Monthly Archives: July 2016

OSU/STEM Outreach Speaker Amplifier Project, Part III

 

Posted 29 July 2016,

In my last post on this subject, I described the work to be completed to finish the OSU/STEM Outreach speaker amplifier project, as follows:

  • physically cut the perfboard down to a size that will fit into the enclosure, and figure out how it is to be secured.
  • Figure out how to mount the channel monitor LEDs so that they can be seen from outside the enclosure.  The plan at the moment is to mount them on the outside edge of the left and right screw terminals, respectively, and widen the AUD OUT opening enough so they can be easily viewed from outside.
  • Figure out how to route power and audio signals to the LED monitor board without using the external power and audio output screw terminals. The power wiring should be simple, as power and ground are available on the breakout pins provided by Adafruit.  However, the audio output signals are more problematic, as it isn’t obvious how to get to these circuit points, and I really don’t want to start drilling holes in a multi-layer PCB.  After a careful visual inspection and some probing with my trusty DVM, I think I have located where I can safely tap into the PCB run from the last SMT resistor to the positive audio terminal for each channel – we’ll see!
  • Make sure the enclosure top fits OK, and all three LEDs are indeed visible.
  • Install semi-permanent L/R audio output cables with alligator clip terminations.  In actual use, I don’t really expect two paper speakers to be connected at once, but I’ll be ready if that situation occurs ;-).

Cut the perfboard down (and mount the LEDs):

With the help of my trusty Dremel tool and a carbide cutoff wheel, I trimmed the perfboard down to a size and shape that would fit into the enclosure, using the top surface of the audio input jack as a handy mating surface on one end, and with the monitor LEDs themselves as mounting legs on the other end, as shown in the following photos.

Cut-down perfboard, mounted front view

Cut-down perfboard, mounted front view

Cut-down perfboard, mounted, side view

Cut-down perfboard, mounted, side view

Power and audio signal wiring:

160729_LEDMonitorBoard1

Audio and power wiring

Audio and power wiring

Enclosure:

160729_LEDMonitorEncl4

Completed project shown connected to OSU paper speaker

Completed project shown connected to OSU paper speaker

Speaker connection cables:

Enclosure side view showing audio output connections

Enclosure side view showing audio output connections

All Together Now…

 

So, at this point the amplifier project is all done except for the ‘proof of the pudding’, which in this case is a real field test with real participants and student helpers.  If the field test results are satisfactory, I’ll probably build at least two more complete systems (I got 3ea amp boards from Adafruit and haven’t managed to kill any of them yet), and donate them to the OSU Engineering Outreach project.

Frank

 

 

 

 

OSU/STEM Outreach Speaker Amplifier Project, Part II

Posted 27 July 2016

In my last post on this subject, I had gotten to the point where I had a single-channel LED monitor circuit working, although it was still a bit unkempt, to say the least.  In this post I describe the effort to get both channels working and to neaten everything up prior to installing the circuit into the speaker amplifier enclosure.

First off, I ‘improved’ the LED monitor circuit a bit, based on the results from testing the one-channel version.  I discovered there was a DC term riding on the class-D input signal, and this was getting amplified through the LED monitor circuit and swamping the audio output at the LED.  So I added a 0.01uF DC blocking cap on the input line.  Unfortunately, this caused another problem, in that now the RC circuit cap had no discharge path so it almost immediately charged up and again swamped the audio signal.  One more addition – a 330K Ohm resistor across the low-pass cap to allow it to discharge.  Then, I adjusted the DC gain of the amp to provide a good LED response to the audio.  I wanted the LED’s to come alive just about the time that a well-fabricated speaker produced clearly audible output.  The idea here is that a helper can tell immediately from the LEDs whether or not there is sufficient output power present to produce an audible response, and therefore if no response is detected, there is a problem with the speaker or the wiring from the amp to the speaker – but not upstream of that point.  After some fiddling around, I settled on a gain of about 70 for the LED monitor.  With that setup, and with the 20W amp gain set appropriately, I could easily drive the OSU paper speaker from my laptop, with the laptop’s audio output slider at about 50%.  The ‘New Improved’ LED monitor circuit is shown below:

Revised LED monitor schematic - one of two channels shown.

Revised LED monitor schematic – one of two channels shown.

The audio input to the monitor circuit is the rail-to-rail high-frequency (about 300KHz) audio-modulated PWM signal, as shown below

Amplifier output with audio input signal present

Amplifier output with audio input signal present

After the RC filter, only the audio signal is present, as shown in the following photo

LED monitor circuit audio input. Scope is set to 200mv/div

LED monitor circuit audio input. Scope is set to 200mv/div.

The following photo shows the output.  Due to the use of a single-supply op-amp and without any DC bias arrangement, only the positive input signal excursions produce an output – but that’s OK in this circuit as all we are doing is driving an LED.

