Day 2.1: Ultrasonic Sensor

Bonus Material for Diligent Students (Released on 3 July 2024)

We appreciate the students who challenged ideas on the afternoon of Day 2. The SWS3009B Teaching teams researched after work and finally got a clear idea about our new Ultrasonic Sensor (HC-SR04A).

Code Explaination

In Day 2's lecture, we used an example code which is provided by the manufacturer of the HC-SR04A; the main methodology (we will call this Method 1) is like this:

Method 1

  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  duration = pulseIn(echoPin, HIGH);
  distance = (duration*.0343)/2;

Here is the code explanation: The third and fourth lines of code generate a 10us high pulse to trigger the trigPin, causing the ultrasonic transmitter to send out eight 40KHz ultrasonic pulses (source: manufacturer). The first and second lines are just to ensure the pulse is off and have no effect. The fifth line of code turns off the pulse. The pulseIn(echoPin, HIGH) function in the seventh line of code is used to detect the duration of the high-level signal on the echoPin, measured in microseconds (us). According to the manufacturer's manual, the duration of the high-level signal on the echoPin (i.e., the width of the echo pulse) represents the time it takes for the ultrasonic pulse to travel from the transmitter to the receiver. Thus, the formula in the eighth line of code is: distance = (duration * 0.0343) / 2.

Here, duration is the width of the echo pulse, and 0.0343 cm/us is the speed of sound in the air. We divide by 2 because this time includes the round trip of the ultrasonic pulse from the sensor to the object and back.

Therefore, the width of the echo pulse is directly related to the distance: the farther the distance, the wider the echo pulse; the closer the distance, the narrower the echo pulse.

Method 2

Many students wonder why using a completely different logic to measure distance can also be successful: that is (we will call this Method 2), recording the duration between the trigPin sending the pulse and the echoPin receiving the pulse, and then using the same formula, distance = (duration * 0.0343) / 2, can also yield an accurate distance?

Below is our verification of the HC-SR04A sensor's functionality using an oscilloscope, which we hope will clarify your doubts.

Verification of Signal Waveform from HC-SR04A

We connected the sensor's trigPin to the red channel and the echoPin to the green channel, then ran the code in Method 1, resulting in the following output as shown in the figure:

Main Time Base = 25ms, real distance = infinite, calculated distance = 806 cm

As consistent with our Code Explanation, we can see that a pulse signal is sent to the trigPin before the rising edge on the echoPin. Following this signal, there is a blind time (a buffer time allocated by the manufacturer for the hardware to send the pulse, source: manufacturer). After this period, the hardware should have emitted the ultrasonic wave, so the echoPin, as designed, turns high. We can reduce the Main Time Base (typically denoted as seconds per division (s/div)) and zoom in on the image to take a closer look:

Main Time Base = 1ms, real distance = infinite, calculated distance = 806 cm

When the distance of the ultrasonic wave is reduced to 100 cm (as per the code from Day 2, where the red light turns on), the resulting waveform looks like this:

Main Time Base = 1ms, real distance = calculated distance = 100 cm

When the distance of the ultrasonic wave is reduced to 2 cm (the theoretical minimum detection distance), the resulting waveform looks like this:

Main Time Base = 1ms, real distance = calculated distance = 2 cm

This validates the design principle of the HC-SR04 and confirms that the code design in Method 1 is correct. However, using Method 2, we can still obtain accurate results:

When measuring the duration between the trigPin and the echoPin, if the end condition for the duration is the falling edge of the echoPin, the duration obtained by the students through this method will also be linearly proportional to the detection distance. If students use Method 2 but the end condition for the duration is the rising edge of the echoPin, the duration obtained will be a fixed value (buffer time), which could be wrong. Below is a diagram showing the difference in duration detected by the two methods in Method 2:

Main Time Base = 1ms, real distance = calculated distance = 100 cm

We hope this clarifies your doubts. Additionally, regarding the issue we encountered this afternoon where the waveforms of the trigPin and the echoPin were exactly opposite, we found after repeated verification that it was caused by a malfunction in the oscilloscope's Channel 2. We have reported this for repair.

the 'mirrored' waveform! So confusing!

Thank you all for raising questions. The teaching team has also learned a lot and reflected on many things during the teaching process. It has been a very rewarding experience for us! :-)