data for your vibration testing endeavors. At Dongguan Precision, we are dedicated to providing you with not only advanced vibration test systems but also the knowledge to utilize them effectively.