Facilius Est Multa Facere Quam Diu

This is another article in the series about the KR01 robot.

The title translates as “It is easier to do many things than one thing consecutively“, attributed to Quintilian, a Roman educator about two thousand years ago. It sounds curiously like a motto for either multi-tasking or multi-threaded processing. But also for how I’ve approached designing and building the KR01 robot.

One thing I’ve learned about building a robot is that, at least for me, the hardware and software is ever-changing. I guess that’s what makes the journey enjoyable. In my last post I ended up with too much philosophising and not enough about the robot, so this one makes up for that and provides an update of where things are at right now.

But before we get into the hardware and software I thought to mention that I’ve been quite happily welcomed into the weekly videoconferences of the Dallas Personal Robotic Group (DPRG) and about a week ago did a presentation to them about the KR01:

The DPRG is one of the longest-standing and most experienced personal robotics club in the US, with a great deal of experience across many aspects of robotics. They’re also some very friendly folks, and I’ve really been enjoying chatting with them. New friends!


So… the biggest issue with the KR01 was imbalance. There was simply no room on the chassis for the big, heavy Makita 3.0Ah 18 volt power tool battery so I’d, at least temporarily, hung it off the back on a small perforated aluminum plate.

[You can click on any of the images on this page for a larger view.]

Earlier design: the black platform at the aft end held the Makita battery

The KR01 without a battery weighed 1.9 kilograms (4 lbs 3oz), so at 770 grams (1 lb 11 oz) the Makita battery and its holder comprised about 40% of the the robot’s total weight. By comparison, my little KRZ01 robot weighs 160 grams, including the battery. Since that photo was taken the robot has gained a bit of weight, now up to 2.1 kg. But that imbalance remained.

With the battery hanging off the back, when trying to spin in place the KR01 would typically sit on one back wheel and rotate around that wheel, but which wheel was almost arbitrary. I’d kinda knew something like this might happen but I was willing to keep moving forward on other parts of the robot (“facilius est…“), because fixing that problem meant making some big hardware changes.

Since I’d gotten to the point where I was actually testing the robot’s movement, I finally needed to bite the bullet and bought another piece of 3mm Delrin plastic. This time I positioned the battery as close to the physical center of gravity of the robot as possible. Then I spent a lot of time reorganising where things fit, as well as finally adding all of the sensors I’d been planning. I think I might have gone overboard a bit. The current design is shown below.

The redesigned KR01 with space for the battery closer to the center of gravity

The copper shielding is my attempt at cutting down on the amount of high-pitched ambient noise put out by the speaker (hidden underneath the front Breakout Garden Mini, next to the servo). This didn’t seem to make much difference but it looks kinda cool and a bit NASA-like so I’ll leave it for now. Yes, the KR01 can now beep, bark, and make cricket sounds. It also has a small 240 x 240 pixel display screen (visible at top center) and two wire feelers to theoretically protect the upper part of the robot, basically an emergency stop. I have no idea how well that will work. The inside of my house is pretty hazardous for a small defenseless robot.

Side view of the KR01 robot’s chassis, showing the power/enable switches and a Samsung 250gb SSD drive lodged underneath

Earlier versions of the robot had a 15cm range infrared sensor for the center, which being digital, replied with a yes or no. It worked as advertised, but 15cm wasn’t enough distance to keep the robot from running into things, even at half speed, so I’ve since replaced it with a longer range analog infrared sensor (the long horizontal black thing in the cutout in the plastic bumper, shown below) that I’ve coded to react to two separate ranges, “short” (less than 40cm) and “long” (triggered at about 52 cm). This permits the robot to slow down rather than stop when it gets within the longer range of an object.

The robot currently has a 15 cm range infrared sensor on each side but I’m planning to replace them with a pair of Sharp 10-150cm analog distance sensors, which hopefully will permit some kind of wall-following behaviour. I’m eagerly awaiting another package in the post…

Front view of the KR01 robot’s chassis, with polycarbonate bumper-sensor and infrared distance sensors

The KR01 now sports a variety of sensors from Adafruit, Pimoroni’s Breakout Garden, Pololu and others, including:

  • a servo-mounted 4m ultrasonic sensor, or
  • a servo-mounted Time of Flight (ToF) laser rangefinder with a range of 4m and accuracy of 25mm
  • four Sharp digital 15cm range infrared distance sensors
  • a Sharp analog 80cm infrared ranging sensor
  • an infrared PIR motion detector (for detecting humans and cats)
  • an X-Band Bi-Static Doppler microwave motion detector (for detecting humans and cats through walls)
  • a 9 Degrees of Freedom (DoF) sensor package that includes Euler and Quaternion orientation (3 axis compass), 3 axis gyroscope, angular velocity vector, accelerometer, 3 axis magnetometer, gravity vector and ambient temperature
  • a 6 channel spectrometer
  • two 5×5 RGB LED matrix displays
  • one 11×7 white LED matrix display
  • an HDMI jack for an external monitor (part of the Raspberry Pi)
  • WiFi capability (part of the Raspberry Pi)
  • a microphone
  • a speaker with a 1 watt amplifier

This is all powered by a Makita 18V power tool battery. The clear polycarbonate bumper (inspired by David Anderson’s SR04 robot) has six lever switches, with two wire feelers protecting the upper part of the robot.

All that just for the territory of my lounge. Or maybe my front deck.


So while working on hardware I’ve also been working on the software. I’ve been writing a Behaviour-Based System (BBS) based on a Subsumption Architecture as the operating system for the KR01 in Python. The concept of a BBS is hardly new. Rodney Brooks and his team at MIT were pioneering this area of research back in the 1980s; it’s an entire field of research in its own right. Here’s a few links as a beginning:

The idea with a BBS is that each sensor triggers either a “servo” or a “ballistic” behaviour. Servo Behaviours (AKA “feedback and control systems”) immediately alter the robot’s movement or make a temporary change in its behaviour, such as speeding up or slowing down, in a relatively simple feedback loop. Ballistic Behaviours (AKA “finite state machines”) are small sub-programs that are (theoretically) meant to run from start to completion without interruption. The below video shows a ballistic behaviour that might occur should the robot find itself facing a wall: it backs up, scans the neighbourhood for a place where there’s no barrier, then attempts to drive in that direction. Yes, that is a sonar “ping” you hear.

This video shows a simple obstacle avoidance behaviour.

My understanding (i.e., the way I’m writing the software) is that for every sensor there is an associated servo or ballistic behaviour, and that each of these behaviours are prioritised so that the messages sent by the sensors contend with each other (in the subsumption architecture), the highest priority message being the one that the robot executes. It does this in a 20ms loop, with ballistic behaviours taking over the robot until they are completed or subsumed by a higher-priority ballistic behaviour. It’s the emergent behaviour as a consequence of these programmed behaviours that gives the robot its personality. When the robot has nothing to do it could begin a “cruising around” behaviour, whistle a tune, or go into standby mode awaiting the presence of a cat.

Not that my cat pays much attention to the robot. In the robot-plus-cat experiments that have been performed in our home laboratory he sniffs at the robot a bit and nonchalantly stays out of its way.

He wasn’t fooled for a moment by the barking.

Leave a Reply

Your email address will not be published. Required fields are marked *