A matter of voltage

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Posted 20 March 2018

I think it is important that Wall-E2 have an accurate measurement of battery voltage, so that he knows when he should be looking for his next charging fix, and more importantly, so he can stop and yell for help if the battery voltage gets dangerously low.   In addition, I would like to monitor the battery voltage during charge, so Wall-E2 can report & display charging progress to any interested humans (like me). ;-).

From what I’ve read, it appears a LiPo cell can go down to about 3V without damage, or 6V for my 2-cell stack.   So, my operating voltage range is from full charge (approx 8.4V) to empty (6.0V).   My first cut at battery voltage monitoring was a simple 1/3 – 2/3 resistive voltage divider tied to an analog input; simply measure the voltage, multiply by 3, and voila – battery voltage!

Only it didn’t work that way; once the battery voltage dropped below about 7V, the drop across the Arduino Mega’s voltage regulator wasn’t sufficient to maintain regulated 5V, so the Mega’s bus voltage began to drop.   At Vbatt = 6V, the Mega was still running OK, but the bus voltage was down to 4V, and the A/D reference was no longer what it should be – rats!

In addition, once I started looking at this issue, I realized I was throwing away most of the A/D dynamic range with the divider idea.   with a 5V A/D reference and a 1/3 divider, the A/D input voltage only varies between 2.0 and 2.8V for an input range between 6 and 8.4V.   In other words, I’m only using 0.8V of the available 5V range or about 16%.

So, I thought that maybe I should implement a level shifter, so the sensed voltage varies from 0 to 2.4 as the battery voltage varies from 6 to 8.4 – and then use the Mega’s internal 2.56V reference for the A/D operation.   This would mean an immediate increase in dynamic range usage from 16% to almost 94%, and would increase resolution from about 15mV/count to about 2.5mv/count.   To do the level shifting, I’ll need a 6V zener, such as the  1N5233B, available from Mouser for a few pennies each.

One last voltage issue to be addressed is the problem with the Mega’s onboard regulator dropping out for battery voltages between 7 and 6V – this is almost half of the available voltage range.   Eventually I decided to address this problem by replacing (or rather, bypassing) the onboard regulator with a low dropout (LDO) regulator such as the  LF50CV-DG  , available from Mouser for less than $1 each.   The  LF50CV-DG can maintain 5V output down to well below my 6V battery voltage cutoff limit, so it is a good match.

23 March 2018 Update:

I just received the LF50CV-DG regulator and  1N5233B parts from Mouser, so I’m in the process of installing them onto Wall-E2.   The regulator will take the place of the MOSFET low-drop diode I installed on the Pololu Wixel Shield some time ago as part of my old PB1000C-based charging subsystem, and is now no longer needed.   The following photos show the installation:

Wixel shield showing MOSFET diode to be replaced by LDO 5V regulator

LF50CV-DG LDO 5V regulator and 1N5233B 6V Zener diode installed on Wixel shield

Rear of Wixel shield showing regulator output connection to +5V bus

25 March Update:

While testing the above arrangement, I managed to somehow kill my Mega 2560 SBC (I think my old power supply did it in, but I’m not sure).   So, in the process of recovering from this mess, I also decided to replace my old Wixel shield for the latest version (v1.1) with updated level-shifting circuits and carry-throughs for the added pins on the UNO R3, Mega, and cousins.   The new layout is shown below

Updated Wixel shield board with LDO 5V regulator and level-shifter circuit installed

Once I got everything back together, I started over with testing the LDO 5V regulator and level shifter performance, and ran into another problem.   The original idea was to use the 1N5233B 6V zener to level shift 6-8.4V to 0-2.4V so the range would fit into the range obtainable using the Mega’s internal 2.56V ADC reference.   This worked  almost perfectly, but the combination of a slightly lower Vz (5.84 vs 6.00V) and a slightly lower Vref (2.42 vs 2.56V) caused the ADC to hit full scale (1023 counts) at about 8.26V (2.44Vref + 5.84Vz = 8.26V).    Most unfortunate, as I really needed to accurately measure Vbatt to at least 8.4V, nominal end-of-charge voltage for a 2-cell LiPo stack.

So, I needed to expand the measurable voltage range at least a little bit on the top end.   With the installation of the LF50CV LDO 5V regulator, I could now do that by reverting to the internal 5V reference, as the LDO easily maintains 5V output all the way down to 6V, the cutoff voltage for my battery stack.   But, this wastes half the available ADC range, as the ADC input voltage for Vbatt = 8.4 is only 8.4-5.84 = 2.56V.   So, after some more Googling through Arduino-space, I realized I could tie the Mega’s AREF pin to the Mega’s 3.3V output line and use ‘analogReference(EXTERNAL)’ to obtain an ADC range from 0-3.3V, corresponding to a Vbatt range of 0 to (5.84+3.3)= 9.14V – perfect!

