Servo - June 2008

Building a Sonar System

SCREENSHOT 1. This is a look at a LV-MaxSonar-EZ0 ranging pulse that is emitted from the ultrasonic rangefinder’s PW pin. This pulse happens to be 9562.3 μS wide. With 147 μS representing one inch, this pulse equates to 65.0496 inches to the target, which happens to be the coffee tabletop to ceiling distance.

is ready to process input on its

RX pin. Recall that if the RX pin is left open or forced logically high, the ranging process will begin and continue until the RX pin is pulled logically low. With that, let’s run a scenario with the RX pin open at power-up plus 250 ms.

With the RX pin open or forced logically high following the setup time, the first ranging operation will be a 49 ms calibration cycle. The next ranging operation will be the first ranging operation that will report ranging data to the I/O interface. Thus, the very first ranging data will appear at the EZ0’s data portal 98 ms past the 250 ms power-up setup period. All subsequent ranging operations will consume 49 ms each. What this all means is that the LV-MaxSonar-EZ0 can perform a ranging operation every 49 ms. If your application requires control of the ranging process, the EZ0 will scan the RX line at the end of every ranging cycle. This allows you to force the RX I/O pin logically low and take control of the scheduling of the subsequent ranging cycles.

Each ranging cycle is initiated by a logical high level at the RX I/O pin. Thirteen 42 kHz waves are transmitted at the beginning of a ranging cycle. After the 13 waves have been sent, the PW I/O pin is pulled to a logically high level. The PW pin will go logically low when a target object is detected. As you would expect, the maximum length of a PW pulse is 37. 5 ms, or the equivalent of just over 255 inches. The maximum ranging distance of the LV-MaxSonar-EZ0 is 254 inches.

The 37. 5 ms pulse width will occur when no target objects are detected. Assuming we didn’t detect a target object, we still have 11. 5 ms of time left in the ranging cycle; 6. 8 ms of the remaining ranging cycle time is used to adjust the analog voltage that will appear on the AN pin to the correct level. We still have 4. 7 ms left. At this point, the EZ0 has presented its pulse width ranging data and its analog ranging data to the I/O interface. The RS-232 ranging data is all that’s left to present. The serial ranging data is sent during the final 4. 7 ms of the ranging cycle.

To guarantee successful ranging operations, all we have to do is make sure that there are no targets closer than seven inches to the ultrasonic rangefinder during its calibration time. Also, the EZ0 is not an outdoor cat. So,

we must be sure to keep it out of harm’s way as far as weather goes. We now have enough information to begin writing some interface code. I’m going to write the EZ0 driver in C using HI-TECH PICC- 18 and I’ll target the PIC18F2620. Before we start writing code and assembling hardware, we can use my CleverScope to check out the LV-MaxSonar-EZ0’s pulse ranging data mechanism.

Ranging with a CleverScope

Let’s use our human eyes to interpret the EZ0 ranging pulse I captured in Screenshot 1. The pulse width as measured by the CleverScope extents is 9562.3 μs. A bit of simple math will yield the distance from my coffee tabletop to the ceiling:

9562.3 μs/147 μs per inch = 65.0496 inches

This is an easy way to receive instant gratification from a ultrasonic rangefinder. However, to make the ranging information work for us, we must employ the resources of a microcontroller. My rangefinder support hardware is visually obtainable in Schematic 2. As you can see, I’ve tied the LV-MaxSonar ultrasonic rangefinder’s PW output pin to the PIC18F2620’s CCP1 capture pin.

Ranging with a PIC18F2620

We need to electronically measure the pulse width presented to the PIC18F2620’s CCP1 capture input. The algorithm is simple and so is the code. We must set up the PIC18F2620 capture engine to trigger an interrupt on the rising edge of the PW ranging signal. Meanwhile, TIMER1 is running free with a period of 1 μs. Thus, a count is supplied to the CCP1 holding registers every microsecond. The