Come join the Darkside Skywalker…..The Switch from PIC to AVR

I’ve been playing around with the Arduino Diecimila and the ATMEGA168 over the last couple of weeks to better familiarise myself with the AVR ATMega168 MCU, I’ve been using PICs on and off the last few years, but the decision was made to use the ATMEGA as the MCU of choice for the Strobit Triggr project, mainly due to the open source tool chains available, and the simply programmer required.

In short I’m glad I’ve made the switch and I must say I’m loving the learning experience.  I’ve moved from the Arduino software development platform as I found it very limiting and am now using the open source avr-gcc (win-avr) and Eclipse, using the AVR plugin and CDT plugin as my development platform of choice, I’m comfortable with eclipse as my editor as I’ve been using this for my Java development for the last 5 or so years.

As a task I set for myself to learn the onboard peripherals,  I’ve created a Weather Shield for the Arduino, so far it has the RFM12B RF module, DS1307 RTC, HH10D Humidity Sensor, a HP03D, combined barometric pressure and temperature sensor, and soon to have a light sensor and Dallas 1-Wire interface for talking to the Dallas Weather Station that I’ve had lying around in a box for the last 10 years, (yes one of the original ones released by Dallas in 1998, I’ve been waiting to move in my house for a long time),  I’ll post the weather shield design up on a separate topic later, but suffice to say, I’ve enjoyed playing with the SPI, I2C, ICP, UART and onboard timers.

Prototype

Prototype boards RF module Testing

Prototype Specs:

  • PIC16F88 running on internal 8MHz Clock – low cost and very popular, free development tools available, i.e. C compiler.
  • Works in unlicensed 915MHz ISM Band(Australia/US), can work at 433Mhz by changing RF module(US/EU/Australia).
  • RF uses FSK Modulation – Less prone to interference sources.
  • All aspects of the RF module is configurable via the firmware, i.e. frequency, Tx power, receiver bandwidth, modulation, datarate etc.
  • Indoor Range – ~30m+ works all through my house non line of sight, i.e. through multiple single brick walls, with transmitter at furtherest end, close to AV, TV & WIFI (i.e. interference sources) and receiver roaming throughout the house in different rooms, microwave oven also in the mix. No packets lost or dropped so far, but more testing needing to be done.
  • Outdoor Range – Untested, but from indoor tests I would guess 150-250m easily.
    Update:
    Outdoor Test Results here
  • Antenna – Simple 8cm whip (i.e. a single piece of wire), these RF units are matched to 50Ohm so an SMA antenna could be used
  • Sync to 1/125 – in theory it could easily sync upto 1/1000 as the Rf modules are capable of the bitrate necessary, however range will be affected by the higher the speed, also the processor / oscillator arrangement may need to be revised to handle the higher interrupt rate to process data, i.e may need to use a chip with onboard SPI support etc.
  • Can act as Master or Slave.
  • Can be programmed to use any frequency from 902Mhz to 928Mhz (using the 915Mhz module), using the 433Mhz Module will allow similar channels.
  • Can be triggered by a contact closure on Trigger Input. i.e. from camera
  • Can be manually triggered by test button.
  • Trigger output is isolated upto 400V, i.e. can safely trigger old stobe units with high voltage on hotshoe / sync terminals
  • Powered by x3 AA batteries. Currently no power management in the firmware.
  • Cost per prototype board AUD$ ~25

Note on the costs:

  • Most components are sourced via local retail outlets so are probably definately more expensive that sourcing elsewhere, i.e. this can be built a lot cheaper i.e. Sub $10 in parts.
  • Some of the components I had lying around at home so I’ve just used market prices in my estimates.
  • No freight was added to construction costs.
  • Some components have minimium order quanties such as the RF module.

