NewPing Library: HC-SR04, SRF05, SRF06, DYP-ME007, Parallax PING))) - v1.7

Okay, I believe I've got the new digital filtering method down to as lean as it's going to get with all the kinks worked out. For the coders out there, here's the method:

unsigned int NewPing::ping_ave(uint8_t it) {
    int uS[it], last, median;
    uint8_t i, j, cnt = 0;
    unsigned long total = 0;
    uS[0] = last = ping();   // Send first ping (done here to assist insertion sort).
    for (i = 1; i < it; i++) {                                                    // Ping iterations.
        if (last != NO_ECHO) cnt++;                                               // Array median tracking.
        delay(PING_AVE_DELAY - ((last == NO_ECHO ? _maxEchoTime : last) / 1000)); // Delay between pings.
        last = ping();                                                            // Send ping.
        for (j = i; j > 0 && uS[j - 1] < last; j--) uS[j] = uS[j - 1];            // Insertion sort (big to small).
        uS[j] = last;                                                             // Add last ping to array in correct order.
    }
    if (last != NO_ECHO) cnt++; // Array median tracking.
    median = uS[cnt >> 1];      // Find median.
    cnt = 0;                    // Reset counter.
    for (i = 0; i < it; i++) {                           // Loop through results.
        if (abs(median - uS[i]) < US_ROUNDTRIP_CM * 2) { // Exclude values outside +/-2cm range (digital noise).
            total += uS[i];                              // Building truncated mean.
            cnt++;                                       // Increment counter.
        }
    }
    return (total / cnt); // Return the ping distance mean minus digital noise (truncated mean).
}

To actually implement it you would need to define PING_AVE_DELAY to 29 and add the method to the class (I set the iterations [it] to default to 5). I know not everyone is adept at doing this, but this is a preview for those that are. Also, if there's any statistical gurus out there that want to poke holes in my code, feel free (statistics is not my thing).

The method uses two passes to calculate a truncated mean with outliner pings removed (digital noise). The first pass collects the pings, keeps track of successful pings, and sorts on-line using the very simple and fast insertion sort. Before the second pass it then calculates the median ping (with the unsuccessful pings filtered out).

Once the median value is found, it then does another pass to calculate the truncated mean. Outliner pings are those outside +/-2cm (these are not calculated as part of the mean). Because insertion sort is done on-line instead of in another pass (like a typical bubble sort), even the max of 255 ping iterations can be done without any perceived slow-down (although 255 is a little excessive, 3-9 is typically plenty). Insertion sort is very fast with a small quantity of values to sort. So fast that highly complex sort algorithms like quicksort resort to insertion sort when there's only a few values to sort (like in this case). Also, insertion sort has a very small stack footprint (only one variable) and extremely small code size (because it's so simple, 2 lines of code). It's really the only sort to use on Arduino.

In laymen terms, the new method does a user-specified number of pings as fast as it can (29ms per ping) ignores out of range pings, sorts the ping values, gets the middle distance ping value, filters out pings outside +/-2cm of the middle distance ping, and returns the average of these pings in microseconds.

unsigned int us = sonar.ping_ave(); // Defaults to 5 iterations, returns the truncated mean in microseconds.
unsigned int us = sonar.ping_ave(3); // Do 3 iterations, returns the truncated mean in microseconds.
unsigned int cm = sonar.convert_cm(sonar.ping_ave(9)); // Do 9 iterations and get the truncated mean in cm.

A few explanations about the method specific to the programming. First, some things may not appear as clean as they could be. In many cases this is done to make the code smaller once compiled. By making these changes around 100 bytes was saved on the final compiled code. I believe this is a fair compromise with the limited memory in the ATmega. Secondly, there are some items that may not be statistically perfect. An example is the median when there's an even number of values. The code only picks one median value, even if there technically should be the average of two. This again was done to reduce compiled code size and it also doesn't make much difference as there's a lot of fuzzy logic in a truncated mean anyway.

Basically, the function could be re-worked to be more clear, logical, and maybe slightly more statistically accurate. But, that's technically how the function started. I tweaked it over the course of a few days to try and squeeze as many bytes as I could out of it.

With that said, if there's something wrong, a suggestion, or another performance tweak that doesn't make the compiled code larger, please let me know (that's one of the reasons I'm posting the method before it's released).

Tim

Here's also a simplified version that just finds the median.

unsigned int NewPing::ping_median(uint8_t it) {
    int uS[it], last;
    uint8_t j, i = 0;
    uS[0] = NO_ECHO;
    while (i < it) {
        last = ping();           // Send ping.
        if (last == NO_ECHO) {   // Out of range ping.
            it--;                // Skip, don't include as part of median.
            last = _maxEchoTime; // Adjust "last" variable so delay is correct length.
        } else {                       // Ping in range, include as part of median.
            if (i > 0)                 // Don't start sort till second ping.
                for (j = i; j > 0 && uS[j - 1] < last; j--) // Insertion sort loop.
                    uS[j] = uS[j - 1]; // Shift ping array to correct position for sort insertion.
            else j = 0;                // First ping is starting point for sort.
            uS[j] = last;              // Add last ping to array in sorted position.
            i++;                       // Move to next ping.
        }
        if (i < it -1) delay(PING_MEDIAN_DELAY - (last >> 10)); // Millisecond delay between pings.
    }
    return (uS[it >> 1]); // Return the ping distance median.
}

This saves 112 bytes of complied code size so it's worth considering. I could add both a ping_median() and ping_mode() but I feel that's a little excessive. Any statistic types out there may want to chime in on if a pure median calculation is good enough in removing digital noise. Or, if the addition of the truncated mean with removed +/-2cm outliners is worth the 112 byte loss. Median and truncated mean yield different cm results about 10% of the time, but which one is better or more correct? When they return two different values there's not one value that's clearly more accurate more consistently as far as I can tell.

I'm leaning towards the above ping_median method as a digital filter. It's 112 bytes smaller and seemingly just as effective (albeit sometimes different).

Tim

NewPing v1.5 released, here's what's new:

Added ping_median() method which does a user specified number of pings (default=5) and returns the median ping in microseconds (out of range pings ignored). This is a very effective digital filter. Optimized for smaller compiled size (even smaller than sketches that don't use a library).

Download NewPing v1.5

Tim

By the way, I went with just the median method for digital filtering. It's fast, the overhead is much smaller, and the results were just as effective as finding the mean of a centered sample. Using ping_median() is just as simple as using ping() too. There's nothing to figure out, just change ping() to ping_median() in your sketch and it will automatically do 5 pings and return the median ping. To specify a different number of samples, just specify the number of iterations you want like this: ping_median(9). Here's a sample sketch that uses ping_median:

#include 

#define TRIGGER_PIN  12  // Arduino pin tied to trigger pin on the ultrasonic sensor.
#define ECHO_PIN     11  // Arduino pin tied to echo pin on the ultrasonic sensor.
#define MAX_DISTANCE 200 // Maximum distance we want to ping for (in centimeters). Maximum sensor distance is rated at 400-500cm.

NewPing sonar(TRIGGER_PIN, ECHO_PIN, MAX_DISTANCE); // NewPing setup of pins and maximum distance.

void setup() {
  Serial.begin(115200); // Open serial monitor at 115200 baud to see ping results.
}

void loop() {
  delay(50); // Wait 50ms between pings (about 20 pings/sec). 29ms should be the shortest delay between pings.
  Serial.print(sonar.ping_median(3) / US_ROUNDTRIP_CM); // Send 3 pins and return the median ping converted to centimeters.
  Serial.println("cm");
}

Enjoy!

Wanted to say thanks for the hard work. Really appreciate this library!

Fantastic job with this new library!

I am especially liking the median feature of v1.5 to eliminate rogue results, but how can this be integrated into the 15 sensor example sketch which does not use ping() but sonar.ping_timer(function) instead?

fairorgan: Fantastic job with this new library!

I am especially liking the median feature of v1.5 to eliminate rogue results, but how can this be integrated into the 15 sensor example sketch which does not use ping() but sonar.ping_timer(function) instead?

To actually combine the two would be simple. However, in the process it would lose the interrupt aspect of the ping_timer() method. I've been toying with the idea, and considering new ways of doing it. One thing I've considered is a mass ping of all sensors at the same time followed by interrupt waiting for the ping echo. With 15 sensors there could be crazy cross-talk. But with 4 sensors it would probably work.

Also consider that with 15 sensors, if it were to poll each sensor 3 times it would take 3 times as long. Not sure how much processing time you have left after pinging 15 sensors. But, multiply the time it takes by 3 or 5 and you may no longer have time to do anything else, or the pings are just too slow to sense something in time if your robot is moving at a high enough speed.