LED monitor circuit LED drive signal. Only the positive audio transitions produce an output. Scope set to 2V/div

LED monitor circuit LED drive signal. Only the positive audio transitions produce an output. Scope set to 2V/div

While tweaking the monitor circuit, I had made quite a mess of my little perf-board layout.  So, after getting the schematic in good shape I basically tore everything down and started over again with the aim of building up the two-channel layout.  After everything was done, the result was the circuit shown in the following photo

Two-channel LED monitor circuit detail

Two-channel LED monitor circuit detail

two-channel LED monitor circuit in operation with the Adafruit 20W amp and the OSU paper speaker

two-channel LED monitor circuit in operation with the Adafruit 20W amp and the OSU paper speaker

After re-working the perf-board layout, adding the second channel, and carefully checking the wiring against the circuit, I was pleased to see that both channels operated as desired, and either channel could easily drive the OSU paper speaker.  The following short video shows the speaker being driven by the right channel, and both channel signals being monitored by the LED monitor circuit.

 

At this point the remaining work is:

  • physically cut the perfboard down to a size that will fit into the enclosure, and figure out how it is to be secured.
  • Figure out how to mount the channel monitor LEDs so that they can be seen from outside the enclosure.  The plan at the moment is to mount them on the outside edge of the left and right screw terminals, respectively, and widen the AUD OUT opening enough so they can be easily viewed from outside.
  • Figure out how to route power and audio signals to the LED monitor board without using the external power and audio output screw terminals. The power wiring should be simple, as power and ground are available on the breakout pins provided by Adafruit.  However, the audio output signals are more problematic, as it isn’t obvious how to get to these circuit points, and I really don’t want to start drilling holes in a multi-layer PCB.  After a careful visual inspection and some probing with my trusty DVM, I think I have located where I can safely tap into the PCB run from the last SMT resistor to the positive audio terminal for each channel – we’ll see!
  • Make sure the enclosure top fits OK, and all three LEDs are indeed visible.
  • Install semi-permanent L/R audio output cables with alligator clip terminations.  In actual use, I don’t really expect two paper speakers to be connected at once, but I’ll be ready if that situation occurs ;-).

Once I’m sure everything is working OK and that the whole thing won’t die on me the first time someone looks at it sideways, then the plan is to volunteer for an upcoming speaker fabrication session and try the amp in the ‘real world’.  If it works as I fully expect it to, then the idea is to donate two or three to the OSU Engineering Outreach program, in the name of my company (EM Workbench LLC).  Stay tuned!

Frank

 

OSU/STEM Outreach Speaker Amplifier Project, Part I

Posted July 25, 2016

Lately I have become involved in the Engineering Outreach program here at The Ohio State University, as a volunteer helper at ‘hands-on’ engineering project presentations to grade-school and middle-school students in the Columbus, Ohio area.  A week or so ago I helped out in a session where the students (middle-schoolers at a science day-camp) got to fabricate and test an audio speaker from a paper template, some magnet wire, and a couple of small permanent magnets, and I was struck by the difficulty the kids were having in actually hearing anything coming out of their freshly-fabricated speakers when they connected them to the output of their phones and/or iPads.  I could understand if I couldn’t hear anything – I’m an old power pilot and my ears are shot from thousands of hours of piston engine noise – but the kids with their brand-new ears couldn’t hear anything either!  There was an audio amplifier available for the kids to use, but it wasn’t much help either – there wasn’t any indication that the amp was actually doing anything, so it could have died long ago and nobody would ever know – bummer.  Anyway, I came away from that project with the distinct impression that this particular project wasn’t doing much for the kids, and maybe I could do something to help.  So, before I left the room, I made a point of ‘borrowing’ all the parts required to fabricate my own paper speaker – the paper template, magnet wire, and permanent magnets along with the other small bits and pieces.

My first line of inquiry in my own home lab was to determine what the best fabrication technique was for the speaker itself.  My thinking was that since this project has been around forever in one guise or another, it sorta had to be successful in some sense, or it would have died out long ago.  Therefore, I reasoned that a more careful approach to the actual fabrication might yield better results.  After doing some inet research, I realized that one of the critical aspects of paper speaker construction is to make sure the speaker coil assembly (a section of plastic straw hot-glued to the speaker cone with the magnet wire wound around it’s lower end) was free to move vertically – i.e. it wasn’t forced down against the baseplate by the tension of the speaker legs.  In other words, the speaker legs had to be arranged such that the speaker coil assembly floated 2-3 mm above the base.  After playing with this a while, I realized that the proper technique was to arrange the speaker legs so as to properly suspend the coil assembly first, and then place the permanent magnet ‘dot’ under the coil assembly second, rather than the other way around.  Assembly in this sequence tends to minimize lateral friction of the coil assembly straw section against the side of the permanent magnets, hopefully resulting in higher mechanical/audio transfer efficiency.

After constructing the speaker as carefully as possible, I hooked it up to my laptop audio output, and voila! – no audio :-(.  Even with the audio output at max volume (which, when directed to my laptop’s internal speakers is enough to vibrate my workbench), it was almost impossible to hear anything from the speaker.  Even with careful construction with no time constraints, and a top-end audio source, getting the speaker to work was a very marginal deal.

