Raspberry pi servo and ultrasonic sensors jitter

Question

I would like to know if someone else experienced the same problem with their servo motor combined with ultrasonic sensors on a Raspberry Pi (I own a 3B).

Essentially, I have mounted two ultrasonic sensors (HC-SR04) with 180° apart from each other on top of a servo motor (HS-422) to act as a radar.

Here's a sample code. The Servo and Sonic classes were made by me. They are fully functional and they use Joan's pigpio.

import pigpio, time
from Files.servo import Servo
from Files.sonic import Sonic

pi = pigpio.pi()

servoMotor = Servo(name = 'Test', gpio_list = [14], pi)
sensor = Sonic(name = 'Test2', trig = 2, echo = 3, pi)

servoMotor.rotate(0)
time.sleep(1)
try:
    while True:
        for i in range(181):
            servoMotor.rotate(angleR = i)
            time.sleep(0.01)
            print(i, sensor.distance())
        for i in range(181):
            servoMtor.rotate(angleR = 180 - i)
            time.sleep(0.01)
            print((180 - i), sensor.distance())

except KeyboardInterrupt:
    pi.stop()

The problem: When I make the servo rotate angle by angle (180 times), everything works just fine. When I instance my ultrasonic sensors along with the servo and return their distance(), everything starts to lag, and the servo moves really slowly.

I tried to print the distances to see if it was a servo jitter problem, but my suspicion was confirmed: it's all lagging.

As if the RPi was too slow.

Sometimes, I get a moment of "normal" speed for like 20° and then it slows down again. This is what confirmed to me it isn't a gpio library problem nor a sensor problem.

Is there a way out of this? Is the RPi really lagging?

Answer 1

Thank you for your reply.

I believe the sole fact that the sensors take about 10 ms to return a distance means for a whole 180°, it would take about:

180 * (0.010 + 0.010) = +/- 3.60 sec

I added another 0.01 because of my loop. The thing is, it takes more than 15 sec to do a 180° which doesn't make any sense. I also tried to put my hands around the sensors to make the duration to gather data lower but it didn't work.

With a lot of googlin', I found out that python isn't ideal for this type of usage (real time ?) and that although pigpio is based on hardware, it seems if there's some computing in parallel, things don't go your way. For reference, here's the distance() code:

 def distance(self):
    self.pi.write(self.trig, 1)
    time.sleep(0.00001)
    self.pi.write(self.trig, 0)
    while self.pi.read(self.echo) == 0:
        pass
    start = time.time()
    while self.pi.read(self.echo) == 1:
        pass
    stop = time.time()
    return ((stop - start) * 17000)

• Would multithreading help?

When I first tried pigpio, calculating the duty cycle myself for the Servo would definitely make it jitter. When I opted for pigpio's servo functions, it went all smooth !

• Would controlling the ultrasonic sensors with pigpio's set_servo_pulsewidth() make any difference?

I will be trying both of these solutions and report them back here.

Answer 2

When I instance my ultrasonic sensors along with the servo and return their distance(), everything starts to lag and the servo moves really slow.

That isn't what engineers usually mean by the word "jitter" you used in your title.

It seems to me that sensor.distance() might take a longer time than you expect. Maybe the Ultrasonic sensors need time to get a stable reading, particularly if there is no obstacle nearby. Sound propagation might be significant compared to the 10 milliseconds sleep. That would slow down the for loops significantly.

A round trip to an obstacle 1.7m away takes 10 ms for an ultrasonic pulse.

If your sensor and library is able to measure distances up to say 10m (round trip 20m), it would have to wait around 60ms before returning a result when nothing is near.

At the very least, I'd time how long sensor.distance() takes to return a result under a wide range of physical/environmental circumstances.

Of course, since your "Sonic" library is not shown, we have no way to tell what limits are imposed by the software and hardware or what is meant by your parameters like echo = 3.

Note that there is a sister website for Raspberry Pi Q&A that might be a better place for this sort of question.