Author: Shaun

Just got the servos in the arms and head working. Gives him a bit of character now he looks where he’s going 😊

Lance just finished printing Wall-e’s head and arms and I couldn’t resist a little portrait. His tracks are out for a refit; attaching grippy TPU treads onto the slippery PLA.

Wall-e wouldn’t be Wall-e without his screen; it’s a critical part of his character!

We’ve had the screen working with test programs over SPI for a while but one of our voltage sensors was a dud. That meant we hadn’t been able to get the iconic “CHARGE LEVEL” screen working in full.

Today I swapped out the voltage sensor and added a screen update loop to the code.

Then all of a sudden there’s Wall-e! He feels real now!

Since we’re using unprotected LiPo batteries, which would be seriously explosively damaged by over-discharge, we’ve worked some I2C voltage and power monitors into our bot this year.

We’re using these INA219 boards in-line with the battery cables to measure  voltage, current and power.

Out of the box, the sensors can read the bus voltage (i.e. potential difference between ground and the IN- connection).  To get them to read current and power, you need to set a configuration value in one of the registers.

One gotcha we hit was that the bus voltage register is not “right aligned”, some of the low-order bits are used for status flags so you have to take the voltage reading and shift the value 3 bits to the right to extract the voltage and then multiple by 4mV to scale it.

With that out of the way and the calibration register programmed, we now have sensible-looking readings from the battery pack that is powering the Pi:

A: 7.95V <nil> A: 0.459A <nil> A: 3.615W <nil>
A: 7.94V <nil> A: 0.431A <nil> A: 3.420W <nil>
A: 7.97V <nil> A: 0.424A <nil> A: 3.339W <nil>

and, we should be able to alarm if the voltage of the pack drops too low.  (We have a two-cell pack for the Pi, so anything less than 6V would mean that our pack would be damaged.)

Since we had some strange power-related gremlins last year, we split the motor and Pi power so that the motors are powered by a completely separate battery pack.  That means that we have two INA219s; one for each pack.

Wall-e wouldn’t be complete without a screen.  We’re using this  128×128 colour OLED screen that works with the PiOLED kernel driver.

After enabling SPI using raspi-config and wiring it up to the SPI bus and a couple of GPIOs needed to access its reset and data/command pins:

we were able to get it working with the fbtft driver, which exposes the screen as a standard framebuffer device.

I hadn’t worked with the framebuffer before but it turned out to be fairly simple to use.  Basically, it exposes the screen as a special type of file; if you open that file and write a couple of bytes to it, it updates a pixel on the screen and then moves the cursor to the next pixel.  Once you’ve written 128 pixels, it moves to the next line.  You can use the seek operation to move the cursor to a different place in the file, which is the same as moving the cursor to a different place on screen.

This particular screen supports 16-bit colour, with 5 bits for red, 6 bits for green and 5 for blue, so the process for writing a colour to the screen is something like this:

• Calculate your red, green and blue intensity.
• Scale red and blue to the range 0-31 (i.e. 5 bits of precision)
• Scale green to 0-63 (i.e. 6 bits).
• Pack the bits into a 16 bits: rrrrrggggggbbbbb and then break the 16-bits up into two bytes: rrrrrggg and gggbbbbb
• Write those two bytes to the address of the pixel; first the gggbbbbb and then the rrrrrggg byte.

Since we’re writing our code in golang, I searched around for a golang drawing library and found the gg library.
As a prototype, I used that to draw a mock-up of Wall-e’s screen and then scanned the resulting gg Image, extracting the pixels and writing them to the frame buffer in the 16-bit format:

The code for the above looks like this:

func drawOnScreen() {
	// Open the frame buffer.
	f, err := os.OpenFile("/dev/fb1", os.O_RDWR, 0666)
	if err != nil {
		panic(err)
	}

	// Loop, simulating a change to battery charge every half second.
	charge := 0.0
	for range time.NewTicker(500 * time.Millisecond).C {
		// Create a drawing context of the right size
		const S = 128
		dc := gg.NewContext(S, S) 
		dc.SetRGBA(1, 0.9, 0, 1) // Yellow

		// Get the current heading
		headingLock.Lock()
		j := headingEstimate
		headingLock.Unlock()