Tim

Hi Tim,

I was actually only thinking of using two SR-04 sensors (at most three), but like your use of the array in the 15 sensor example for holding the data. Would I be better just duplicating the new ping_median once per sensor and not use an array nor interrupt method?

My project: I am controlling a robot’s head position via servos, which currently have either remote control (via joystick) or automatic random positioning of x and y axis. I would like to add the ability for close object tracking of the x axis, so thought the best way would be two ultrasonic sensors pointing slightly apart concealed on the head itself, and in the event of an object coming within say a couple of metres range, the head will stop random movements and track the object in x axis by moving the servo to keep the signal from both ultrasonic sensors the same, until either the object goes out of range or the robot gets bored.

Nick

First of all, thanks for the great job you are doing!

Now, i wanted to comment two issues i am having.

First one is the "0" thing. I still get it too often, but i have found some "logic" sequence on it.

Here are some results:

210 210
211 211
210 210
211 211
0 211
0 211 211
0 0
210 211
210 210
211 210
210 210
210 210
0 210
0 210 210
0 0
210 0
210 0 210
210 210
210 210
210 211
0 211
0 211 211
0 0
209 0
209 0 209
209 209
210 211
210 210
210 210
0 210
0 210 210
0 0
210 211
210 210
210 0
210 0 210
211 210
0 211
0 211 211
0 0
210 213
210 210
211 210
210 210
211 211
0 210
0 210 210
188 0
188 0 188
210 0
210 0 210
210 210
210 210
210 209
209 210
0 210
0 210 210
0 0
210 0
210 0 210
210 209
210 210
210 211
0 210
0 210 210
0 0
209 211
210 210
210 210
210 210
210 211
0 210
0 210 210
0 0

First and second colums are read values, and third is just the difference between first and second, so can be ignored. There seems to be an issue that affects after X cycles and makes it return value "0".

Here is the code that i am running:

#include 

NewPing sonar1(2, 3, 400); // Sensor 1: trigger pin, echo pin, maximum distance in cm


#define pingSpeed 35 // Ping frequency (in milliseconds), fastest we should ping is about 35ms per sensor, 100ms is default


void setup() {
  Serial.begin(115200);

}

void loop() {

   unsigned int current_distance1 = sonar1.convert_cm(sonar1.ping());
   delay(100);
   unsigned int current_distance2 = sonar1.convert_cm(sonar1.ping());
         Serial.print(current_distance1);
      Serial.print(" ");
      Serial.println(current_distance2);

   if (current_distance1 > current_distance2) {
    unsigned int result = (current_distance1 - current_distance2);
    if (result > 10){
      Serial.print(current_distance1);
      Serial.print(" ");
      Serial.print(current_distance2);
      Serial.print(" ");
      Serial.println(result);

 }
   }
   if (current_distance1 < current_distance2) {
    unsigned int result = (current_distance2 - current_distance1);
    if (result > 10){
      Serial.print(current_distance1);
      Serial.print(" ");
      Serial.print(current_distance2);
      Serial.print(" ");
      Serial.println(result);

 }
   }
   delay(29);
}

Maybe it helps you solve this issue.

Second thing, is that after a determined number of cycles, the board stops working. I mean, it just freezes. It might take something like 4 minutes or so. Im not too sure about what the reason is, but just wanted to comment it.

This code is being run on an UNO with an SRF05.

BR

I have some very good news for ppl with so many "0".

The issue is not the library (NewPing). The issue are the connections between the sensor and the board. I just found out that, if i hold tight the connections in the sensor with my hand, i get these results:

38 54
38 54 16
34 196
34 196 162
199 198
177 197
177 197 20
172 195
172 195 23
171 171
195 170
195 170 25
109 59
109 59 50
96 46
96 46 50
58 75
58 75 17
55 195
55 195 140
196 83
196 83 113
176 0
176 0 176
171 175
169 195
169 195 26
195 169
195 169 26
194 61
194 61 133
90 137
90 137 47
196 57
196 57 139
111 61
111 61 50

With same code as posted in post above this one, moving the sensor all around my room.

So, hope this helps!

BR

Your code works fine for me. I guess a loose connection could the the source of the problem. But, it could also be that you're trying to get a ping that's just too far away. These sensors really don't work all the way out to 400 or 500cm. They seem to work okay to about 200cm for most, but I have some sensors that won't read anything consistently beyond around 75cm. This is the fault of the sensors, not the library as the same thing happens with other libraries or with no library at all. Try putting something 50cm away from the sensor and see what happens then. It's just suspicious that it stops working around 200cm and then in your next test all the results are within 200cm. Maybe it would work without you holding the sensor if all the readings were <50cm? But, if it is a lose connection, that could also be why it's crashing after a few minutes, it's simply shorting out. BTW, I've never got a zero reading that wasn't for a logical reason and I've never had any of my Arduino or Teensy 2.0 crash while using the NewPing library. And, I've sometimes ran them for several hours or even overnight without a single issue.

Tim

teckel: Your code works fine for me. I guess a loose connection could the the source of the problem. But, it could also be that you're trying to get a ping that's just too far away. These sensors really don't work all the way out to 400 or 500cm. They seem to work okay to about 200cm for most, but I have some sensors that won't read anything consistently beyond around 75cm. This is the fault of the sensors, not the library as the same thing happens with other libraries or with no library at all. Try putting something 50cm away from the sensor and see what happens then. It's just suspicious that it stops working around 200cm and then in your next test all the results are within 200cm. Maybe it would work without you holding the sensor if all the readings were <50cm? But, if it is a lose connection, that could also be why it's crashing after a few minutes, it's simply shorting out. BTW, I've never got a zero reading that wasn't for a logical reason and I've never had any of my Arduino or Teensy 2.0 crash while using the NewPing library. And, I've sometimes ran them for several hours or even overnight without a single issue.

Tim

Yeah, as i say, it was all due to loose connections. I just got everything soldered, and now i get 0 only when its out of range. Both tests you see above where done in the same room, where i should get no zeros at all since its pretty small.

I guess the "arduino stops working" would be due to some data generated by the loose connection that would cause an overflow.

Everything working pretty much perfect right now!

BR

fairorgan: I was actually only thinking of using two SR-04 sensors (at most three), but like your use of the array in the 15 sensor example for holding the data. Would I be better just duplicating the new ping_median once per sensor and not use an array nor interrupt method?

My project: I am controlling a robot’s head position via servos, which currently have either remote control (via joystick) or automatic random positioning of x and y axis. I would like to add the ability for close object tracking of the x axis, so thought the best way would be two ultrasonic sensors pointing slightly apart concealed on the head itself, and in the event of an object coming within say a couple of metres range, the head will stop random movements and track the object in x axis by moving the servo to keep the signal from both ultrasonic sensors the same, until either the object goes out of range or the robot gets bored.

If you just wanted to use an array to store the ping results, you could easily modify the 15 sensor sketch to work with ping_median instead of ping_timer. The problem is that the sketch would no longer be interrupt driven, and would be waiting for ping echos instead of doing anything else (like positioning the servo on the robot or doing probably much of anything else). For example, if you had 3 sensors and each sensor would take 5 samples to find the median that would be a total of 15 pings with at least a 29ms delay between each or a total of around 435ms (about 1/2 second) every ping cycle where your robot can do nothing but read the sensors. Not at all interrupt driven like the 15 sensor sketch where very little time is wasted monitoring the pings and you're free to do other things what seems like the same time.

A better solution is to continue to use the 15 sensor sketch as a base, but use oversampling to read the sensors multiple times, then average the results. Here's an example sketch:

#include 

#define SENSORS        3 // Number or sensors.
#define ITERATIONS     5 // Iterations for each sensor.
#define PING_NUM ITERATIONS * SENSORS // Calculates the number of pings.
#define MAX_DISTANCE 200 // Maximum distance (in cm) to ping.
#define PING_INTERVAL 29 // Milliseconds between sensor pings (29ms is about the min to avoid cross-sensor echo).

unsigned long pingTimer[PING_NUM]; // Holds the times when the next ping should happen for each sensor.
unsigned int cm[PING_NUM];         // Where the ping distances are stored.
uint8_t currentSensor = 0;         // Keeps track of which sensor is active.