After making this change I ran some measurements to verify the input range and accuracy, as displayed in the following Excel plot

Measured vs Calculated Vbatt, with raw ADC values

As can be seen in the above plot, the measured and calculated voltage plots are almost perfectly overlaid, and well within the accuracy requirements for effective battery management.

Summary:

As usual, what started out as a simple plan (in this case, to accurately measure the battery voltage) rapidly metastasized into a full-blown hardware and software project, complete with howls of anguish and gnashing of teeth.   The first idea was to use a simple 1/3 resistive voltage divider input to a ADC port referenced to 5V. This worked OK, but failed at battery voltages below 7V because the Mega’s onboard voltage regulator requires an approximately 2V input-output offset.   Since I needed to measure Vbatt down to 6V, this was never going to work.   In addition, the available measurement accuracy sucked because the 2.5V range of interest was being compressed into 2.5/3 = 0.833V, and with a 5V reference I was using less than 20% of the available ADC counts. The next idea was to replace the onboard regulator with the LF50CV LDO regulator, and use a 6V zener to level shift the range of interest to under 2.56V so that the Mega’s internal 2.56V reference could be used.   This  almost worked, but I ran out of ADC counts before I ran out of battery voltage – oops.   The third (and last, I hope) idea was to change the ADC reference from internal 2.56V to external 3.3V using the AREF pin tied to the Mega’s 3.3V regulated output.   This allowed the top voltage to go to a little over 9V, just about perfect for this application.

 

 

Stay tuned,

Frank

 

 

To a man with a hammer, …

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Posted 17 March 2018,

To paraphrase the saying, “to a man with a hammer, every problem looks like a nail”, “to a man with a 3D printer, every problem looks like a 3D printing opportunity”.   And that’s pretty much what happened when I ran across the problem of adapting some  80mm wheels to my Wall E-2 robot, which came originally with 65mm versions.   The extra 15mm diameter/7.5mm radius doesn’t sound like much, but it makes a  huge difference when navigating over carpet or other small obstacles (like my wife’s slippers).

After a  lot of work, I finally was able to print four reasonable quality adaptors, and thought I was home free.   Unfortunately, I soon learned that despite my best efforts, the printed adaptors were no match for physics; the wheel eventually worked its way off the motor shaft, just as before – it just took a little longer ;-).

After the usual number of curses, imprecations, and woe-is-me’s, I finally decided to use whatever was left of my engineering brain to actually look at the physics of the situation.   When I did so, I realized that my adaptor idea was never going to work.   While the adaptor did indeed (after the aforementioned ‘lot of work’) provide for a better fit between the 80mm wheel receptacle and the motor shaft, it also moved the wheel another 9mm or so away from the robot chassis, which put the wheel center of pressure (CP) well outside the adaptor-to-motor shaft parting plane.   This meant that the wheel would always be trying to pry the the adaptor off the shaft, and it didn’t take all that long for it to succeed :-(.   The following photo illustrates the problem

80mm wheel with 3D-printed adaptor on the left, same wheel directly attached to motor shaft on the right

So, contemplating this problem while drifting off to sleep I was struck by a solution; I could use a small roll pin inserted through the wheel and motor shafts to literally pin them together.   The geometrical physics would still cause the wheel to flex the shaft, but the forces wouldn’t be able to overcome the strength of the metal roll pin.   Because I knew I would forget this insight if I left it until morning, I staggered out of bed and jumped onto the McMaster-Carr site (they have  everything!) to look for an appropriately sized roll pin.   I found a 1 x 6mm roll pin that would be perfect for the job, and if I ordered them now they would probably already be on my doorstep when I woke up in the morning.

McMaster-Carr metric roll pins

However, while I was doing the necessary measurements on the motor shaft, I noticed the motor shaft had a axial hole in it, and so did the wheel; hmm, maybe I could simply run the roll pin through the axial hole, instead of cross-wise?   Then I thought – wait that hole looks to be slightly smaller than 3mm – maybe I could simply drill/tap it for 3mm and use a 3mm screw (of which I had plenty in different lengths) instead of a roll pin?

So, in just a few minutes I had drilled & tapped the axial hole in the motor shaft of one of my spare motors, drilled out the wheel hole for 3mm clearance, and firmly screwed the wheel to the shaft (the right-hand wheel in the first photo above) – cool!

Now all I have to do is modify all four wheel shafts for 3mm clearance, and all four motor shafts to accept a 3mm screw – piece of cake!

As can be seen in the above photos, the 80mm wheels are now much closer to the chassis.   The wheel guards are now much too wide, but I may keep them that way for the moment, as I have already adjusted the charging station lead-in rails to accommodate the (now unnecessary) greater wheelbase – oh well 😉

So, the moral of this little story is:   Just because you have a 3-D printer doesn’t mean the solution to every problem is a new 3-D printed piece; and maybe to keep one’s eyes/brain open for even better solutions as they might come along when least expected!

Stay tuned,

Frank

 

 

 

 

 

 

 

New TP5100-based Battery Pack for Wall-E2

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Posted 13 March 2018

In a recent post, I described my study of the widely available and dirt-cheap TP5100 1/2-cell LiPo battery charger as a possible replacement for my current Adafruit PB1000C-based battery charger.   Based on the results of this study, it was clear the TP5100-based system was superior in all respects to my home-brew system:

  • Twice the charge current (2A vs 1A) means significantly shortened charge times
  • Much smaller and simpler
  • Charger current path independent of load path – much lower IR drop
  • Battery always connected to the system, so no requirement for ultra-low-drop MOSFET diode
  • Much simpler software – no requirement to monitor status of two separate chargers
  • No electromechanical relay to screw up.

I constructed a small charger module using some perfboard and a couple of 2-place screw terminals, as shown below (with the previous module shown for size comparison).

New TP5100-based charger module, with previous Adafruit PB1000C-based module below for size comparison. The orange box contains 4 Panasonic 18650 cells.   Note the separate charge & load circuits

The following figures show the old and new schematics:

Old battery pack schematic

New battery pack schematic

Now that the load current doesn’t have to go through the charging module, I was able to replace all main battery wiring with #20 wire for lower IR drops, as shown below

Power wiring replaced with #20 wiring, and 2-pin Deans connectors

#20 wiring to main battery buss. Note in-line safety disconnect

 

The change to the new battery pack also considerably simplified the system hardware and software.   The changes to the system schematic are shown below:

Old system schematic. Note the ultra-low-drop MOSFET diode required to keep Arduino Mega alive during charge. and the number of pins required for charge monitoring.

New system schematic. No requirement for diode, as full battery voltage is available at all times. Also, only two pins are required for charge monitoring

The operating system software has also been simplified.   Now, instead of monitoring both cell voltages and four different status lines, only two lines have to be monitored.   Also, there is now no requirement to correctly sequence the ‘Charger Connect’ and ‘Coil Enable signals in order to accomplish correct charging station connect-disconnect behavior.   Now the system simply shuts off the motors when the robot connects to the station, and turns them back on again to disconnect.   As an added benefit, the six charge status LEDs have been repurposed to show a crude approximation (based on battery voltage only for the moment) of charge status.

All these changes have caused one minor hiccup in the implementation of the charging station; the new charging voltage is +12V vs +5V as before.   As you may recall, the charging station implements a square-wave modulated IR signal, and this signal is produced by a Teensy 3.2 and some associated circuitry, all of which expect +5V.   This will require either a dual-output supply, or the addition of an on-board 12-to-5V regulator. This is still up in the air, but I suspect it will land on a simple 3-pin regulator.

So far, all the hardware changes (except for the charging station changes) have been accomplished, but the software changes have yet to be implemented and tested. Stay tuned!

Frank

 

TP5100 2-cell LiPo Charger Module Study

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Posted 24 February 2018

I have been working on Wall-E2, my autonomous wall-following robot, for almost three years now, and it seems like I have been struggling with the battery and charger arrangement for that entire time.   I started out with 4 AA batteries, but quickly moved on to a pair of Sparkfun 2000mAh ‘flat-pack’ cells with Sparkfun chargers, with a relay to switch the batteries from series (RUN) to parallel (CHG) wiring.   This worked, but not very well.   The flat-pack batteries weren’t a good match for motor control, and I kept burning up charger modules as well.   After struggling with this through several iterations, I finally abandoned it entirely in favor of a 7.4V 20C LiPo RC battery and an external charger. This worked much better, but forced me to manually disconnect the battery from the robot and charge it externally – not at all what I wanted.   Later on I made another run at the 2-cell series/parallel switching strategy for charging, this time using Adafruit Powerboost 1000C charge modules, each capable of 1A charge rates. Again this worked (actually quite well), but I recently discovered that it has a fatal flaw – this design imposes significant IR drops on the way from the battery terminals to the motors.

So, I have once again been searching for a solution to the battery/charger problem.   While wandering through the Googleverse the other day, I ran across a mention of the 1/2-cell TP5100-based charger module (about 9:50 from start), available for next to nothing on eBay.

Unfortunately, the available technical information on this module is also next to nothing, and what does exist is all in Chinese.   Still, this module has the potential for vastly simplifying my charger setup, so I thought it was worth the effort to perform a thorough study.

In a previous post, I described an Arduino controlled charge/discharge test setup for testing operation of my 2-cell parallel/series switched setup, so I decided to modify it for evaluating the TP5100 module, as shown below

View of TP5100 module showing RUN & CHG indicator connections

Charger test setup, in discharge mode (note 1.1A discharge current)

Charger test setup, in charge mode (note 1.8A charge current)

TP5100 Module Test Circuit

Using this setup, I was able to cycle the battery between a 7.5V load and the TP5100 charge module.   In order to keep the cycle times down to a dull roar, I set the software to switch to charge when the battery voltage dropped below 7.5V, resulting in the plots shown below.

In this case, the discharge current was about 1.1A, and the observed charge current was about 1.8A.  The TP5100 modules seems to work as advertised – with a 12V 5A power supply and a partially charged battery, it successfully charged my 2-cell LiPo pack terminated the charge at about 8.4V (I’m not sure if it is terminating based on current or voltage).

Over the next couple of days, I performed three complete charge/discharge cycles using this same setup.   Discharge was terminated at 6V, and charge was terminated when the TP5100 ‘complete’ output changed from open-circuit to active-low. As can be seen in these charts, performance was very consistent – almost 6 hours run time  into a 7.5V load, and about 4 hours for a complete recharge.

 

So here’s what I know now about the TP5100 module

  • When the ‘1-cell/2-cell’ jumper selector is shorted to select 2-cell, the output voltage stabilized at 8.4 with a 10-15V DC input (I used a 12V 5A supply for the tests).   Below about 10V, the output voltage falls below 8.4V
  • with a partially charged battery stack, output current was about 1.8A at the start, tapering to below 200mA at termination
  • There is an onboard Red/Blue LED and solder holes for an external bi-color LED.   The onboard LED states are:
    • RED = Charging
    • Blue = Finished
  • Both the onboard and external LED connections are tied to +V via the same 1K current limiting resistor.   This resistor is routed to the center hole of the 3-hole external LED breakout.   The rightmost hole is tied to an open-collector gate that goes LOW upon charge termination, and the leftmost hole is  tied to an open-collector gate that goes LOW upon charge initiation.   In my testing circuit above, these lines are labelled ‘Fin’ and ‘Chg’ respectively and were routed to digital inputs with 20K pullups on the Arduino UNO.
  • This is  NOT a balance charger – so there may be differences in cell voltages over time.   If this is a potential issue, then separate cell protection modules like these should be installed.

Here’s an annotated photo showing the pertinent features:

So, it looks like this TP5100 module will work fine for my 2-cell LiPo application, with the addition of an external 2-cell protection module like the one noted above.   Not only will this solve my original IR drop problem, but it is  much smaller and simpler too, as shown in the following size comparison shot.   Oh well, at least I had a lot of fun building up and testing the original charger module ;-).

charger module and TP5100 size comparison

Stay tuned!

Frank

 

 

 

The operation was a success, but …

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Posted 21 February 2018

In early January of this year I posted about finishing the integration of my new-improved battery charger & battery pack into my new-improved robot chassis.   Between then and now I have been working on getting the new robot chassis mated up with the charging station (the new robot chassis is wider, and I also changed to larger diameter wheels) in preparation for renewed field testing.

Unfortunately, just as I was getting ready to move into field testing, my robot started acting funny.   About half the time, it wouldn’t disengage from the charging station and instead would reboot.   At first I thought the added weight of the new battery pack and robot chassis was causing the motors to stall, so I changed the code to have the robot disconnect at full motor speed rather than 1/2 as before.   This made the problem even worse; now not only wouldn’t Wall-E2 disengage from the charger, it wouldn’t even move forward or backward under it’s own power!   Clearly something was badly wrong, but I had no clue what it was.

Applying my time-honored troubleshooting – I simply put Wall-E2 aside for a few days and let my subconscious work backwards through all the changes since Wall-E2 had last worked properly.   After enough time had elapsed, my subconscious reported back and said:

“You are an idiot.   All of the complexity you added in your quest for an on-board charging system has placed that wonderful high-capacity battery pack at the far end of a long series of (relatively) high resistance circuitry, and the IR drop caused by full-speed motor currents is killing you!”   “Oh, and by the way, you’re ugly too!”

Well, my subconscious is almost never wrong, and it only took me a little bit of testing to confirm it’s theory.   I set the code up to go forward and backward at full speed, and monitored the CPU’s 5V regulated output line with my trusty oscilloscope.   As soon as the motor command was executed, the 5V line drooped to less than 3V, and the CPU rebooted – oops!

So now I knew  what was happening, and I (or my subconscious anyway) had a good idea  why.   To confirm the why, I bypassed all the charge-management circuitry and wired my 7.4V 7200mAh battery pack directly into the main robot power line, as shown in the photo below

7.4V 7200mAh battery pack wired directly into robot power

With this setup, the robot not only was able to move forward and backward at full speed, the thing damn near took my arm off when I tried to stop it – whoa!

So, the bottom line is that all the work I put in designing and implementing a really cool on-board dual-cell charge management system had the ultimate effect of making the battery unusable.   The operation was a success, but the patient died! ;-).

So, where to go from here?   It appears that I have to completely revise my thinking about battery charging and maintenance for Wall-E2.   Instead of being in series with the battery, any charging/maintenance system must operate in parallel, and be completely out of the path between the battery pack and the load when the robot is running.   Now I realize this is the reason most RC/Hobbyist multi-cell battery packs have a balance charging cable in addition to the main power cable; charging is done completely independently of the output path.

When I first started the charger project, my original goal was to avoid having to remove the battery from the robot to charge it; I wanted Wall-E2 to   connect to power and charge itself without human intervention.   At the time, I felt the only way to do this was to have the charging circuitry on board, so that only a single DC connection from the charging station was required.   I thought the only way to make this happen was to use two of the Adafruit SBC1000 charger modules to charge each of the two cells independently.   Unfortunately, the SBC1000’s grounds aren’t isolated, so this meant that I had to disconnect the two battery pack cells from each other to charge them independently and then switch them back together again to run the robot after charging.   This worked (rather elegantly if I do say so myself), but had the unintended side-effect of putting too much high-loss circuitry and wiring between the battery pack and the motors.

Now that it is clear that I can’t interfere with the current path to the motors, I know I have to abandon the current charging module design, but what are the alternatives?

  • The TP5100 is a little module that can balance charge a 2-cell LiPo stack at 2A.   It has a dual-color LED output that I might be able to use for charge termination.   Unfortunately, the specs are all in Chinese, so it may take some experimentation to figure out.
  • I can use an external balance charger like the EV Peak e4 ‘cube’ automatic balance charger, and feed the three required wires (ground, B1+, B2+) out through the front of the robot to the charging station. This solves the problem of carrying the charger around, but significantly complicates the interface to the charging station.

Stay tuned!

 

Printing an ABS Shaft Adaptor for 80mm Wheel

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Posted 25 January 2018

Over the last few days I have been struggling with a project to 3D print a small adaptor to allow me to mount some 80mm wheels I bought some time ago to my Wall-E2 autonomous wall-following 4-wheel drive robot.

The original robot came with 56mm wheels and this gave Wall-E2 very little ground clearance.   I found some 80mm wheels that I thought would do the trick nicely, but when I tried them, it quickly became apparent that the shaft receptacle on the wheel was significantly larger than the motor shaft, leading to a very bad wobble and catastrophic wheel departures – oops!

Lightweight 4WD Drive Aluminum Mobile Dolly Car Robot Platform for Arduino

 

original 56mm wheel and companion motor

Ebay ‘Arduino Robot’ motor dimensions

80mm wheel

After some troubleshooting, I discovered that the new 80mm wheels have a shaft receptacle that measures 5.9mm long and 3.6mm wide, while the motor shafts are 5.4mm long and 3.5mm wide.   The width is OK, but the longer length is causing the problem.

After thinking (and cursing) a bit, I decided to try printing an adaptor.   The larger diameter wheels are also considerably thinner (20mm or so vs 30mm), so there should be room for an adaptor part, as shown below

shaft adaptor for 80mm wheel

And threw it on my PowerSpec 3D PRO (Flashforge Creator Pro knockoff).   After just a little fiddling, I got some nice parts, and thought I was done.   A couple of days later, I noticed one of the parts was just a little loose on its shaft, so I said to myself – “I’ll just print off another one”.   Unfortunately, what came off the printer was really ugly, and completely unusable, even though that same printer had produced nice parts just a few days ago – WTF!?   Clearly I had forgotten what magic I had wrought the first time, so now I had to go back and recreate it – bummer.   As part of my penance for this crime, I am writing this post so the next time I want to do this, I’ll have the print settings recorded.

Print Settings for ABS

The significant factors in how to get good prints with ABS on this printer appear to be

  • Print speed
  • Extrusion factor

The first thing I did was slow the print speed down, but this had only a minor effect on print quality.   Going slower helped, but even very slow speeds (like 10mm/sec) didn’t result in clean edges on the male part of the adapter. However, the female portion was very clean, which left me a bit puzzled – why one part but not the other?   I finally realized that the difference was that the female piece was perfect because the hole perimeter was the first thing laid down at each slice, then the outside perimeter, and finally the fill material was added last.   This meant that the hole perimeter had a chance to cool and solidify before the fill material impinged on its outer surface, and this meant that the perimeter stayed in the same shape as originally laid down.   When printing the male part, however, the outer perimeter was laid down first, and then the fill material was immediately added, before the outer perimeter had a chance to cool and solidify, even at the slower speeds.   The material making up the fill was pushing the outer perimeter out of shape.

This led me to focus on the extrusion factor.   Reducing the extrusion factor from 1.00 to 0.95 had a significant positive impact on the print quality of the male portion of the adaptor.   Reducing it again to 0.90 resulted in an even better print, as did a further reduction to 0.85.   However, at the 0.85 value, I started to see some degradation in the quality of the female portion, so I backed off to 0.90 as the final value.   The following image shows the last seven prints.   All were printed at either 20mm/sec or 10mm/sec, and the last three on the right were printed with 10mm/sec and extrusion factors of 0.95, 0.90, and 0.85 respectively.

The last seven prints. the last three on the right were printed at 10mm/sec and with extrusion factors of 0.95, 0.90, and 0.85 respectively

Original and new wheels, with completed adaptor shown

Bottom Line (PowerSpec 3D Pro, Simplify3D):

  • Material: Gray ABS
  • Extruder temp: 230 (not critical 220-240 should be OK)
  • Bed temp: 110 (not critical, 100 should do too)
  • Speed: 20 or 10mm/sec (maybe faster would be OK, but not much)
  • Extrusion factor: 0.95 or 0.90

 

 

 

 

A New Chassis For Wall-E2, Part III

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Posted 13 January 2018

Back in November of last year I started moving Wall-E2 to his new home in a larger chassis, and subsequently I was able to integrate the new-improved charging system in to this new chassis.

So, today I started some field testing, and discovered a major problem – the motors that came with the new chassis don’t have sufficient torque to handle the operating environment – they stall out much too quickly when encountering any movement issues at all.   This wasn’t a problem for the previous incarnation of Wall-E2, so clearly the motors that came with the new chassis have different speed/torque specs than before.   And, of course, the old motors won’t fit into the new chassis (actually, they do fit, but not with the new battery pack).    What a bummer!

So, I started researching dc motors to determine if I could find motors with the speed/torque specs of the old chassis, but with the physical specs of the motors that came with the new one.

Motors in previous ‘Pirate’ chassis:

The specs for the motors that came with the DF Robots ‘Pirate’ chassis are shown below:

Specifications for the motors using in the ‘Pirate’ robot chassis from DFRobots

With a gear ratio of 160, the motors should produce about 160 RPM at At Wall-E2’s normal operating range of 6-7.5V, and can produce a maximum torque of 0.8 kgf-cm (kilogram force – cm).

Motors in new chassis:

A DRIVE MOTOR GEARBOX REDUCTION RATIO: 1:48
DRIVE MOTOR NO-LOAD SPEED: 220RPM

I couldn’t find the max torque specification, but assuming it’s the same basic motor body, the much lower gear ratio should result in the observed higher no-load speed and lower torque.

So, I’m fairly confident that I now know  why the motors in the new chassis aren’t performing as well as the old ones – a much lower gear ratio.   Now all I need to do is find another motor source with the same form factor as the new ones, but with a gear ratio more like the ones from the older chassis:

Pololu:

I found a 120:1 gear ratio right-angle motor at Pololu

Pololu 120:1 right-angle motor dimensions

Pololu 120:1 right-angle motor

These motors have almost the same exact dimensions as the ones in the new chassis,  but they have a shorter (5 vs 9mm) and larger diameter shaft (7 vs 5mm).   These are not insurmountable difficulties, but surely I can find a better fit.

Ebay:

So, eventually I ended up back on eBay, the same place where I got the new chassis.   But, this time I was a little more informed about what gear ratio I wanted, and so was able to reasonably quickly locate a set of 4 motors (without wheels – I have too many already!), as shown below

4ea 1:120 right-angle shaft motors with 5mm shaft diameter

So, now all I have to do is wait 2-3 weeks for them to arrive, and I should be back in the low(er)-speed, high(er)-torque business!

Stay tuned!

Frank

 

 

Wall-E2 battery charger module integration

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Posted 01 January 2018

What a way to start off the new year!   The battery charger module for my autonomous wall-following robot Wall-E2 has been completed and tested, and now has been integrated into the robot – yay!!

If you have been following this saga, you will recall that I started working on an internal charging module for Wall-E2 well over a year ago, back in November 2016 with this post.   Since then I have gone through several iterations, revisions, and mis-steps (including a semi mind-boggling deep-dive into the details of the Adafruit PowerBoost 1000C specifications in this post).   Last month I finally got a complete system (two PowerBoost 1000C’s integrated onto a single PCB with appropriate control and battery switching circuitry) working, and was able to run extensive charge/discharge cycle testing using a simple test circuit and an Arduino Uno to run it. So, now all I had to do was stuff the whole thing back into the robot.   This task was made possible by my earlier decision to upgrade Wall-E2’s ride to a slightly larger chassis, so instead of trying to cram 2Kg of battery/charger into a 1Kg space, I now had the pleasure of fitting 2Kg into a 3Kg space – nice!    Here are some photos of the integration process.

Battery module shown in the ‘maintenance’ configuration.

Another shot of Battery module in the ‘maintenance’ configuration.

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Front cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Rear cover removed to show how the battery module fits into the robot. Note there is plenty of room for cable runs

Now that the battery/charger module has been integrated into the robot chassis, I will have to make some minor changes to the robot operating system to accommodate changes I have made along the way, but these should be easy and straightforward.   Then, it will be back to field testing, I hope.

Stay tuned!

Frank

 

 

Wall-E2 battery charger module testing, Part II

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Posted 22 December 2017

After having worked out (hopefully) all the bugs in the Arduino Uno program and associated test hardware, I have moved on from testing just one of the Adafruit PowerBoost 1000C   charging module in isolation to testing the entire 2-cell battery pack system, albeit with the smaller 2500mAh LiPo’s rather than the full-up 18650 stacks.

Full-up test of the 2-cell charging system using 2500mAh LiPo cells

Full charge of 2500mAh cells

After several days of testing, I never really got consistent results with this setup – I seemed to be always chasing intermittent problems of one sort or another.   Then I finally figured it out (I think) – the problem wasn’t my setup, it was the proto plug-board I was using.   This board was a cheap no-name plugboard I got off eBay, and apparently I got exactly what I paid for! ;-(.   So, I reverted to my tried-and-true (but HUGE!) AP Products A.C.E. 236 plugboard that I have had around the lab for a couple of decades, at least.   When I transferred the setup to this plugboard, everything started working better.   In particular, the battery charging current went from about 0.5A to about 1.5A – a much more believable (and practical) figure than before.   Here’s the new test setup.

New test setup with my trusty AP ACE 236 plugboard. Note charging current shown on current meter

After getting the hardware problems squared away, I started getting reliable charge/discharge cycle data, as shown in the curves below

Two complete charge/discharge cycles. Note the time axis is in minutes here

The next step in the process will be to replace the PKCell 2500mAh flat-pack LiPo cells with the 6800mAh pack (two series banks of 2ea Panasonic 18650 3400mAh cells in parallel) to be used in the robot.

26 December Update:

Chg/Dischg testing with the Panasonic 18650 packs from the robot.

As shown above, I switched out the 2500 mAh flatpacks for the Panasonic 3400 mAh cells from the Wall-E2 robot.   Initially I was somewhat disappointed with charging performance, as I was only seeing about 1 – 1.5A initial charge current for both 6800 mAh cells, instead of the 2+ A I expected to see.   Eventually I narrowed the problem down to stray resistance in the testing circuit itself, through the plugboard, the plugboard wiring, and the relay being used to switch charging power to the battery module.   When I bypassed these elements and connected the external MeanWell 5V power supply to the battery module terminals, the max current increased to slightly over 2A, as expected.   This is actually very good news, as it means that the resistance of the PCB traces supplying power to the PowerBoost 1000C modules is low enough to not materially affect the max charging current – yay!

Still, even with the max charging current up at 1A/cell and with the PB 1000C’s PROG3 charge termination resistor reduced from 100K to 33K, it takes every bit of 10 hours to charge the battery pack from 3V to 4.2V, as shown below.

