Monthly Archives: May 2017

Charging Station System Integration – New Sunshade Testing

Posted 19 May 2017

While working with John Jenkins on the modulated IR beam idea, I decided to run some tests with the current 4-detector design, to see how the new sunshade with center divider was affected by ambient IR on a bright sunny day.  So, I disabled Wall-E2’s motors and then placed it at several different critical spots in the entry hallway.  At each location I used my little IR test generator to mark the beginning and the end of the test for that location, and then moved on to the  next one.  The locations are shown in the following photos, in the order that the tests were run.

Position 1: Near where Wall-E2 transitions from wall-tracking to IR beam homing

Position 2: This is where Wall-E2 has been winding up when it homes on the outside sunlight instead of the IR beam

Position 3: Here Wall-E2 should be firmly fixated on the IR beam

These results, combined with my earlier IR response tests with a single phototransistor are encouraging, because it is clear that at least in this case, Wall-E2 should have no difficulty discriminating between the ambient IR and the charging station IR beam.

I ran some homing tests, which Wall-E2 handled with ease; unfortunately God had already turned the lights out on this side of the world, so the tests weren’t in the presence of daylight IR interference.  I’ll do some more real-world discrimination testing tomorrow, and I am hopeful that the new sunshade-with-divider version will be successful, at least for this part of the house.

Stay tuned,

Frank

 

 

IR Light Follower for Wall-E2, Part XI – Center Divider Investigation

Posted 16 May 2017

One of the suggestions John Jenkins made during his visit, in addition to the idea of modulating the IR beam to suppress ‘flooding’ from ambient IR sources, was the idea of placing an opaque divider between the left and right halves of the detector array.  He thought that I might even be able to reduced the detector array from four to two phototransistors and still get good homing performance, assuming that each of the two detectors had sufficiently wide beamwidths to accommodate off-axis IR beam intercepts.  The current detectors have a +/- 12º beamwidth, but are arrayed in such a way as to provide well over 60º aggregate coverage.  To do the same thing with just two detectors would require parts with considerably wider beamwidths.  the TAOS TSL267 (another one of JJ’s suggestions) has an approximately +/-30º beamwidth at the half-response points, so they seem almost ideally suited for this application.  As a major added bonus, the TAOS parts feature a photo-diode integrated with an op-amp to address the dynamic range issue mentioned by John.  The diode operates in its linear range, and the op-amp amplifies the IR signal to useful levels.  The only fly in the ointment is that the op-amp gain isn’t adjustable, and its output limits at fairly low light levels – bummer!

In order to investigate the opaque divider idea, I decided to run some bench angular response tests using my robot’s 4-detector array with and without a center divider.  I printed out a copy of the compass rose graphic I had hanging around from my magnetometer project (one of my more spectacular failures) and set up a bench test with Wall-E2 and my little IR test source, as shown below.

Angular response test setup

Closeup of the ‘sunshade’ cowling (black rectangular opening) around the 4-detector array

Closeup of sunshade with divider installed

The results without the center divider were about as expected, with about +/- 45º coverage, as shown in the Excel plot below

Response vs angle for 4-detector array, without center divider

+/- 60 deg response vs angle for 4-detector array, without center divider

With the divider, the response appears to be about the same (note here that the ‘with divider’ response is flipped left/right from the ‘no divider’ case – oops!)

+/- 30 deg response vs angle for 4-detector array, without center divider

Note in the above ‘detail’ view that the response curves for the two center detectors (DET2 & DET3) seem to be very symmetric about the 0º point, as expected.  What this also shows is that just these two detectors could probably be used for homing, if off-axis beam detection weren’t a consideration.

+/- 30 deg response vs angle for 4-detector array, with center divider

The ‘with divider’ plot above shows a significant difference from the ‘no divider’ plot, right at the center.  In the ‘no divider case, the DET1 & DET2 responses are very nearly the same, but in the ‘with divider’ case they are significantly different, and almost perfectly anti-symmetric about the center.  This should allow more precise detection of small left/right deviations from the beam centerline, and therefore more precise homing.