NewPing sonar[PING_NUM] = {      // Sensor object array (Each sensor's trigger pin, echo pin, and max distance to ping).
  NewPing(12, 11, MAX_DISTANCE), // Sensor 0, sample 1
  NewPing(2, 3, MAX_DISTANCE),   // Sensor 1, sample 1
  NewPing(4, 5, MAX_DISTANCE),   // Sensor 2, sample 1
  NewPing(12, 11, MAX_DISTANCE), // Sensor 0, sample 2
  NewPing(2, 3, MAX_DISTANCE),   // Sensor 1, sample 2
  NewPing(4, 5, MAX_DISTANCE),   // Sensor 2, sample 2
  NewPing(12, 11, MAX_DISTANCE), // Sensor 0, sample 3
  NewPing(2, 3, MAX_DISTANCE),   // Sensor 1, sample 3
  NewPing(4, 5, MAX_DISTANCE),   // Sensor 2, sample 3
  NewPing(12, 11, MAX_DISTANCE), // Sensor 0, sample 4
  NewPing(2, 3, MAX_DISTANCE),   // Sensor 1, sample 4
  NewPing(4, 5, MAX_DISTANCE),   // Sensor 2, sample 4
  NewPing(12, 11, MAX_DISTANCE), // Sensor 0, sample 5
  NewPing(2, 3, MAX_DISTANCE),   // Sensor 1, sample 5
  NewPing(4, 5, MAX_DISTANCE)    // Sensor 2, sample 5
};

void setup() {
  Serial.begin(115200);
  pingTimer[0] = millis() + 75;          // First ping starts at 75ms, gives time for the Arduino to chill before starting.
  for (uint8_t i = 1; i < PING_NUM; i++) // Set the starting time for each sensor.
    pingTimer[i] = pingTimer[i - 1] + PING_INTERVAL;
}

void loop() {
  for (uint8_t i = 0; i < PING_NUM; i++) { // Loop through all the sensors.
    if (millis() >= pingTimer[i]) {        // Is it this sensor's time to ping?
      pingTimer[i] += PING_INTERVAL * PING_NUM;   // Set next time this sensor will be pinged.
      if (i == 0 && currentSensor == PING_NUM - 1) oneSensorCycle(); // Sensor ping cycle complete, do something with the results.
      sonar[currentSensor].timer_stop();          // Make sure previous timer is canceled before starting a new ping (insurance).
      currentSensor = i;                          // Sensor being accessed.
      cm[currentSensor] = NO_ECHO;                // Make distance NO_ECHO in case there's no ping echo for this sensor.
      sonar[currentSensor].ping_timer(echoCheck); // Do the ping (processing continues, interrupt will call echoCheck to look for echo).
    }
  }
  // The rest of your code would go here.
}

void echoCheck() { // If ping received, set the sensor distance to array.
  if (sonar[currentSensor].check_timer())
    cm[currentSensor] = sonar[currentSensor].ping_result / US_ROUNDTRIP_CM;
}

void oneSensorCycle() { // Sensor ping cycle complete, do something with the results.
  unsigned int uS[ITERATIONS], last;
  uint8_t i, j, x, it, ii;
  for (x = 0; x < SENSORS; x++) {
    uS[0] = NO_ECHO;
    it = ITERATIONS;
    ii = 0;
    for (i = 0; i < ITERATIONS; i++) {
      last = cm[x + (i * SENSORS)];
      if (last != NO_ECHO) {   // Ping in range, include as part of median.
        if (i > 0) {           // Don't start sort till second ping.
          for (j = ii; j > 0 && uS[j - 1] < last; j--) // Insertion sort loop.
            uS[j] = uS[j - 1]; // Shift ping array to correct position for sort insertion.
        } else j = 0;          // First ping is starting point for sort.
        uS[j] = last;          // Add last ping to array in sorted position.
        ii++;                  // Next ping in insertion sort array.
      } else it--;             // Ping out of range, skip and don't include as part of median.
    }
    Serial.print(x);
    Serial.print("=");
    Serial.print(uS[it >> 1]);
    Serial.print("cm ");
  }
  Serial.println();
}

The above sketch is 3 sensors with 5 times oversampling. Instead of just printing the results in the oneSensorCycle function, it does the same insertion sort/median calculation as NewPing does with the ping_median method. It pings each sensor (0,1,2) then repeats 5 times. For example, the ping order is: 0,1,2,0,1,2,0,1,2,0,1,2,0,1,2. This sketch will also accommodate different oversampling (iterations). Just change the ITERATIONS define and sonar array to match.

