SLAA732A February   2017  – April 2021 PGA460 , PGA460-Q1

 

  1. 1Trademarks
  2. 2Overview
  3. 3External Performance Factors
    1. 3.1 Range Requirements
    2. 3.2 Detectable Target and Objects
    3. 3.3 Ambient Environment
  4. 4Component Selection
    1. 4.1 Sonar Configuration
    2. 4.2 Transducer Selection
    3. 4.3 Driver Selection
    4. 4.4 Passive Tuning
      1. 4.4.1 Impedance Gain-Phase Analyzer
      2. 4.4.2 Tuning Capacitor
      3. 4.4.3 Damping Resistor
      4. 4.4.4 Tunable Transformer
  5. 5PGA460 Parameters
    1. 5.1 Center Frequency
    2. 5.2 Pulse Count
    3. 5.3 Current Limit
    4. 5.4 Time-Varying Gain and Digital Gain
    5. 5.5 Threshold
  6. 6End-of-Line Calibration
    1. 6.1 Transducer Parameters
      1. 6.1.1 Optimal Frequency and Sound Pressure Level Measurements
        1. 6.1.1.1 Frequency Diagnostic Feature of PGA460
        2. 6.1.1.2 External Microphone
  7. 7Revision History

3.3 Ambient Environment

Changes to temperature, humidity, and air pressure influence the speed of sound and the transmission impedance characteristics of the transducer just as a variable parallel load at the transducer would. Temperature has the greatest impact on the performance of ultrasonic sensors. Sound and heat are both forms of kinetic energy, whereby an increase to temperature yields an increase to the rate of molecular vibration. Because of the fluctuation in molecular vibration, sound waves are able to travel from 300 to 400 m/s. Use Equation 1 to calculate the speed of sound in air (v) as a dependency to temperature (T).

Equation 1. v = 331 m/s + 0.6 m/s/°C × T

Table 3-4 shows the speed of sound across temperature.

Table 3-4 Speed of Sound in Air Across temperature
Temperature (°C)Speed of Sound (m/s)
–40307
–30313
–20319
–10325
0331
10337
20343
30349
40355
50361
60367
70373
80379
90385
100391
110397
120403

When converting the round-trip time of an ultrasonic time-of-flight based echo, the speed of sound must be considered in order to prevent ±15cm of error to the distance equivalent of the target.

The resonant frequency of the transducer decreases as temperature increases. Therefore, to compensate for the point at which the phase change will occur, the transducer must be driven at an offset frequency, or external passive components must be introduced beyond a certain temperature to retune the resonance towards the nominal frequency. The PGA460 device offers a temperature decoupling mode to introduce additional passives in parallel to the transducer beyond a user-specified temperature.