After thinking about this for a while, I decided that I was maybe working the wrong end of the system.  What I needed to do was to implement an audio amplifier that would provide sufficient output power so that even a marginally constructed speaker would work properly; the only real limitation on power that I could see would be the current limit imposed by the magnet wire itself – as long as I didn’t physically melt the wire, I should be OK! ;-).

My first try at an amplifier was based around a power MOSFET I had lying around, as shown in the following photo.  1607_MOSFET_AMP

This worked ,but only up until the point at which the drain resistor started smoking!  That’s the problem with a linear amplifier driving a speaker – LOTS of power being dissipated.

So, back to the drawing board, with more inet research.  This time, I came across the very nice Adafruit Class-D speaker amplifier shown below.  The kit has everything needed to build a complete speaker amp, as shown in the second image below.  The amplifier runs Class-D, so it is very efficient, and it depends on the speaker coil inductance to reject the high-frequency switching signal, leaving only the audio.

 

Adafruit 20W Class-D speaker amplifier, based on the MAX9744 amplifier chip

Adafruit 20W Class-D speaker amplifier, based on the MAX9744 amplifier chip

Adafruit 20W Class-D speaker amplifier application

Adafruit 20W Class-D speaker amplifier application

The folks at Adafruit were even thoughtful enough to provide the STL file for a 3D-printable enclosure for the amp, as shown below:

3D-printable enclosure for the 20W Class-D amp

3D-printable enclosure for the 20W Class-D amp

And, since I am the proud owner of not just one, but two 3D printers (a Printrbot Simple Metal, and a PowerSpec 3D Pro/aka Flashforge Creator X), the existence of a ready-made design for the enclosure saved some time.

After receiving my amps from Adafruit, and after the requisite amount of fumbling around, I managed to get the amp running and connected to a couple of old speakers.  When I connected up my laptop’s audio output I was quite pleased that, even at the default 6dB gain setting, the amp easily drove the speakers to the point where I had to reduce the audio input volume to avoid complaints from the wife in the next room.

Adafruit amp test setup

Adafruit amp test setup

At this point, it was clear that the Adafruit amp should be an excellent solution to the paper-speaker driver problem, but I wasn’t quite done yet.  There were still several remaining issues:

  • The amplifier used in the previous sessions had no ‘power ON’ indication, so there was no way to tell if the thing was actually operating or not.  The Adafruit amp, as delivered doesn’t have one either, so this had to be corrected somehow.
  • The previous amplifier had no ‘audio activity’ indication, so even if you somehow knew that it was indeed getting power and working, there wasn’t any way to tell if there was any audio output without attaching a speaker; and if there was no sound, there was no way to tell if the problem was the speaker or the amp.
  • The original enclosure design by Adafruit didn’t actually fit – the slots for the power and audio-in jacks weren’t high enough (later design change?). In addition, there were no provisions for either the ‘power ON’ or ‘audio activity’ indicators.  Fortunately I not only have 3D printers, but an account on TinkerCad so the design could be adjusted.
  • Since I still haven’t actually connected the amp to my paper speaker, I don’t really know yet if the default 6dB gain setting is sufficient to drive the speaker; I may still have to change to the analog gain setup and use a higher gain setting (fortunately, Adafruit thoughtfully provided a 1K potentiometer and a set of jumper pads to achieve this, if necessary).

Power-ON Indication:

The power-ON indication issue was fairly easy to address – all I had to do was add an LED-resistor combination across the external power input lines.   The only problem with this solution was finding a way for the LED to be visible outside the enclosure – and the solution was to modify the enclosure design to expose the external power screw terminals (they were hidden in the original design), and in the process leave a bit of space open between the power input jack and the screw terminal, a space just wide enough for a small, rectangular LED as shown below.

Power ON indicator LED installed between power input jack and external power screw terminals

Power ON indicator LED installed between power input jack and external power screw terminals

Audio Activity Indicator:

The Adafruit amplifier is Class-D – i.e. the output is a pulse-width-modulated (PWM) signal, as shown below:

Amplifier output with audio input signal present

Amplifier output with audio input signal present

Amplifier output with no audio input signal present

Amplifier output with no audio input signal present

As can (or more accurately, cannot) be seen from the above photos, there is no appreciable difference between the ‘audio’ and ‘no audio’ waveforms.  The class D amplifier technique depends on the low-pass nature of the speaker coil to suppress the high-frequency switched waveform terms, leaving only the audio term.  Unfortunately, this means there really isn’t anything there to directly drive an audio activity indicator – the typical LED is plenty fast enough to follow the high-frequency PWM waveform, thereby masking the audio signal.  One possible technique would be to sample the speaker coil current (which pretty much by definition doesn’t contain the high-frequency switching terms), but this requires that a working speaker be attached.  This won’t work, because the whole idea of an audio activity indicator is to confirm that an audio input is present and has been amplified sufficiently to adequately drive a speaker before one is connected.  So, I decided to try a simple RC low-pass filter, followed by a basic audio amplifier to drive the activity LED.  The circuit for one channel is shown below.