The advantage is that this sketch is totally interrupt driven. So, the ATmega has all kinds of time to do other things. While pinging, every 24uS it checks for a ping echo, but the rest of the time your sketch can do other things (like rotate a servo, move, etc). Once a full 3 sensor, 5 oversample pass is made, it goes to the oneSensorCycle function where the median values for the 3 sensors are calculated, which is very quick and only takes about 30uS. Basically, the ATmega is free to do other things at the same time it's pinging. Just be sure to not use any delay statements in your sketches!

Tim

fairorgan got me to thinking about exactly how much processing time is spent using the ping_timer method. Which also helps to understand what else your sketch could be doing in the remaining time. So, I wrote a sketch to tests this. The sketch runs for 30 seconds and logs how much time is spent both initiating the ping and checking for the return echo. It's not perfect, as there's some overhead in doing these calculations making these estimates a little on the high side. Anyway, here's the results with pings happening every 29ms.

5cm pings = <1% load 20cm pings = 2% load 70cm pings = 7% load 125cm pings = 12% load 200cm pings = 19% load 500cm pings = 45% load

When reducing the maximum sensor distance value when calling the NewPing constructor, you can greatly reduce the maximum ping_timer load. For example, if you leave it at the default of 500cm, using ping_timer every 29ms will use up to 45% processor load. However, lowering the max distance to 200cm means the maximum load can be only 19%, or lowering it to 50cm makes the max load only 5%. So, setting a maximum distance to the longest distance you need will greatly free up the ATmega to do other parts of your sketch.

The ping frequency also effects the processor load. The above numbers were all done at a 29ms ping rate. However, if the ping rate is every 100ms, the load is drastically reduced to only 5.5% (down from 19%). 200cm maximum distance and pinging every 50ms uses a maximum of 11% processor load.

So, when creating an interrupt driven sketch and using NewPing, setting the maximum sensor distance and ping rate to what you really need for your project will greatly effect how much processing time you have available to do other parts of your sketch. Doing a maximum ping rate of 29ms and leaving the default maximum distance at 500cm will use almost half of the ATmega processing time (45%). If your project really only needs to sense out to 200cm and pinging once every 100ms is all the faster you really need, you've lowered the processing time down to only 5.5% (almost 1/10th the load).

Keep in mind that these are maximum load amounts. If the a ping echo is sensed at a shorter distance, the load can be greatly reduced. For example, the 45% load for a max distance of 500cm at a 29ms ping rate is only if there's a NO_ECHO (zero value). If under the same settings there's a constant ping echo at 10cm, the load is only 1%. So, these are truly worse case numbers. But, to avoid your sketch to be sluggish at times, you should consider the worst case scenario.

Tim

Thinking... Instead of measuring the time of a reflection returned from an object. What if we had 2 devices synchronized by GPS or an accurate clock. One sending the ping, the other receiving it. If we can measure the delay within 1-2us that would be accurate. Roughly 0.01". Obviously that is not possible because of the physics of a sound wave.

What is the theoretical limit using the frequency of the ultrasonic sensors in this configuration? It's the same as the normal method. Let's use 100khz as an example. Each cycle is 10us. So 1/10"? Certainly <1/2". Averaging multiple readings.

Wouldn't this greatly increase the range compared to using only 1 device?

You could send a ping every 2ms once you know the rough distance. Then you'd have 500 measurements to average in only 1 sec, regardless of the distance.

Obviously the limitation would not be the accuracy of the PPS clock, since GPS is +-10ns. 10ns is about 10ft at the speed of LIGHT! We'll leave that for another LightPing Library...

How can I use this method with the receiver in a Quadcopter? The problem is the sound from the motors and blades. The obvious answer is to put the transmitter in the air, the receiver on the ground where you want to land, then Xbee to send the data. This works much better, with a huge increase in range, compared to reflecting from the ground when it's grass.

Thoughts?

sbright33: Thinking... Instead of measuring the time of a reflection returned from an object. What if we had 2 devices synchronized by GPS or an accurate clock. One sending the ping, the other receiving it. If we can measure the delay within 1-2us that would be accurate. Roughly 0.01". Obviously that is not possible because of the physics of a sound wave.

What is the theoretical limit using the frequency of the ultrasonic sensors in this configuration? It's the same as the normal method. Let's use 100khz as an example. Each cycle is 10us. So 1/10"? Certainly <1/2". Averaging multiple readings.

Wouldn't this greatly increase the range compared to using only 1 device?