		// Move the current origin over to the right.
		dc.Push()
		dc.Translate(60, 5)
		dc.DrawString("CHARGE LVL", 0, 10)

		// Draw the larger power bar at the bottom. Colour depends on charge level.
		if charge < 0.1 {
			dc.SetRGBA(1, 0.2, 0, 1)
			dc.Push()
			dc.Translate(14, 80)
			DrawWarnign(dc)
			dc.Pop()
		}

		dc.DrawRectangle(36, 70, 30, 10)

		for n := 2; n < 13; n++ { if charge >= (float64(n) / 13) {
				dc.DrawRectangle(38, 75-float64(n)*5, 26, 3)
			}
		}

		dc.Fill()

		dc.DrawString(fmt.Sprintf("%.1fv", 11.4+charge), 33, 93)

		dc.SetRGBA(1, 0.9, 0, 1)

		// Draw the compass
		dc.Translate(14, 30)
		dc.Rotate(gg.Radians(j))
		dc.Scale(0.5, 1.0)
		dc.DrawRegularPolygon(3, 0, 0, 14, 0)
		dc.Fill()

		dc.Pop()

		charge += 0.1
		if charge > 1 {
			charge = 0
		}

		// Copy the colours over to the frame buffer.
		var buf [128 * 128 * 2]byte
		for y := 0; y < S; y++ {
			for x := 0; x < S; x++ { c := dc.Image().At(x, y) r, g, b, _ := c.RGBA() // 16-bit pre-multiplied rb := byte(r >> (16 - 5))
				gb := byte(g >> (16 - 6)) // Green has 6 bits
				bb := byte(b >> (16 - 5))

				buf[(127-y)*2+(x)*128*2+1] = (rb << 3) | (gb >> 3)
				buf[(127-y)*2+(x)*128*2] = bb | (gb << 5)
			}
		}
		_, err = f.Seek(0, 0)
		if err != nil {
			panic(err)
		}

		lock.Lock()
		_, err = f.Write(buf[:])
		lock.Unlock()
		if err != nil {
			panic(err)
		}
	}
}

As mentioned in my previous post, this year we needed (an excuse) to learn KiCad and build a custom PCB.  Thankfully, we did succeed in soldering it up , despite the tiny pitch on some of the components.

The PCB dives into a few parts.  I expect you’ll all recognise the Pi header in the top left.  Above that, in yellow on the annotated image, we have the SPI peripherals: the screen and the IMU (which we use mainly for the gyroscope).

Below the header, in pink, we have the Parallax propeller chip, a fast microcontroller that we use to decode the signals from the motors.   Each motor can put out 200k pulses per second, which isn’t really possible to handle from the GPIO pins because Linux can’t really handle that many interrupts per second.

To the right, in yellow, we have connectors for the “noisy” off-board components.  These sit over their own ground plane, so that, if we want to, we can drive them from a completely isolated power supply. From top to bottom:

• “noisy” 5v power
• motor driver control 1
• motor encoder 1
• motor driver control 2
• motor encoder 2
• servo controller
• 2 x power monitors

To bridge the gap between the microcontroller and the noisy world of the motors, (in red) we have a pair of ISO7742 chips.  These provide two input and two output signals, which are level shifted from 3.3v to 5v and are  isolated through an internal capacitor.  Unlike an optoisolator, they were super-simple to use, requiring 3.3v and 5v power and grounds, a couple of decoupling capacitors and some pull-ups on their enable pins.

Similarly, below that, we have an isolated i2c line for driving the servo board (which runs from the “noisy” 5v power supply.

In the bottom left (in green) we have 6 connectors for optical time-of-flight sensors.

The time of flight sensors, Propeller, servo controller and voltage monitors are all i2c controlled, which poses a couple of problems:

• i2c busses tend to become unstable with more than a handful of devices (because each device adds capacitance to the bus, making it harder for any device to drive the bus)
• we have no control over the addresses of many of the devices; for example, all the time-of-flight sensors use the same address.

To address those problems, we included an i2c multiplexer in the design (to the left of the Propeller), allowing us to switch any combination of devices on and off the bus.

Despite having very little space to play with, we were able to squeeze in a bit of prototyping area, which we’ve used to address errata.  For example, I found that I’d missed a couple of pull-ups on the i2c port that the propeller was attached to.  A bit of thin kynar wire to the rescue: