Understanding Voltage-Mode Accelerometers

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Definition: What is a Voltage-Mode Accelerometer?

Voltage-mode accelerometer is a piezoelectric accelerometer with built-in signal conditioning electronics that converts the high-impedance charge from the piezoelectric element into a low-impedance voltage output. This term is essentially synonymous with IEPE accelerometer (Integrated Electronics Piezo-Electric) and ICP® (Integrated Circuit Piezoelectric, a trademark of PCB Piezotronics). The “voltage-mode” designation emphasizes that the output is voltage (typically mV per g) rather than charge (pC per g), distinguishing it from charge-mode accelerometers.

Voltage-mode accelerometers have become the overwhelming standard in industrial vibration monitoring, accounting for over 95% of applications due to their simplicity (no external amplifier needed), ease of use (simple two-wire connection), and cost-effectiveness. Understanding that voltage-mode, IEPE, and ICP all refer to essentially the same technology helps navigate product specifications and literature.

Key Characteristics

Integrated Electronics

• Microelectronic amplifier built into sensor housing
• Converts high-impedance charge to low-impedance voltage
• Typically uses FET or IC amplifier
• Powered by constant current from measurement instrument

Output Format

• Sensitivity: Typically 10-1000 mV/g
• Common: 100 mV/g industry standard
• Signal Type: AC voltage proportional to acceleration
• Impedance: Low (< 100 ohms output impedance)

Power Requirements

• Constant Current: 2-20 mA typical (4 mA common)
• Supply Voltage: 18-30 VDC
• Bias Voltage: 8-12 VDC on output
• Two-Wire: Power and signal on same cable

Advantages

System Simplicity

• No external charge amplifier required
• Direct connection to instrument
• Lower total system cost
• Reduced complexity and failure points

Cable Capability

• Low-impedance drives long cables (to 300m)
• Standard inexpensive coaxial cable
• Good noise immunity
• Flexible installation

Ease of Use

• Simple plug-and-play operation
• Minimal setup required
• Standardized interface
• Widely compatible with instruments

Comparison with Charge Mode

Voltage-Mode (IEPE) Advantages

• Simpler system (no external amplifier)
• Lower cost
• Long cable capability
• Better noise immunity
• Easier to use

Charge-Mode Advantages

• Higher temperature capability (to 650°C vs. 175°C)
• Radiation hardness (nuclear applications)
• No active electronics to fail
• Specialized applications only

Selection

• Voltage-Mode: 95%+ of industrial applications
• Charge-Mode: Only when temperature > 175°C or radiation present

Common Specifications

Sensitivity Options

• 10 mV/g: High vibration, shock (±500g range)
• 50 mV/g: General purpose (±100g range)
• 100 mV/g: Industry standard (±50g range)
• 500-1000 mV/g: Low vibration, precision (±5-10g range)

Frequency Response

• Low frequency: 0.5-5 Hz (-3 dB, AC coupled)
• High frequency: to resonance (10-70 kHz depending on size)
• Usable to 1/3 resonance frequency

Temperature Rating

• Standard: -50 to +120°C
• Extended: -50 to +150°C
• High-Temp: -50 to +175°C
• Above 175°C: charge-mode required

Variants and Technologies

Design Variations

• Compression-mode IEPE (most common, economical)
• Shear-mode IEPE (premium, better performance)
• Differential output (better common-mode rejection)
• Low-noise (ultra-low noise floor for precision)

Package Types

• Industrial (hermetically sealed, rugged)
• Miniature (small size for space-limited applications)
• Triaxial (three orthogonal axes in one package)
• Subminiature (< 10 grams)

Terminology Clarification

Equivalent Terms

• Voltage-Mode: Generic descriptor
• IEPE: Integrated Electronics Piezo-Electric (standard term)
• ICP®: Integrated Circuit Piezoelectric (PCB Piezotronics trademark)
• CCLD: Constant Current Line Drive (Brüel & Kjær term)
• Deltatron: Brüel & Kjær brand name
• All: Fundamentally same technology with built-in electronics and constant-current power

Best Practices

Selection

• 100 mV/g sensitivity for general machinery
• Industrial-grade housing for harsh environments
• Appropriate temperature rating for application
• Hermetically sealed for contaminated environments

Installation

• Stud mount for high-frequency measurements
• Adhesive for semi-permanent installations
• Magnetic for route surveys
• Proper mounting critical for performance

Maintenance

• Periodic calibration (annually for critical)
• Cable inspection
• Mounting verification
• Function checks before critical measurements

Voltage-mode accelerometers (IEPE/ICP) are the workhorse sensors of modern industrial vibration monitoring, combining the advantages of piezoelectric transduction with integrated electronics for simplicity and reliability. Their dominance in the field reflects the optimal balance of performance, cost, and ease of use for the vast majority of rotating machinery condition monitoring and diagnostic applications.

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