The principle of the ultrasonic distance measurement depends on the velocity of the ultrasonic wave in the air. The velocity of the ultrasonic wave in the air is influenced by the air temperature. In strict applications, consider the temperature factor in the distance calculation to achieve a more accurate result.
Figure 1. NTC Measurement
As shown in the figure above, a low-cost NTC resistor is used in this design. See the tables below for more details about the resistor.
| Part Name | Manufacturer Part Number | Signal Name | Pin in MCU |
|---|---|---|---|
| NTC resistor | NCP15WF104F03RC | Temp | PB4 |
The table below shows the accurate relationship between the temperature, NTC resistance, and voltage at the NTC resistor.
| Temperature [°C] | Resistance [kΩ] | Rtemp=Rth*R108/(Rth+R108) [kΩ] | Vtemp=3.3*Rtemp/(Rtemp+R105) [V] |
|---|---|---|---|
| -40 | 4397.119 | 97.77635415 | 1.631448664 |
| -35 | 3088.599 | 96.8638264 | 1.623714387 |
| -30 | 2197.225 | 95.64692183 | 1.613288055 |
| -25 | 1581.881 | 94.05427613 | 1.599444843 |
| -20 | 1151.037 | 92.0066313 | 1.58130936 |
| -15 | 846.579 | 89.4356414 | 1.557983569 |
| -10 | 628.988 | 86.28235307 | 1.528495644 |
| -5 | 471.632 | 82.50622778 | 1.491842525 |
| 0 | 357.012 | 78.11873649 | 1.447303274 |
| 5 | 272.5 | 73.15436242 | 1.394186047 |
| 10 | 209.71 | 67.71173033 | 1.332337992 |
| 15 | 162.651 | 61.92666314 | 1.262040385 |
| 20 | 127.08 | 55.96265633 | 1.184108877 |
| 25 | 100 | 50 | 1.1 |
| 30 | 79.222 | 44.20327862 | 1.01156382 |
| 35 | 63.167 | 38.71309762 | 0.920988893 |
| 40 | 50.677 | 33.63287031 | 0.830547692 |
| 45 | 40.904 | 29.02969398 | 0.742449177 |
| 50 | 33.195 | 24.92210669 | 0.658353867 |
| 55 | 27.091 | 21.31622223 | 0.579836168 |
| 60 | 22.224 | 18.18300825 | 0.507720425 |
| 65 | 18.323 | 15.48557761 | 0.442500329 |
| 70 | 15.184 | 13.18238644 | 0.384351988 |
| 75 | 12.635 | 11.21764993 | 0.332845055 |
| 80 | 10.566 | 9.556283125 | 0.287849619 |
| 85 | 8.873 | 8.149862684 | 0.248678511 |
| 90 | 7.481 | 6.96029996 | 0.214743133 |
| 95 | 6.337 | 5.959355634 | 0.185598275 |
| 100 | 5.384 | 5.108934943 | 0.160400116 |
| 105 | 4.594 | 4.392221351 | 0.138844928 |
| 110 | 3.934 | 3.785094387 | 0.120352653 |
| 115 | 3.38 | 3.269491198 | 0.104477332 |
| 120 | 2.916 | 2.833378678 | 0.09092524 |
| 125 | 2.522 | 2.459959814 | 0.079229656 |