Awhile back I’d tested out the PWM3901 Optical Flow Sensor, a Breakout Garden board from Pimoroni. Its a vision sensor that contains a tiny camera, typically used on flying drones. Coupled with a drone’s GPS sensor and Inertial Measurement Unit (IMU), an optical flow sensor assists in navigational control by observing the motion of the ground below it, taking successive pictures and comparing how far an image has moved relative to the previous one. Since the camera itself doesn’t know how far it is from the surface it can only provide relative numbers, not distances in measurement units like feet or meters.
Since the PWM3901’s chip was designed for drones, its camera’s effective range is 80mm to infinity. This means that if used on a ground-based robot its lens must be at least 80mm from the ground, and its view must not be impeded. This can pose a challenge for the design of a robot, whose bottom must either be at least 80mm from the ground or contain a large open area for the sensor, ideally right in the center of the robot. But even then we’d be using the sensor at the extreme lower limit of its range; it’s really tuned for being way up in the sky.
Now it turns out that PixArt, the maker of the sensor chip, also manufactures a very similar sensor called the PAA5100JE-Q that is tuned for working close to the ground, apparently used in service robots and robot vacuum cleaners. Whereas the Pimoroni carrier board is 24 by 24 mm, the PixArt sensor itself is only 6mm square. Depending on your screen resolution the image to the right is roughly the size of the sensor. According to the PAA5100JE datasheet, the chip is advertised as having a tracking speed of up to 45 inches per second and a range of 10-35mm, which seems ideal for hanging from the bottom of a robot. They consider it suitable for use on “carpet, granite, tiles, wooden floors with dry or wet surfaces.”
So I went ahead and sent an email to Pimoroni, suggesting that since the two sensors are very similar it might be possible to develop a new version of their existing carrier board, but using the PAA5100JE.
I discussed the idea of a ground-based optical flow sensor with the DPRG during the weekly video conference and there was a general consensus that this sounded like a great idea, especially for mechanum-wheeled robots where wheel odometry was problematic. Its president Carl is currently building a mechanum-wheeled robot and the availability of such a sensor would potentially enable visual odometry.
I received an email from Pimoroni indicating that they were going to develop a new Breakout Garden board using the PAA5100JE, and in March told me they’d send me one prior to it being released. The photo of the boards being fabricated was quite exciting to see. So I waited patiently for the package to arrive in the post from the UK. In New Zealand I think we’re accustomed to waiting for overseas shipments to arrive.
The New Sensor Arrives
Last Friday a small mysterious package arrived containing the new PAA5100JE-Q Near Optical Flow Sensor. When placed next to the original PMW3901 it’s clear they’re related, the only discernable differences seem to be that the PAA5100JE has a different lens, the illumination LEDs seem a bit larger (though this may not be significant), and the SMT components to the right of the lens have been reorganised. Perhaps the nicest thing is that I didn’t have to wait for a software to be developed as the open source library for the PWM3901 seemed to work just fine.
Ground Testing
So I thought the first thing to do was test the new sensor to see how I might use it for visual odometry. The output is simply an x,y value that is pushed from the sensor when it senses motion. But there are no units to that x,y, nor could there be since its camera doesn’t know how far away the image is. We’ve got to help it a bit.
One of the first surprises was that the optical flow varied quite significantly depending on the surface below it, which seems to be related to the level of surface detail.
 Treated Pine Fence |  Concrete |  Unfinished Birch Ply |
 Hardwood Floor |  Lawn |  Persian Rug |
 Painted Wood |  Tile |  Hardwood Deck |
About a week after the sensor arrived I received an email from Pimoroni stating that their engineers had finished the modifications to the Python library and that it was now available. This uses the same file as the PWM3901 but you import the PAA5100 class instead, e.g.,
from pmw3901 import PAA5100
I updated the library on my Raspberry Pi and changed the import statement in my test file, then went ahead and retested the sensor across my nine test surfaces. It turns out this made a significant difference in the results.
PWM3901 vs. PAA5100JE
It’s clear that the ranges of the PWM3901 (80mm to infinity) and the PAA5100JE (10-35mm) represent two distinct design choices. If your robot’s base is close to the ground, that is, between 10 and 35mm, the latter will work just fine.
If you’re building something like a Mars Rover or some other design where the lowest part of the chassis is above 35mm (making it unsuitable for the PAA5100JE) and greater than 80mm, then the PWM3901 would work better. It’s that span between 35mm and 80mm that may present a design problem: either hang the PAA5100JE lower than the robot’s chassis (reducing ground clearance) or somehow position the PWM3901 up higher on the robot so that it is at least 80mm from the ground. Not ideal in either case.
This blog entry isn’t really finished as I need to do more testing, but the KROS-Core Python library project has been taking up most of my time of late, so I’m publishing it now just to get it out the door (somebody has asked to see it). So I’ll come back to this later when I get a bit more time…