Panasonic 18650 6.8AH robot battery pack. Initial charge rate approx 1A per 2-cell parallel stack

Panasonic 18650 6.8AH robot battery pack discharge, 15 ohm load

28 December 2017 Update:

While testing these battery packs for charge and discharge performance, I came to realize that I had over-complicated the test circuit.   The original test circuit is shown below:

This circuit uses a relay to switch +5V power to the charger modules in the battery pack module, and also to switch the load in and out.   As I gained more experience, I realized the relay contact resistance was substantially reducing the charge current, so I wired +5V directly to the battery pack; this worked because the Arduino running the test circuit keeps the battery pack parallel/series relay disengaged (meaning the battery cells are arranged in series) until the load voltage drops below a set threshold.   So, this realization resulted in the following updated circuit schematic.

As shown above, I added the connections to the battery charger module, and the LED displays.

However, I have now come to realize that the other half of the relay isn’t necessary either, as the ‘Robot +’ line isn’t connected to anything until the test manager computer (Arduino Uno) recognizes the end-of-charge condition and changes the Coil Enable signal from HIGH to LOW, disabling the battery pack’s internal relay and changing the battery configuration from parallel (CHARGE) to series (RUN).   So, now the test circuit can be reduced to just the LEDs and the load resistors, as shown below.

While I was making the other changes, I also cut the load resistance in half, from 15Ω to 7.5Ω to more accurately simulate the actual robot motor loads, and (finally!) managed to capture a complete discharge cycle, as shown below.

Complete Robot Battery Pack Discharge Curve, 7.5-ohm load.

The 7.5Ω load for this run provides a good approximation for the maximum current drain experienced by the robot under most operating conditions, so it is now safe to say that the robot should be able to run at least 6 hours on a charge, and that a full charge will take about 10 hours.   So, the robot will spend more time on the charger than on the road, but that’s life in the robot lane.

Stay tuned!

Frank

 

 

 

Wall-E2 battery charger module testing, Part I

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Posted 15 December 2017

For the last couple of months I have been working on the battery subsystem for my Wall-E2 autonomous wall-following robot.   Along the way I upgraded the robot chassis to provide more room for the on-board charger & battery pack, and created a PC board design for the module that charges the 2-cell LiPo stack in parallel and then switches to a serial configuration to run the motor.   This post describes the first part of a testing program to validate the performance of the Adafruit PowerBoost 1000C charger module, and the idea of switching seamlessly from charging to powering a load.

By way of background for this post, I ran into some difficulties when I tried to integrate the full-up 2-cell, 2-PB1000C battery pack/charger combination into my Wall-E2 robot, and troubleshooting the problems using the complete operating system software proved to be somewhat tedious, I decided to start at the other end with a very simple test program running on an Arduino Uno, and a single PB1000C charger.

The Arduino test program is very simple; when it is initialized, it checks the battery voltage to see if is above or below a preset threshold.   If above, it energizes a DPDT relay to connect the battery to a 15Ω load; if below, it de-energizes the relay, which disconnects the battery from the load and also connects 5V to the ‘USB’ (5V input) terminal of the PB1000C, thereby charging the battery.   After that, the program alternately switches the battery between the load and charging states, recording the battery voltage every four seconds.

Battery Testing Hardware

Arduino Uno and test circuit.   Note the yellow ‘charging’ LED on PowerBoost 1000C shown at middle right

After working the inevitable bugs out of the Arduino program, I got some reasonable battery cycling data, as shown in the following plots.   The battery used in all these tests was a PKCell LP795060 LiPo cell, rated for 3.7V and 2500mAH.   It is rated for about 5hrs at 0.2C discharge, but I was only seeing about 3hrs at 0.1C, most probably because I was terminating the discharge at 3.4V rather than the 2.75V used in the discharge tests.

Two complete chg/discharge cycles

Chg/Dischg cycles using PB2 (same battery as previous tests). Short cycles may imply battery wear-out

After the last test above, I was concerned that either the PowerBoost 1000C was not operating properly, or the battery itself was failing, or both.   So, I changed the discharge termination threshold from 3.4 to 3.0V to be more consistent with the 2.75V cutoff used by the battery manufacturer.   After making this change only, I got the following plot

Charge/discharge cycles after changing the discharge termination threshold from 3.4 to 3.0V

As can be seen from the above plot, the discharge and charge times were extended dramatically.   Discharge (at approximately 220 mA or 0.1C) took well over 3hrs, and charge took about the same amount of time.   So, it appears that both the PowerBoost 1000C modules are still working OK even after the abuse I put them through by plugging them into a PCB with wiring errors ;-).