LED monitor circuit for one channel

LED monitor circuit for one channel.  Pin numbers are Channel1/Channel2

Here is a short video showing the LED monitor circuit in action

 

So, at this point I have a working speaker amplifier, a power-ON indicator, and at least the prototype of an audio activity monitoring circuit.  Next up – finish the LED monitor circuit, finish the enclosure design and fabrication, and assemble everything for delivery.

 

 

 

 

 

 

 

 

Giving Wall-E2 a Sense of Direction, Part VII

Posted 07/11/16

My last post on this subject described my successful effort to mount my Mongoose IMU on Wall-E2, my wall-following robot.  I showed that the IMU, mounted on an 11cm wood stalk on Wall-E2’s top deck, when calibrated using my Magnetometer Calibration tool, provided reasonably accurate and consistent magnetic heading measurements.

This post attempts to extend these results by replacing the long wooden stalk with a more compact plastic mounting bracket (actually my original IMU mounting bracket) as shown below

Mongoose IMU mounted on 2nd deck using original mounting bracket

Mongoose IMU mounted on 2nd deck using original mounting bracket

In an effort to determine what, if any, effect the stalk mounting had on IMU calibration, I decided to acquire and plot calibrated (as opposed to raw) magnetometer data using my mag cal tool, and compare it to the results of the calibration performed on the stalk-mounted configuration.  Since I don’t currently save the ‘calibrated’ point-cloud (there’s no need, as all it does is show how well (or poorly) the raw mag data point cloud is transformed using the generated calibration matrix/center offset values), I first had to import the saved raw data from the stalk-mounted configuration and then regenerate the calibration values (and the resultant ‘calibrated’ point cloud).  Once this was done, then I can capture the new calibrated (i.e. calibrated using the previous stalk-mounted calibration values) but now in the lower mounting position.  If the stalk mounting had no additional isolation effect, the two point clouds should look identical.  If the stalk mounting did have some effect, then the two clouds should look different.

I started by launching the mag cal tool and importing the raw mag data captured 07/06/16. Then I computed the calibration factors and the resulting ‘calibrated’ point cloud, as shown in the following screenshot.

Raw mag data from the stalk-mounted config, and the resulting calibrated point cloud

Raw mag data from the stalk-mounted config, and the resulting calibrated point cloud

As can be seen from the image, the data calibrated quite well, starting with a visibly offset point cloud with an average radius of about 450, and ending with a well-centered and symmetric point cloud with a radius close to 1.

Next, I captured a set of data from the bracket-mounted IMU, using the calibration values from the 6 July stalk-mounted config (this required a bit of reprogramming to pare back the reporting from Wall-E2 to just the magnetometer 3-axis data).  The data was captured by manually rotating Wall-E2 about all 3 axes in a way that produced a well-populated ‘point cloud’ in the mag cal tool app.  During this run, Wall-E2 had power applied, and all motor drives enabled.

Bracket-mounted IMU calibrated magnetometer data vs 06 July stalk-mounted computed calibration data

Bracket-mounted IMU calibrated magnetometer data vs 06 July stalk-mounted computed calibration data, with Wall-E2 power on and motors running.

From the above screenshot it is quite clear that the stalk and bracket mounting configurations are essentially identical in terms of their calibrated performance.  This means I could, if I so chose, simply use the stalk-mounted calibration values and party on.  Moreover, if I do chose to re-calibrate, I wouldn’t expect to see much change in the calibration values.

Here’s a short movie showing the calibration process:

 

After noting that the ‘stalk’ calibration values appeared to be reasonably valid for the bracket-mounted configuration, I re-ran the heading error tests on my bench-top heading range, with the following results:

Bracket-mounted IMU Heading Error, Power On, Motors Running

Bracket-mounted IMU Heading Error, Power On, Motors Running

For comparison, here is the ‘stalk’ heading error chart

Stalk-mounted Mongoose IMU, with power and motor drive enabled.

Stalk-mounted Mongoose IMU, with power and motor drive enabled.

And the original problem measurement from back in March with the IMU mounted on the first deck at the front of the robot:

Heading performance for front-mounted IMU, power off.

Heading performance for front-mounted IMU, power off.

From the above, it is kind of hard for me to believe that this much error could possibly be corrected just via the calibration matrix and center offset adjustments, so I suspect the current performance depends as much on moving the IMU from directly over the front motors to the 2nd deck (a minimum of 10cm from the rear motors, and about 15 from the front ones) as it does on the calibration values.  I could verify this by re-mounting the IMU on the front and seeing if I could calibrate out the errors, but I’d rather let sleeping dogs lie at this point ;-).

Frank

 

Giving Wall-E2 A Sense of Direction, Part VI

Posted July 06, 2016

In my last post on this subject, I had used my newly-completed Magnetometer Calibration Tool to generate calibration factors for my HMC5883L-based ‘Mongoose IMU board, and compare the ‘raw’ vs ‘calibrated’ performance in a ‘free-space’ (actually my wood lab workbench) environment.  The result of the comparison showed that the ‘calibrated’ performance was pretty much unchanged from the ‘raw’ setup, indicating that the test setup (on my wooden workbench) wasn’t significantly affected by ‘hard’ or ‘soft’ interference.

