Posted 23 May 2017

As you may recall from previous posts, I have been collaborating with my long-time friend and mentor John Jenkins on the  idea to use square-wave modulation of the charging station IR beam to suppress ‘flooding’ from ambient IR sources such as overhead incandescent lighting and/or sunlight.

More than likely, If it is possible to implement a software processing algorithms to recover steering information from potentially corrupted data, it will have to be housed on a dedicated processor.  So, I decided to set up a separate test setup using two processors – one to generate a square-wave modulation waveform, and another to receive that waveform through an IR link.  The link can then be modified in a controlled way to simulate link losses and/or ‘flooding’.  The initial hardware setup is shown below.

Initial test bed verification using 12cm separation

Scope shot showing transmitted and received waveforms, 12cm separation

Algorithm test bed with IR link range set to 78cm

Then I ran my little test program on the receiver processor that simply acquires 100 samples at roughly 20 samples/cycle and then prints out the results.  The following two images are Excel plots of the results for 12cm & 78cm separation.

As can be seen from these two plots, the 12 & 78cm separation values provide a reasonably good simulation of the ‘very good’ and ‘reasonably crappy’ signal conditions.

Next I verified that I can successfully ‘flood’ the receiver with my portable battery-operated IR signal generator.  I monitored the transmitted  and received waveforms, without and then with flooding.  In both cases, the  bottom trace is the 5V square-wave transmitted signal, shown at 2V/div, and the top trace is the received signal shown at 1V/div.  The ground for both traces is the same line on the scope screen.

78cm separation, no flooding signal. Bottom trace is transmit @ 2V/cm, top is receive @ 1V/cm, ground for both is same line

78cm separation, with flooding signal. Bottom trace is transmit @ 2V/cm, top is receive @ 1V/cm, ground for both is same line

Applying flooding signal with battery-operated IR signal generator

As can be seen in the scope photos, I can indeed produce almost 2V of ‘flooding’ using the IR signal generator, so I should be able to determine whether or not a particular recovery algorithm is successful at suppressing flooding effects.

Stay tuned

Frank

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

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 +/- 12 º beamwidth, but are arrayed in such a way as to provide well over 60 º 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 +/-30 º 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!

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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 +/- 45 º 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 0 º 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

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  Î© 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

Posted 07 April 2017

After getting the wall-following mode re-implemented in the ‘new-improved’ Wall-E2 operating system, I re-enabled the IR homing part of the code, only to discover that Wall-E2 thought it could see the IR homing beam  everywhere, in the atrium, even though the IR homing LED wasn’t even in the room (and was turned OFF, besides)!  This didn’t make a whole lot of sense, until I realized Wall-E2’s sensors were ‘seeing’ IR from the overhead floodlamps.  I confirmed this by having my ‘lovely assistant’ wife turn the atrium lights on & off and monitoring the detector outputs.  Same thing for the entry hallway (see data below)

I thought I had solved this problem way back when I first started working with the IR detection/homing idea back in October of last  year.  In that initial post  where I investigated  the  OSEPP IR Light Follower’, I found I had to turn OFF the LED track lights in my lab in order to avoid swamping the detector array.  Over the next month or so this project evolved into a custom designed array of  Oshram SFH309FA-4 phototransistors (see  http://gfpbridge.com/2016/11/ir-light-follower-wall-e2-part-vi/), and as part of this I discovered that the Oshram devices weren’t at all sensitive to the LED track lighting in my lab.  From this I concluded (erroneously as it now turns out!) that I didn’t need a ‘sunshade’ at all, which allowed for a much simpler installation on the robot (see  http://gfpbridge.com/2016/11/ir-light-follower-wall-e2-part-vii/).  Finally, when this module was transferred from the 3-wheel test bed to the Wall-E2 and combined with the charging status display panel, we arrived at the ‘final’ arrangement shown in the following photo

IR detector module (red) shown beneath charging status display panel (blue)

This worked great for all my in-lab IR homing tests with my all-LED overhead track lighting, but as soon as I started testing out in the rest of the house, the overhead incandescent and halogen lamps swamped out the detectors. So, it was back to the drawing board – again :-(.

The first thing I did was to confirm that the IR detectors were indeed getting swamped by the overhead lighting, and that it was possible to prevent this by some sort of shielding.  I started with a wrap-around cardboard shield that completely covered the IR detector module, as shown below, and placed the robot on the floor of the entry hallway in a normal ‘wall-following’ position

Sunshade V0. Not very practical, but does set the baseline for the rest of the tests

Test position for 07 April 2017 sunshade tests

With this setup, Wall-E2 was pretty much deaf to the overhead lamps, showing no reaction to cycling the lights.  Of course, this configuration would also completely prevent it from detecting the charging station IR beam, but….

Next I modified the above V0 cover to allow a bit more visibility, as shown below:

Sunshade V1 oblique view

Sunshade V1 front view

This version is still a bit too restrictive for normal IR beam detection/homing, but was worth a shot for testing purposes.  With this setup, the overhead lighting  is noticeable, as shown below.

Sunshade V1 at sunshade test position. Lights ON four times for 5, 5, 5 and 10 sec

As can be seen from the plot, the V1 sunshade responds to the overhead lamp IR with an average A/D reading about 700 out of 1024.

Next, I modified  the sunshade again so that it would just allow the IR detector module to ‘see’ straight ahead, on the theory that that would be the minimum requirement for successful IR beam detection/homing.  With this setup, the overhead lamp response was stronger, but still not completely overbearing;  the IR detector response to the overhead lights averages A/D values of about 600 out of 1024.

Sunshade V2 oblique view

Sunshade V2 front view.  Note IR detectors visible from directly in front

IR detector response to overhead lamps. ON three times for 5 sec each

When I ran a homing test  with the V2 sunshade in my lab (with LED overheads, not incandescent), the robot homed successfully from about 1 m away, with the following IR detector/homing performance

In-lab (LED overhead lighting) charging station homing performance with V2 sunshade

In the above homing test, the IR detector values started out at approximately 900, 200, 450 and 750, respectively.  This means I could set the IR beam detection threshold at, say, 400 and still have a 200-count noise margin in both directions (200 down from the overhead IR flooding value, and 200 up from the typical 1-meter IR beam intensity).  From the movie it is obvious that the minimum IR reading is switching back and forth between at least two detectors, so I think it is safe to say that IR homing would occur even in the presence of a 600 unit noise level, as the 200 or lower reading switched between detectors.

Next, I moved the charging station into the entry hall so I could test IR homing performance with the overhead lights on and off.  My prediction was that Wall-E2 should be able to home successfully in both cases.  For the test, the charging station lead-in rails were oriented in the preferred ’tilted’ orientation, as that should give the best homing performance, as shown in the photos below:

Hallway IR homing test setup

Robot shown in captured position. Note lead-in rail ’tilt’ relative to the wall

I made three runs; Lights ON, lights OFF, and Lights ON, as shown in the three video clips below:

As can be seen in the videos, Wall-E2 successfully homed to the charging station with the overhead lights OFF, but not with them ON – bummer!

So, further testing will be required to determine the particulars of the two failed runs, and what – if anything – can be done about it.

After moving my laptop  out into the hallway so I could capture telemetry from the robot while also filming the runs, and checking everything out, I made two successful IR homing runs – one with the overhead lights ON, and another one with them OFF.  I captured telemetry from both runs, and was clearly able to distinguish between the lights ON and OFF scenarios.  The videos and the telemetry plots are shown below:

08 April 2017 Hallway Test2 with V2 Sunshade – Lights OFF

08 April 2017 Hallway Test2 with V2 Sunshade – Lights ON

From the above plots, the lights ON & OFF conditions are readily recognizable.  In the ‘OFF’ condition, the ‘background noise’ is at about 600-700, and the IR beam is at 100-200.  In the ‘ON’ case, the noise level is higher (lower count), at about 150-250, but the IR beam is lower too – around 50-150.  I guess this makes some sort of sense, as the IR energy from the charging station beam is a separate, additive source relative to the overhead lighting.  Also, both plots show good motor response curves – the left & right motor speeds are obviously being adjusted rapidly in response to IR detector changes.

So, at this point I’m pretty convinced that the V2 sunshade is working, so I plan to print up a permanent version that will (hopefully) simply slide on to the front of the existing charge status display panel.

Stay tuned!

Frank