基于LM2917N与ESP32的金属车轮RPM测量技术求助

1. Calculating LM2917N Parameters (C1, C2, R1) #

The LM2917N converts input frequency to a proportional voltage using this core formula:
Vout = (Vcc * R1 * C2 * F) / 2
Where:

• Vcc: 12V (system supply)
• F: Input pulse frequency (Hz)
• Vout: Output voltage from LM2917N

Step 1: Define Target Output #

Choose whether to scale the LM2917N output directly to 0-3.3V or use a voltage divider later. For direct scaling (no extra components):

• Max F = 17.67Hz → Target Vout = 3.3V
  Rearrange the formula to solve for the R1*C2 product:
  R1*C2 = (2 * Vout_max) / (Vcc * F_max)
  Plugging in values:
  R1*C2 = (2 * 3.3) / (12 * 17.67) ≈ 0.0311s (31.1ms)

Step 2: Pick Standard Components #

Select off-the-shelf values that meet the R1*C2 requirement:

• Example 1: C2 = 10µF → R1 = 3.1kΩ (closest standard to 3110Ω)
• Example 2: C2 = 22µF → R1 = 1.4kΩ (closest standard to 1414Ω)

Step 3: Input Filter Capacitor (C1) #

Use C1 = 0.1µF to filter noise from the NPN sensor output. If you see erratic readings, increase to 1µF.

2. Adjusting Output Voltage to ESP32's 0-3.3V Range #

Two practical options:

Option A: Direct Scaling #

Use the R1/C2 values calculated above to set LM2917N's max output to 3.3V. This is simple but leaves no headroom for unexpected frequency spikes.

Option B: Voltage Divider #

For more headroom (e.g., set LM2917N's max output to 6.6V), add a 1:1 voltage divider:

• Connect R_top = 10kΩ between LM2917N pin3 (output) and ESP32 ADC pin
• Connect R_bottom = 10kΩ between ESP32 ADC pin and ground
  Adjust R1/C2 to target 6.6V at max frequency:
  R1*C2 = (2 * 6.6) / (12 *17.67) ≈0.0622s (62.2ms)
  Example: C2=22µF → R1=2.8kΩ (closest standard to 2827Ω)

3. Mapping ADC Voltage to RPM #

The system is linear, so mapping is straightforward:

Step 1: Frequency to RPM Conversion #

Each revolution produces 2 pulses, so:
RPM = (F * 60) / 2 = 30 * F
Where F is pulses per second.

Step 2: Simplified Linear Mapping #

Since max RPM (530) corresponds to 3.3V on ESP32, use direct proportion:
RPM = (530 / 3.3) * V_ESP ≈ 160.6 * V_ESP
For ESP32's 12-bit ADC (0-4095 raw value), convert to voltage first:
V_ESP = (raw_adc_value * 3.3) / 4095
Combine into one formula for raw ADC to RPM:
RPM ≈ 0.128 * raw_adc_value

Example Checks #

• Raw ADC = 4095 → RPM ≈ 524 (close to 530, accounting for component tolerances)
• Raw ADC = 769 → RPM ≈98.4 (matches 100 RPM target within tolerance)

Additional Practical Notes #

• NPN Sensor Interface: Connect sensor output to LM2917N pin1, sensor ground to system ground, sensor Vcc to 12V. The sensor's 12V high/0V low outputs are compatible with LM2917N's input.
• Tolerances: Use 1% tolerance resistors/capacitors for better accuracy.
• Calibration: After assembly, test with a known RPM to tweak the mapping formula if needed.

内容的提问来源于stack exchange，提问作者lucian_v