Stay tuned!

Frank

 

 

IR Phototransistor Sensitivity/Dynamic Range Study

Posted 05/14/17

In a previous post, I mentioned that John Jenkins, mentor and old friend, had some ideas regarding my Wall-E2 robot’s problems with homing in on an  IR beam in the presence of ambient IR sources like overhead incandescent lighting and/or sunlight streaming in through windows and doors.   John pointed out that this was really a system dynamic range issue, and it was likely that, as currently configured, the IR phototransistors were running out of dynamic range (saturating) well before the 10-bit A/D’s on the AtMega SBC.  In order to get the detector sensitivity up to the  point where Wall-E2 could ‘see’ the IR beam far enough (1.5-2m) away to avoid hanging up on the lead-in rails (see this post and this post for details), I had to use a very high value collector resistor (330K), which reduces the dynamic range significantly.

In a subsequent email conversation, John suggested that the proper way to handle this problem was to reduce the collector resistor to the point where the detector doesn’t saturate under the worst case ambient IR conditions, and then add amplification as necessary after the detector stage to get the required sensitivity.  John’s point was that as long as the detector response is relatively linear (i.e., it’s not saturated), then there shouldn’t be any loss of information through the stage, so a later linear amplification stage will allow the desired IR signal to be detected/processed even in the presence of interference.  However, if the detector stage saturates due to interference, then it’s basically ‘game-over’ in terms of the ability to later pull the desired signal out of the noise.

This wasn’t exactly what I wanted to hear, as adding the required post-detector amplification stage wasn’t going to be particularly easy – there’s not much left in the way of free real-estate on Wall-E2’s main platform (there’s plenty of room on the second level, but putting stuff there requires inter-level cabling and is a major PITA).  As I was thinking this, I had a flashback to similar conversations with John from 40-45 years ago, when we were both design engineers in a USG design lab; the usual outcome of such ‘conversations’ (to the uninitiated these might be mistaken for shouting matches) was that my circuit got ‘simplified’ by the addition of 50% more parts – but could then withstand a nuclear attack in the middle of a snowstorm at the South Pole!

Anyway, back in the present, John suggested I run some experiments designed to determine the ratio of IR field intensity to collector current for a the phototransistors I was using, so the proper collector resistor value to be computed to utilize the available detector dynamic range without driving it into saturation.  His suggestion was to not  use a collector resistor at all, but to simply connect the collector to +VDC through a current meter, and then expose the detector to worst-case ambient conditions.  I didn’t particularly like this idea, because I only have a manual non-recording multimeter, and I wanted to record the data for later analysis.  So, I decided to program up one of my small SBC’s with analog input capability (in this case, a Pololu Wixel), and set up to measure the voltage drop across a 10 Ω precision resistor in the collector circuit.  The hardware setup is shown in the following photo

Hardware setup for IR Phototransistor Response Experiment

Before and after collecting ‘field’ data, I made a collection run on the bench, using my IR LED test box, with the following results.  As can be seen in the plot, the maximum voltage drop across the 10Ω resistor was approximately 200mV, or about 20mA.

IR detector response bench test. Nose-to-nose with my IR test source

Bench test with IR LED test generator

 

To collect the ‘field’ data, I placed the SBC in different locations around the house, and recorded A/D values.  For these experiments I was using an example Wixel app that printed out the A/D values for all 6 analog inputs, scaled such that the maximum reading was equivalent to the SBC board voltage (3.3V) in millivolts.  In other words, the maximum A/D reading was approximately 3375, or about 3 mV/bit

Location 1: Looking out glass doors on south side of house

Location 2: Direct sunlight through window on south side of house

Location 3: Kitchen floor, looking toward entrance hall and atrium

Location 4: Entry hallway, looking toward atrium. This is the usual starting location for charging station homing tests

Location 5: Entry hallway near door to atrium

These results were not at all what I expected.  Either there is something wrong with the experimental setup, or the ambient light IR field intensity isn’t anywhere near as strong as expected.  If the data can be believed, the ambient conditions are significantly weaker than the bench-test conditions, which were basically nose-to-nose with my IR test LED.  So, if my planned double-check of the hardware doesn’t find any problems, then I’ll change the resistor value from 10Ω to 100Ω and repeat the tests.