The next step is to mount the Mongoose IMU on Wall-E2, my 4WD wall-tracking robot to see if the magnetometer can be compensated for DC motor magnet fields, power cables, and the like.  I decided to start this process by mounting the IMU on a wooden ‘stalk’ on the second deck, to see if this placement would minimize the above interfering effects.

Mongoose IMU mounted on wooden stalk on 2nd deck

Mongoose IMU mounted on wooden stalk on 2nd deck

Raw and calibrated data. Reference circles on left have radii equal to average raw value radius. Circles on right all have a radius == 1

Raw and calibrated data. Reference circles on left have radii equal to average raw value radius. Circles on right all have a radius == 1

The calibration values can now be saved to a text file convenient for transcription into the user’s calibration routine.  After doing the save, the text file looks like the following;

Magnetometer Compensation Values Saved Wednesday, July 06, 2016 1:50:09 PM

Compensation Matrix
U11: 0.0021076
U12: -0.00015447
U13: 6.5899E-05
U22: 0.0020698
U23: 1.24E-05
U33: 0.0022697

Center Offset
Cx: -10.812
Cx: -120.549
Cx: -239.734

After copy/pasting the above values into my calibration routine and re-running the data collection exercise but recording the calibrated magnetometer readings instead, I got the following ‘raw’ (calibrated magnetometer data, but displayed in the ‘raw’ view) results.

The displayed data in the 'raw' view is new magnetometer data after being calibrated with the results of the first run. The circle radius on the left is 0.92. The data on the left is the old magnetometer data, calibtrated using the results of the calibration value computation from the first set of raw magnetometer data

Comparison of new calibrated data from the magnetometer with the results of the Octave calibration algorithm as applied to the old set of raw magnetometer data.

The displayed data in the ‘raw’ view is new magnetometer data after being calibrated with the results of the first run. The circle radius on the left is 0.92. The data on the left is the old magnetometer data, calibtrated using the results of the calibration value computation from the first set of raw magnetometer data.  As is easily seen from the two views, the calibration values generated by the Octave program produce very good ‘on-the-fly’ calibration results.

After calibration, I re-ran the heading performance tests (main power ON, but no drive to the motors), with the following results

Stalk-mounted magnetometer heading error, main power, no motor drive

Stalk-mounted magnetometer heading error, main power, no motor drive

The next step is to repeat this experiment with the motor drives enabled.  Here’s the results of a quick run.  With the motors enabled, I held Wall-E2 so that it’s wheels didn’t quite touch the surface, and slowly rotated the robot 360 clockwise, starting at the same point (nominally 0 deg as reported by the Mongoose IMU) as in the above plot.

Manually rotated over 360 degrees with motors running. Mongoose stalk mounted on 2nd deck

Manually rotated over 360 degrees with motors running. Mongoose stalk mounted on 2nd deck

As shown in the plot above, the headings reported by the Mongoose IMU increased monotonically as the robot was rotated clockwise from nominal zero. Although just a preliminary result, it is actually quite encouraging, as it indicates that running the motors doesn’t significantly affect the heading value reported by the Mongoose IMU.

Today I had the chance to perform a ‘motors running’ heading error experiment with the stalk-mounted Mongoose IMU.  The robot body was placed on a small plastic box such that the wheels were free to turn without touching the workbench.  Then it was manually rotated in 10 deg increments as before. The experimental setup and the results are shown below.

Test setup for the "Power and Motors" IMU heading error experiment.

Test setup for the “Power and Motors” IMU heading error experiment.

Stalk-mounted Mongoose IMU, with power and motor drive enabled.

Stalk-mounted Mongoose IMU, with power and motor drive enabled.

Comparing the heading error plots, it is pretty clear that enabling the motors does not significantly affect the stalk-mounted IMU.  If I wanted to leave the IMU mounted on the stalk, it appears that I could expect to get reasonable, if not spectacularly accurate, magnetic heading readings ‘in real life’.

However, I really don’t want to leave the IMU mounted on a stalk, so the next step in the process will be to replace the stalk mounting arrangement with a more ‘streamlined’ mounting setup.  For this I plan to use the mounting bracket I printed up for the original front-mounted setup (see image below), but attached to the 2nd deck vs the 1st.

160318MongooseInstalled1_Annotated2

Original mounting location for the Moongoose IMU (arrow points to the IMU)

 

 

 

 

 

 

 

Magnetometer Calibration Tool, Part IV

In my last episode of the Magnetometer Calibration Tool soap opera, I had a ‘working’ WPF application that could be used to generate a 3×3 calibration matrix and 3D center offset value for any magnetometer capable of producing 3D magnetometer values via a serial port.  Although the tool worked, it had a couple of ‘minor’ deficiencies:

  • My original Eyeshot-based tool sported a very nice set of 3D reference circles in both the ‘raw’ and ‘calibrated viewports.  In the ‘raw’ view, the circle radii were equal to the average 3D distance of all point cloud points from the center, and in the ‘calibrated’ view the circle radii were exactly 1.  This allowed the user to readily visualize any deviations from ideal in the ‘raw’ view, and the (hopefully positive) effect of the calibration algorithm.  This feature was missing from the WPF-based tool, mainly because I couldn’t figure out how to do it :-(.
  • The XAML and ‘code-behind’ associated with the project was a god-awful mess!  I had tried lots and lots of different things while blindly stumbling toward a ‘mostly working’ solution, and there was a LOT of dead code and inappropriate structure still hanging around.  In addition to being ugly, this state of affairs also reflected my (lack of) understanding of basic WPF/Helix Toolkit concepts, principles, and methods.

So, this post describes my attempts to rectify both of these problems.  Happily, I can report that the first one (lack of switchable reference circles) has been completely solved, and the second one (god-awful mess and lack of understanding) has been at least partially rectified; I have a much better (although not complete by any means!) grasp of how XAML and ‘code-behind’ works together to produce the required visual effects.

To achieve better understanding of the connection between the 3D viewport implemented in Helix Toolkit by the HelixViewport3D object, the XAML that describes the window’s layout, and the ‘code-behind’ C# code, I spent a lot of quality time working with and modifying the Helix Toolkit’s ‘Simple Demo’ app.  The ‘Simple Demo’ program displays 3 box-like objects (with some spheres I added) on a grid, as shown below

Simple Demo WPF/Helix Toolkit Application

Simple Demo WPF/Helix Toolkit Application (spheres added by me)

Simple Demo XAML View

Simple Demo XAML View – no changes from original

Simple Demo 'Code-behind', with my addition highlighted

Simple Demo ‘Code-behind’, with my addition highlighted

My aim in going back to the ‘Simple Demo’ was to avoid the distraction of my more complex window layout (2 separate HelixViewport3D windows and lots of other controls) and the associated C#/.NET code so I could concentrate on one simple task – how to implement a set of 3D reference circles that can be switched on/off via a windows control (a checkbox in my case).  After trying a lot of different things, and with some clues garnered from the Helix Toolkit forum, I settled on the TubeVisual3D object to construct the circles, as shown in the following screenshots.  I used an empirically determined ‘thickness factor’ of 0.05*Radius for the ‘Diameter’ property to get the ‘thick circular line’ effect I wanted.

Simple Demo modified to implement TubeVisual3D objects

Simple Demo modified to implement TubeVisual3D objects.  The original box/sphere stuff is still there, just too small to see

MyWPFSimpleDemo 'code-behind', with TubeVisual3D implementation code highlighted

MyWPFSimpleDemo ‘code-behind’, with TubeVisual3D implementation code highlighted.  Note all the ‘dead’ code where I tried to use the EllipsoidVisual3D model for this task.

Next, I had to figure out a way of switching the reference circle display on and off using a windows control of some sort, and this turned out to be frustratingly difficult.  It was easy to get the circles to show up on program startup – i.e. with model construction and the connection to the viewport established in the constructor(s), but I could not figure out a way of doing the same thing after the program was already running.  I knew this had to be easy – but damned if I could figure it out!  Moreover, after hours of searching the blogosphere, I couldn’t find anything more than a few hints about how to do it. What I did find was a lot of WPF beginners like me with the same problem but no solutions – RATS!!

Finally I twigged to the fundamental concept of WPF 3D visualization – the connection between a WPF viewport (the 2D representation of the desired 3D model) and the ‘code-behind’ code that actually represents the 3D entities to be displayed must be defined at program startup, via the following constructs:

  • In the XAML, a line like  ‘<ModelVisual3D Content=”{Binding Model}”/>, where Model is the name of a Model3D property declared in the  ‘code-behind’ file (MainViewModel.cs in my case)
  • In MainWindow.xaml.cs, a line like ‘this.DataContext = mainviewmodel’, where mainviewmodel is declared with ‘public MainViewModel mainviewmodel = new MainViewModel();’
  • In MainViewModel.cs, a line like ‘ public Model3D Model { get; set; }’, and in the class constructor, ‘Model = new Model3DGroup();’
  •  in MainViewModel.cs, the line ‘var modelGroup = new Model3DGroup();’ at the top of the model creation section to create a temporary Model3DGroup object, and the line ‘ this.Model = modelGroup;’ at the bottom of the model construction code. This line sets the Model property contents to the contents of the temporary modelGroup‘ object

So, the ‘MainViewModel’ class is connected to the Windows window class in MainWindow.xaml.cs, and the 3D model described in the MainViewModel class is connected to the 3D viewport via the Model Model3DGroup object.  This is all done at initial object construction, in the various class constructors.  There are still some parts of this that I do not understand, but I think I have it mostly correct.

The important concept that I was missing is the above connections have been made at program startup and cannot (AFAICT) be changed once the program starts, but the contents of the temporary Model3DGroup object (i.e. the ‘Children’ objects in the model group) can be changed, and the new contents will be reflected in the viewport when it is next updated.  Once I understood this concept, the rest, as they say, “was history”.  I implemented a simple control handler that cleared the contents of the temporary Model3DGroup object modelGroup and regenerated it (or not, depending on the state of the ‘Show Ref Circles’ checkbox).  Simple and straightforward, once I knew the secret!

So this ‘aha’ moment allowed me to implement the switchable reference circles in my Magnetometer calibration tool and check off the first of the deficiencies noted at the start of this post.  The new reference circle magic is shown in the following screenshots.

Raw and calibrated magnetometer data. Calculated average radius of the raw data is about 444 units, and the assumed average radius of the calibrated data is close to 1 unit

Raw and calibrated magnetometer data. Calculated average radius of the raw data is about 444 units, and the assumed average radius of the calibrated data is close to 1 unit

Raw and calibrated magnetometer data, with reference circles shown. The radius of the 'raw' circles is equal to the calculated average radius of about 444 units, and the assumed average radius of the calibrated circles is exactly 1 unit

Raw and calibrated magnetometer data, with reference circles shown. The radius of the ‘raw’ circles is equal to the calculated average radius of about 444 units, and the assumed average radius of the calibrated circles is exactly 1 unit

The reference circles make it easy to see how the calibration process affects the data.  In the ‘raw’ view, it is apparent that the data is significantly offset from center, but still reasonably spherical.  In the calibrated view, it is easy to see that the calibration process centers the data, removes most of the non-sphericity, and scales everything to very nearly 1 unit – nice!

Now for addressing the second of the two major deficiencies noted at the start of this post, namely “The XAML and ‘code-behind’ associated with the project was a god-awful mess! “.

With my current understanding of a typical WPF-based application, I believe the application architecture consist of three parts – the XAML code (in MainWindow.xaml)  that describes the window layout, the ‘MainWindow’ class (in MainWindow.cs) that contains the interaction logic with the main window, and a class or classes that generate the 3D models to be rendered in the main window.  For my magnetometer calibration tool I created two 3D model generation classes – ViewportGeometryModel and RawViewModel.  The ViewportGeometry class is the base class for RawViewModel, and handles generation of the three orthogonal TubeVisual3D ‘circles.  The ViewportGeometryModel class is instantiated directly (as ‘calmodel’ in the code) and connected to the main window’s ‘vp_cal’ HelixViewport3D window via it’s ‘GeometryModel’ Model3D property, and the derived class RawViewModel (instantiated in the code as ‘rawmodel’) is similarly connected to the main window’s ‘vp_raw’ HelixViewport3D window via the same ‘GeometryModel’ Model3D property (different object instantiation, same property name).

The ViewportGeometryModel class has one main function, and some helper stuff.  The main function is ‘DrawRefCircles(HelixViewport3D viewport, double radius = 1, bool bEnable = false)’.  This function is called from MainWindow.xaml.cs as follows:

       public MainWindow()
        {
            InitializeComponent();

            //connect raw/cal viewports to their respective geometry model classes
            rawmodel = new RawViewModel(vp_raw, this); //ctor creates an empty 3D model & loads it into the raw view's 'GeometryModel' property
            calmodel = new ViewportGeometryModel(vp_cal, this);//ctor creates an empty 3D model & loads it into the calibrated view's 'GeometryModel' property
            vp_raw.DataContext = rawmodel; //this tells vp_raw to use the 'rawmodel' object's 'GeometryModel' property
            vp_cal.DataContext = calmodel; //this tells vp_raw to use the 'calmodel' object's 'GeometryModel' property

           ..... (other stuff)
        }

        private void chk_RefCircles_Click(object sender, RoutedEventArgs e)
        {
            System.Windows.Controls.CheckBox cbx = (System.Windows.Controls.CheckBox)sender;
            bool check = cbx.IsChecked ?? false; //'??' needed in case cbx is null
            rawmodel.DrawRefCircles(vp_raw, rawmodel.GetRawAvgRadius(), check);
            calmodel.DrawRefCircles(vp_raw, 1, check);
        }

The ‘DrawRefCircles()’ function creates a new ModelGroup3D object if necessary, and optionally fills it with three TubeVisual3D objects of the desired radius and thickness, as shown below

        public void DrawRefCircles(HelixViewport3D viewport, double radius = 1, bool bEnable = false)
        {
            //// Create a model group
            if (modelGroup == null)
            {
                 modelGroup = new Model3DGroup();// Create an empty model group if necessary
            }
            else //already exists - remove all children so we can start over
            {
                modelGroup.Children.Clear();
            }

            if (bEnable) //if checkbox state is 'checked', create the ref circle objects
            {
                // Create the materials (colors) we will need
                var greenMaterial = MaterialHelper.CreateMaterial(Colors.Green);
                var redMaterial = MaterialHelper.CreateMaterial(Colors.Red);
                var blueMaterial = MaterialHelper.CreateMaterial(Colors.Blue);

                //create reference circles using TubeVisual3D objects
                //probably should do this using transforms, but don't know how :-(
                double thicknessfactor = 0.05; //established empirically