You could send a ping every 2ms once you know the rough distance. Then you'd have 500 measurements to average in only 1 sec, regardless of the distance.

Obviously the limitation would not be the accuracy of the PPS clock, since GPS is +-10ns. 10ns is about 10ft at the speed of LIGHT! We'll leave that for another LightPing Library...

How can I use this method with the receiver in a Quadcopter? The problem is the sound from the motors and blades. The obvious answer is to put the transmitter in the air, the receiver on the ground where you want to land, then Xbee to send the data. This works much better, with a huge increase in range, compared to reflecting from the ground when it's grass.

Thoughts?

The current ping sensors I've found can't ping that quickly. I'm finding some ping sensors now that can't really ping faster than about 48-49ms, and none can faster than once every 29-32ms. Basically, they initiate a ping, then you can't initiate another ping till some set time has passed.

Also, when you initiate a ping they take a little while before they actually start (around 450uS). This varies greatly from device to device. My library waits for the sensor to be ready so an accurate measurement is taken.

Finally, when sending a large number of pings, there's no way the receiving sensor would be able to know when the ping that arrived was sent. The only way that would be possible is if the ping itself actually contained a timestamp.

Without some kind of time stamp on the ping itself, very accurately synchronized sensors, and custom built sensors that would ping on command, it's not really possible. There are probably easier ways of doing it, albeit probably just as expensive.

Tim

There must be a way to SEND a ping every 5ms. Once you know the rough distance, there’s no need to put a timestamp on each ping. You already know the rough distance, hence how many integer ms it takes. When you send them at 2-5ms intervals you’re now trying to measure the 100us interval. 0.1ms which you add to the integer rough distance you already found. 0.1ms or 0.05ms can be obtained by averaging many samples. Forget about all this for a minute, it’s not my main point. Let’s just use your 50ms interval with no timestamps.

Wouldn’t the distance or range increase greatly with 2 devices, sender and receiver in sync?

sbright33: There must be a way to SEND a ping every 5ms. Once you know the rough distance, there's no need to put a timestamp on each ping. You already know the rough distance, hence how many integer ms it takes. When you send them at 2-5ms intervals you're now trying to measure the 100us interval. 0.1ms which you add to the integer rough distance you already found. 0.1ms or 0.05ms can be obtained by averaging many samples. Forget about all this for a minute, it's not my main point. Let's just use your 50ms interval with no timestamps.

Wouldn't the distance or range increase greatly with 2 devices, sender and receiver in sync?

Around double the distance. So maybe up to 10 meters for very inexpensive SR04 type sensors. There are commercial sensors that have a range of up to 50 feet, so using two sensors that range could be as high as 100 feet.

In the case of an Quadcopter, you would need to have two synchronized timers. I'm not sure if this is the environment you're really considering, but you wouldn't need synchronized timers if you could tie the send and receive sensors to the same Arduino. There could be an issue with wire length, however.

I still think you'd need a time stamp, unless you're only trying to measure acceleration, which may be possible as the distance between the pings would vary due to acceleration.

Tim

Interesting discussion Tim. I disagree about timestamps. Here's how to synchronize 2 Uno's to 2us. As you can see they stay in sync even without GPS once initialized. http://arduino.cc/forum/index.php/topic,120288.0.html

sbright33: Interesting discussion Tim. I disagree about timestamps. Here's how to synchronize 2 Uno's to 2us. As you can see they stay in sync even without GPS once initialized. http://arduino.cc/forum/index.php/topic,120288.0.html

Synchronizing two are not the issue, it's knowing when the ping was sent so you know how long it took to get there. How do you know WHICH ping you're getting? Without knowing when the ping was sent, how can you possibly know how long it took to get there?

Let's say that you've synced two ATmega's to within 15 microseconds (that accuracy would be plenty). One starts sending out pings every 2 millisecond which the other would receive. Even though they're synced, how does the receiving unit know when that ping was sent to be able to calculate the time and therefore the distance? Without the start time reference, I can't see how you would measure distance. For the sake of argument, the receiving unit gets the ping, what values are you using to calculate for distance? You have receive ping time, so what's the start time? You need both start and end time to calculate the time it took to send the ping. You know the pings are 2ms apart, but where do you get the start time?

You could measure acceleration/deceleration by measuring the time between the pings. But, that wouldn't need the two units to be time synced anyway, and would do nothing to measure distance. Only "hotter" or "colder" would be known.

Tim