17 May 2017 Update

After thoroughly reviewing the hardware setup, I concluded that everything was working properly, but that the 10Ω load resistor simply didn’t provide sufficient drop for reliable measurements.  So, I changed out the 10Ω resistor for 100Ω, and repeated the tests, with the following changes:

  • As before, I started the run by performing a ‘nose-to-nose’ test to verify proper operation, but this time I measured the detector current using a multimeter. The  result was about 15mA maximum current.
  • At each of the 5 locations, I started with a short nose-to-nose section (not shown in the plots) to make sure the detector was operating properly
  • At location 2 (the direct sunlight location), I physically oriented the detector for maximum response.
  • After location 5, I placed the detector on the charging station’s IR beam reflector boresight at about 0.5m distance, and physically oriented the detector for max response.
  • I calculated the detector current for each reading, and plotted that rather than the raw reading.  To calculate the detector current for each measurement, I subtracted the detector reading from the corresponding 3.3VDC supply voltage measurement and divided by 100.

The results of these tests are shown in the plots below:

 

These results are pretty interesting.  As I’m sure John would point out, the direct sunlight response of about 0.2 mA is about 20 times the value required to saturate the phototransistor with the current 330KΩ.  No wonder I was having interference problems – oops!

Stay tuned!

Frank

 

 

 

 

 

 

Charging Station Design, Part XIV – Modulated IR Beam Study

Posted May 10, 2017

A couple of weeks ago my old friend and mentor John Jenkins was visiting the area and stopped in for a couple of days.  Naturally I had to show him my Wall-E2 robot, complete with autonomous charging capability.  Just as naturally (if you believe in Murphy’s law), Wall-E2 refused to cooperate.  Instead of homing in on the charging station, it blissfully homed in on the sunlight streaming in through the atrium, bypassing the charging station entirely!

After John got through laughing, he mentioned that he knew of some other IR-following robotics projects where the designers used a modulated IR beam to allow the robot to discriminate between general IR background noise and the intended signal.  So, I decided to start a feasibility study to see if I could incorporate some sort of modulation into the design of the charging station, without completely overtaxing either the charging station, or the robot, or both!

I started with the idea that I could easily add square wave modulation to the IR beam by simply switching the LED off & on at some reasonable rate (say, 500Hz).  The problem occurs on the other end (the robot), as it will somehow have to a) detect the square wave signal, and b) still have the ability to home in on the modulated signal – i.e. still be able to determine signal variation across the four photo-transistor array.

To investigate,  I created a square wave generator using an Adafruit Trinket, and a square wave receiver using an Arduino Uno, as shown in the following photo

Adafruit Trinket SBC used for generating the square wave signal, and an Arduino UNO used as the receiver

Square wave signal from the Trinket

The first step was to see if the Arduino Uno was fast enough to accurately detect the square wave pattern, so I simply grabbed 100 samples from the input pin using digitalRead() with a delay of about 1/10 of the waveform period.  Then I plotted the data in Excel, as shown below:

Digital samples from Arduino Uno ‘receiver’

As can be seen from the plot, there are plenty of samples per cycle, well above the Nyquist rate for the dominant signal term.

Next, I moved the square wave signal to an analog input on the Uno, and reprogrammed to collect analog values vice digital ones, as shown below:

Analog readings with 500Hz square wave input

As can be seen, the analogRead() function is plenty fast enough to accurately reproduce the digital square wave signal, with about 10-13 samples/cycle.  So, at least in principle I should be able to detect a square-wave-modulated IR beam, and determine its ‘field strength’ (analog reading when the beam is ON) for homing purposes.

Stay tuned!

Frank