Back – Triggr Home Next – Firmware

PIC, SPI and the RFM12

I finally had some time this weekend so I started the hardware verification, mainly getting the PIC and the RFM talking together via SPI, of which I have had no previous experience.  The PIC is receiving it’s clock source from the RFM12 via the CLK pin so I dont need any external Xtals.  The RFM12 default clock rate on reset is 1MHz, so the first challenge is to change the speed of the RFM12 CLK to 10MHz, that way I know the PIC and RFM12 are talking.

 As I’m doing most of my software development with BoostC (http://www.boostc.com), there is a library to use the on-chip MSSP (SPI and I2C) functions of the PIC16f873A, so I thought I’d use this and the on-board SPI functions to handle all SPI communications to the RFM12, after a very frustrating few hours and with my scope monitoring the SPI pins, I finally got the SPI to spit out proper CLK, SDO, and receiving something back on the SDI line after I realised I had not had my interrupts enabled to allow for the non blocking SPI functions DOH!

Now that something was getting out and back in, however none of the commands I sent seem to be getting through, well at least not interpreted as commands by the RFM12.  After much head scratching and re-reading of the RFM12 SPI timing diagrams It occurred to me that the 16bits that make up the the RFM12 commands were getting send through in byte sized chunks, i.e. 8 bits at a time were being sent through, then the next 8 bits were send with a delay in between the two bytes, this makes sense as the internal SPI uses an 8 bit register that gets transmitted, although the library I was using allows from the transmit of any number of bytes via a buffer.  I figured this was the problem and quickly wrote some bit banging SPI functions to test the theory, and straight away the RFM12 responded to my commands and now I have the PIC running at 10MHz.

So now that  all is working in the land of Oz as far as my PIC and RFM12 and talking to each other it’s onto finishing the RFM12 library I started last week and testing that with the RFM12 Demo board so I can test Tx and Rx functionality without having two development boards set up.

RFM12 Demo Board

Open Source Wireless Trigger

After getting back into photography after many many years (and because Digital SLRs are so much cheaper)  I’ve been an avid follower of the stobist website (www.strobist.com) for off camera lighting.  To do this off camera lighting you need a way of triggering the remote stobes, some are wired, some are wireless either RF or Infrared, but obviously the easiest is via wireless.  Well there are many solutions, some cheap (ebay triggers) some expensive (Pocket wizards) and you get what you pay for, the cheaper ones (of which I one a couple) are notorious for false triggers, while the Pocket wizards are way out of my price range.

So in the true spirit of Open Source, of which I’m a huge fan, I’ve proposed an open source wireless trigger for the strobist community.  Initial thread found here – www.flickr.com/groups/strobist/discuss/72157602088957861/  So far the response has been great fantastic.

 I’ve now setup a wiki for a brain dump of ideas http://wiki.everythingrobotics.com and will shortly have a mailing list up and running.

 forums are now also up and running http://forums.everythingrobotics.com  hopefully these will eventually fall under the strobist.com domain

BuzzBot – Initial Testing

I started testing last night after finishing initial motor and wheel sensor hookups to my development board.  Couple of minor issues so far. 

Issues 

I’m using a pic based board from Modtronics. These are great and reasonably priced, along with the prototyping daughterboard.  However I have since found that the onboard RS232 is not on the same pin as the PICAXE28X, so I had to wire in an additional programming circuit, no biggy.  The other issues so far were the wheel sensors.  From my initial design I had used 10K resistors to bias the output of the opto-transistor, this was not tested and when testing the logic probe was picking things up ok, but not the PICAXE, after much stuffing around and not being able to find my multimeter (it was still packed away from shifting house) I ended up prototyping the same circuit, still had some issues, so I went  hunting for my multimeter and of course it was on the bottom box lol.  Luckily thanks to my trusty multimeter I found that the sensor was just dropping down to 3.8v when interrupted, anyway the solution to the problem was a larger resistor, a 22K on the output, this tested AOK, now getting ttl levels when switched, I also noted that the distance of the sensor to the reflective object (paper in this case) was fairly critical.  I modded the breadboard and then found one side was working fine, the other was not being seen.  Checked and found that the sensor was slightly further away than the one that was working.  So out with the hot glue gun and moved the sensor.  Both side are now giving me a reading on the PICAXE.

Motor Tests 

Next I adapted some motor control routines on Hippys PICAXE site to test my motor circuit.  First I tested without my motor control board plugged in, didnt’ want any smoke to escape, and all logic tested aok, then came big crunch of running motors live.  I can gladly say that all tests worked A1 with no issues.  Motor was tested though a range of 3 speeds in FWD, REV, Turn right FWD , Turn Left FWD, Turn Right Rev, Turn Left Rev, Spin Left and finally Spin Right.  While Testing the FWD and REV, I checked the Pulses from the wheel encoders to see how much the same each drive was over 100 counts from the wheel encoders and it looks like the Right side is slightly stronger than the left, no real surprise here as no two motor/gearbox combinations are the same.  I had thought about this a few days ago and knew that with any differential drive system I will encounter this type of problem.

Motor Control Algorithm 

This then leads me onto my motor control algorithm.  Over the last few days I have been researching PID Controller and their application to motor control for robots, as I want Buzzbot to drive in a straight line, also been looking at fuzzy logic, but I think will give that a miss at the moment.  I have been thinking of using and adaptation of the PI Proportional controller from the book “Robots – Inspiration to Implementation”  however so far I’m not 100% happy with the PWM and the way it is from the code I adapted, while it is very simple and works,  I would really like to use the onboard PWMout routines, but as I mentioned in an earlier entry it is only limited to two PWM and to two physical pins on the PICAXE – Bummer!!!!   I also thought I can use the old PWM command.  This command requires it to be refreshed, much like what is happening in  my test code, except with a agreater resolution, unlike the PWMOUT that runs in the background, perfect I thought!, but when I went to test it I found it’s only available on the 08 series…double Bummer!!!!!  So now may have to look at teh PulseOut command.

While the existing configuration does give me three speeds, the lowest is time critical and I notice that when I add a debug routine the slowest speed does start pulsing, also I don’t think it will give me the fine control needed on each of the wheel speed controllers.  I’m going to have to think about this a bit deeper.

Conclusion 

All in all everything is working as expected, with a couple of minor hicups along the way.  Biggest issue will be PWM resolution and time of my routines.  I think I’ll need to come up with another way of doing what I want simply and without too much additional hardware if I want a closed loop control system.  While I ponder on this I’ll test the IR Sensor circuits.  These I’ll prototype first before I hardwire in.  Once I finish these I can then start working on behaviour.  I would like to look at using some sort of subsumption model, but will look at it deeper as I get closer.

buzzbot -  Motor Testingbuzzbot -  Motor TestingBuzzBot - Modtronics PIC Board

CNC Bug Bites

Over the past number of years, along with my interest in robotics and software development,  I have also had a keen interest in machining and metal work, I suppose ever since I build my first Stuart Turner stationary engine when I was 15 on my fathers Lathe,  BTW my father is also a very keen model engineer/machinist.  So wanting to merge my interests along came CNC.

I don’t own a lather / milling machine, actually I don’t even have a workshop myself (about to change with my new house being built), although I do have access to them when ever I want its still not the same as ducking out to the shed to whip up a part, in general I find making small components and PCBS a bit of a pain, mainly though lack of tools and proper work area.  So Getting back into the robotics/electronics scene after a long break has also re-kindled my interest in the CNC Area, especially for small robotic parts (plastic and aluminium) and one off PCBs (routing and drilling). 

So in my quest for knowledge, I spent the last few nights browsing what’s out there, and was very surprised to see the huge amount of work that people have done.  I must admit it had been a couple of years since I’ve last looked, and there there are some truly amazing machines out there (check out http://www.cnczone.com/).   Anyway I think I want to start off small first with a low cost pcb router/drilling using a 3-axis with a dremal.  Plenty of photos out there with low cost designs and common parts have inspired me to take some steps and begin some design work, so stay tuned.