I wrote about Phase 02 in September 2013 and listed goals for the Phase 03.
Most of the goals are implemented and finally it is a joy to remote drive the car.
I attached a wide angle lens to the existing camera and that made a huge difference. The camera crops a bit from the 180° angle but clearly the wider the better. I also made my first 3D print to better attach the camera to the servos.
Tegra 3 based Ouya was replaced with a Tegra K1 based Jetson TK1. That made it easy to stream a low latency H264 video over the network. Due to USB 2.0 and network limitations, the best quality video stream that can be enabled from the controller application is 800×600 2 Mbps.
The Microsoft Lifecam decreases the FPS in low light conditions so I added some V4L2 controls. The brightness is set to minimum to get the maximum FPS. In addition to that, the controller application now has manual focus and manual zoom.
The latency is low enough to drive over a long distance. In local wired network, the application latency (from the controller application to the slave application and back to the controller application) is 1-2 ms. Over my local WIFI it increases to 4 ms. In the video above the car was connected to an LTE network and the communication was routed over a distance of 800 km (the relay is in Stockholm and I live near Helsinki). The latency was around 60-80 ms and it did not introduce any noticeable delay. A friend of mine even drove the car from Gold Coast (that is in Australia, almost 15000 km away!). The latency was around 450 ms and while the delay was obvious, he was still able to drive it.
I also measured the actual visual delay, i.e. how long it takes for the controller application to show what the camera sees (“photon to display”). I did that by taping a led to the camera and using an external microcontroller with two light sensors. One sensor was taped to the led on the camera and the other one was taped to the monitor showing the controller application. I then measured the difference of the two sensors.
The visual latency was about 105 ms with 1 ms network latency. The camera is supposed to be 30 FPS so that can introduce a maximum of 33 ms of latency and my monitor is 60 Hz, so it can introduce a 16 ms latency. On average something else still introduced a 80 ms latency. That 80 ms may be the sum of getting the video stream from the USB camera to the Jetson, encoding it, sending it, buffering a frame, decoding it and then finally showing it. While it might be possible to get the latency down, it is already low enough for remote driving within a 1000km range :)
The project will never end and there are already some goals for the Phase 04:
AHRS based car orientation visualisation
60 FPS (stereo?) camera
Control the webcam based on driver’s head orientation
In the Part I I described my real project with the radio: a wireless power consumption meter. But it is supposed to be a low power MCU so why not run it indefinitely with a renewable power source?
Solar power is the easiest form of renewable energy and I decided to try it out. That is of course not an option in the dark closet where the smart electricity meter is so I ended up designing a modular solar powered wireless soil moisture sensor.
Measuring the moisture of the soil is only a secondary objective. The real goal is to see if I can keep the radio running continuously throughout the Finnish winter. In December 2013 we had a total of 24 minutes of sunshine during a period of 18 days, so having solar power available should not be taken for granted.
Another issue is the temperature as common batteries must not be charged below zero degrees Celsius. I decided to go with a large solar panel from Sparkfun and 10 F super capacitors from Digikey. The panel provides a maximum of 9.15V which nicely matches four 2.7V super capacitors in series.
The voltage of solar panels decreases quickly if too much power is drawn from them. All larger solar panel systems use maximum power point tracking (MPPT) which tracks the voltage and limits the power draw if the voltage drops. I tried to find MPPT solutions for small systems but did not find anything suitable for this project. Adafruit’s Solar Lithium Ion/Polymer charger is based on MCP73871 but that seems to be designed for batteries alone. There is also bq25570 from TI which looks otherwise very well suited for this kind of project but it seems to be designed for even smaller solar panels.
The final software will make the measurements once an hour and sleep the rest of the time in the deepest sleep mode. The power consumption in the sleep is only a few micro-amps for the whole thing.
I made some initial tests with a five minute interval behind glass windows and without direct sunshine. During the last day I tried to keep the panel pointed directly to the sun, when possible and that clearly shows in the graph below.
For now I have just printed out the values and created a graph about the measurements. For the electricity measurements I was using Sparkfun’s Phant on my own server but that seemed to lack features and stability. Currently I am testing ThingSpeak and letting them handle the hosting. It is open source so I could move it to my own server as well. So far the ThingSpeak looks good.
Based on the test with shorter measurement interval, I am hopeful about the radio being able to run through the long and dark winter in Finland.
Electricity is expensive and being low power is environmentally friendly. And what is more motivating than seeing the total power consumption in real time? There are plenty of commercial products out there already but will they give you the raw data so that you can plot nice graphics? Not that many. And of course self made is always .. self made.
Lately in Finland the old electricity meters have been replaced with smart meters and they seem to have leds showing the power consumption. The model I have has a led that blinks once for every consumed watt-hour so it is easy and safe to calculate the power consumption by counting the blinks.
There is no power plug that I could use for the radio board so it is running on two AA batteries. I put everything in a small plastic box. Below is an image of the box before I placed it next to the smart meter.
The radio part of the CC430 is turned completely off when not sending and the MCU part is sleeping. The only part running is the comparator that compares the photo resistor’s output to a predefined thresholds. If the threshold is exceeded an interrupt fires and I count the interrupts. Once a minute I reset the counter, turn on the radio and send the count over the air. Running the comparator takes some 200uA and I think I might be able to just bluntly interpret the photo resistor’s output as GPIO. That should drop the consumption below 10uA. Even with the higher consumption it has been running well for a few months now as can be seen in the graph below.
Having to replace batteries is inconvenient and another project of mine will be running outside so I can use a solar panel. More on that in Part II.