                //ring in XY plane
                TubeVisual3D t_xy = new TubeVisual3D();
                //t_xy.Fill = System.Windows.Media.Brushes.Black;
                Point3DCollection p3dc = GenerateCirclePoints(36, radius, CirclePlane.PLANE_XY);
                t_xy.Diameter = thicknessfactor * radius; //1% factor emperically determined
                t_xy.Material = blueMaterial; //to match viewport coord sys colors
                t_xy.Path = p3dc;
                modelGroup.Children.Add(t_xy.Model);

                //ring in Xz plane
                TubeVisual3D t_xz = new TubeVisual3D();
                t_xz.Material = greenMaterial;
                p3dc = GenerateCirclePoints(36, radius, CirclePlane.PLANE_XZ);
                t_xz.Material = greenMaterial; //to match viewport coord sys colors
                t_xz.Path = p3dc;
                t_xz.Diameter = t_xy.Diameter;
                modelGroup.Children.Add(t_xz.Model);

                ////ring in yz plane
                TubeVisual3D t_yz = new TubeVisual3D();
                p3dc = GenerateCirclePoints(36, radius, CirclePlane.PLANE_YZ);
                t_yz.Diameter = t_xy.Diameter;
                t_yz.Material = redMaterial; //to match viewport coord sys colors
                t_yz.Path = p3dc;
                modelGroup.Children.Add(t_yz.Model);
            }

            // GeometryModel is bound to HelixViewport3D with the line
            //   in MainWindow.xaml)
            GeometryModel = modelGroup;
        }

The last line in the above function is ‘GeometryModel = modelGroup;’, where ‘GeometryModel’ is declared in the ViewGeometryModel class as

        public Model3D GeometryModel { get; set; } //this is bound to the viewport

and bound to the appropriate HelixViewport3D window via

Line in MainWindow.xaml that binds the HelixViewport3D to the 'GeometryModel' Model 3D property of the ViewportGeometryModel class

Line in MainWindow.xaml that binds the HelixViewport3D to the ‘GeometryModel’ Model 3D property of the ViewportGeometryModel class (and/or its derived class RawViewModel). The line shown here is for the raw viewport, and there is an identical one in the calibrated viewport section.

Now, instead of a mishmash spaghetti factory, the program is a lot more organized, modular, and cohesive (or at least I think so!).  As the following screenshot shows, there are only a few classes, and each class does a single thing.  Mission accomplished!

Magnetometer calibration tool class diagram. Note that the RawViewModel is a derived class from VieportGeometryModel

Magnetometer calibration tool class diagram. Note that the RawViewModel is a derived class from VieportGeometryModel.  The ViewportGeometryModel.CirclePlane ‘class is an Enum

Other Stuff:

This entire post has been a description of how I figured out the connections between a WPF-based windowed application with two HelixViewport3D 3D viewports (and lots of other controls) and the XAML/code-behind elements that generate the 3D models to be rendered. In particular it has been a description of the ‘reference circle’ feature for both the ‘raw’ and ‘calibrated’ views.  However, these circles are really only a small part of the overall magnetometer calibration tool; a much bigger part of the 3D view are the point-clouds in both the raw and calibrated views that depict the actual 3D magnetometer values acquired from the magnetometer being calibrated, before and after calibration.  I didn’t say anything about these point-cloud collections, because I had them working long before I started the ‘how can I display these damned reference circles’ odyssey.  However, I thought it might be useful to point out (no pun intended) some interesting tidbits about the point-cloud implementation.

  • I implemented the point-cloud using the Helix Toolkit’s PointsVisual3D and Point3DCollection objects.  Note that the PointsVisual3D object is derived from ScreenSpaceVisual3D which is derived from RenderingModelVisual3D instead of a geometry object like TubeVisual3D which is derived from ExtrudedVisual3D, which in turn is derived from MeshElement3D.   These are very different inheritance chains.  A PointsVisual3D object can be added directly to a HelixViewport3D object’s Children collection, and doesn’t need a light for rendering!  I can’t tell you how much agony this caused me, as I just couldn’t understand why other objects added via the ModelGroup chain either didn’t render at all, or rendered as flat black objects.  Fortunately for me, the ‘SimpleDemo’ app did have light already defined, so things displayed normally (it still took me a while to figure out that I had to add a light to my MagCal app, even though the point-cloud displayed fine).
  • Points in a point-cloud collection don’t support a ‘selected’ property, so I had to roll my own selection facility.  I did this by handling the mouse-down event, and manually checking the distance of each point in the collection from the mouse-down point.  If I found a point(s) close enough, I manually moved the point from the ‘normal’ point-cloud to a ‘selected’ point-cloud, which I rendered slightly larger and with a different color.  If a point became ‘unselected’, I manually moved it back into the ‘normal’ point-cloud object.  A bit clunky, but it worked.

All of the source code, and a ZIP file containing everything (except Octave) needed to run the Magnetometer Calibration app is available at my GitHub site – https://github.com/paynterf/MagCalTool

Frank