Make it with Micromite
Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller
Part 16: Introduction to the Micromite Robot Buggy
T
his month, we begin a really
fun project – the Micromite Robot
Buggy (MRB). The whole idea
behind this robot is to demonstrate how
to bring together many of the lessons
learnt so far in the Make it with Micromite
(MIWM) series, and combine them into a
fun, highly customisable project. To help
minimise the overall cost of the robot,
three important design criteria were set:
1. Use the MKC as the robot brain
– all aspects of the robot will be
programmable, and under the control
of MMBASIC code.
2. Use the Bluetooth module to provide
a wireless link between the MKC and
your terminal app.
3. Enable plug in of any other MIWM
module from the series to provide extra
features at no additional cost.
The MRB could be regarded as a mobile
MKC; but for the MKC to become mobile,
some additional hardware is needed. For
example, a minimum of two motors, a set
of wheels, and some form of battery power.
This month, we will start discussing
a new module: the robot chassis. This
will not only provide the new hardware
required, but also allow the MKC and
other MIWM modules to be plugged in.
The robot chassis module comprises
two main parts: an acrylic chassis unit
(for attaching the mechanical hardware),
and a daughterboard (for mounting the
electronics). By simply plugging in your
MKC, Bluetooth module and TFT module,
along with a basic test program, you will
create the MRB shown in Fig.1.
Background
The design of the MRB was inspired by the
versatile Pololu Zumo robot. The Zumo
is popular with Arduino users thanks to
the availability of a Zumo shield (which
enables an Arduino to be directly inserted).
The Arduino then controls all aspects of
the robot.
Practical Electronics | May | 2020
Fig.1. The Micromite Robot Buggy (MRB) (front) next to the popular Zumo buggy, on
which the design is based. Note the red acrylic chassis under the stripboard.
The Zumo is a small (100mm × 100mm)
robot that uses a pair of silicone tracks
to provide traction. It is a very modular
design, with all mechanical and electrical
elements available as individual parts.
Therefore, the Zumo seemed like a suitable
robot from which to create a Micromitecontrolled version.
Initially, I considered replacing the
Arduino Zumo shield with an equivalent
Micromite daughterboard, and mount
this onto the existing Zumo chassis. The
Micromite daughterboard should ideally
be a piece of stripboard using throughhole components and modules, making
it a quick project for this series. However,
early prototypes of the Micromite version
quickly highlighted the fact that in order
to physically accommodate the MKC
(and other modules from the MIWM
series), a slightly bigger chassis would
be required. This would mean that we
could no longer use the readily available
Zumo chassis. This wasn’t too much of
an issue because the Zumo chassis could
easily be replaced with an alternative.
Access to a laser cutter meant I could
create a custom acrylic chassis designed
to meet our exact requirements. The end
result is only slightly larger (150mm ×
150mm) and is shown in Fig.2.
Replacing the Zumo chassis with our
own custom chassis meant that two other
matters needed resolving. This is because
the Zumo chassis has appropriately sized
(small) tracks, and also has a built-in
53
Fig.4. The shorter Zumo tracks have 22 toothed-slots designed
to be used with wheels spaced 48mm apart. The MRB uses
the longer 30-toothed tracks allowing the wheel spacing to be
increased to 85mm.
Fig.2. This custom laser-cut acrylic chassis is used instead of the
smaller Zumo chassis.
battery compartment designed to take four AA rechargeable
batteries. So we need to resolve design issues around tracks
and power.
The silicone tracks need to be increased in size to suit our
bigger chassis. In fact, all that is required is to increase the
distance between the wheels (easy), which means slightly longer
tracks. On the Zumo, each silicone track has 22 ‘teeth’ and it
is wrapped around a pair of toothed-wheels (the wheels and
tracks are available as a kit). The track length ultimately defines
the distance between the wheel-centres and on the Zumo this
equates to a spacing of 48mm. Thankfully, there is an alternative
kit available which has tracks containing 30 teeth (30T, see
Fig.3). This simply means that the tracks are slightly longer than
the Zumo version, and they result in the wheels being spaced
85mm apart. This is perfect for our new chassis, so these 30T
tracks are used in our robot. You can see the increased wheel
spacing and the longer tracks in Fig.4.
Power
Now we turn to power for the robot. Initial prototypes were
designed to use a readily available mobile phone USB battery
pack. These can supply the 5V output required to power the
robot; however, an unforeseen problem was that 99% of these
units are designed to automatically switch off if they are not
connected to a ‘load’ (ie, not connected to a phone). The current
consumption of the Micromite falls well below the battery pack’s
Fig.3. The 30T track and toothed wheels are available as a kit
(Pololu #3033).
54
threshold and hence it would automatically switch off after just
a few seconds. All kinds of ‘dummy loads’ were tried (LEDs
worked the best) but then a new issue arose – how to turn the
robot on and off, especially if the battery pack’s on/off button
was inaccessible. The battery pack also needs to be recharged,
requiring access to its charging socket, which means it would
need to be easily removed from the robot. Furthermore, most
battery packs are physically bigger (thicker) than we want, so
the decision was taken to abandon this idea, and instead use a
thin LiPo battery (Fig.5), partnered with a PowerBoost 1000C
module from Adafruit (Fig.6). This excellent module not only
takes the LiPo voltage (typically 3.7V) and boosts it to 5.2V,
it also has a dedicated LiPo charger onboard to recharge the
battery without ever needing to have direct physical access to
it. An enable pin (EN) on the module provides the ability to
switch the 5.2V boosted output on and off. With a little design
imagination we can use a push button (instead of a switch) to
turn the robot power on; and then use program-code to turn the
power off – more on this next month.
So, in summary, by using a custom acrylic chassis, the longer
30T tracks and a LiPo battery (along with the Adafruit booster/
charging module), we can replace the Zumo chassis with our
own bespoke design.
Building-blocks overview
We explained above that to create the MRB, several new parts
(mechanical and electrical) are brought together along with some
existing MIWM modules. The way these building blocks work
together is summarised in Fig.7. To make things a little simpler
to follow, the diagram is laid out to mimic the actual physical
locations on the robot itself.
Fig.5. A typical 2-wire LiPo battery is both thin and lightweight
making it suitable for our robot. Ensure a minimum capacity of
2000mAh, and that the leads are terminated in a 2-way JST
connector as shown here.
Practical Electronics | May | 2020
T FT
Mod ule
Fig.7 shows that the laser-cut acrylic
chassis is used as a baseplate to which
the following items are attached:
Two motors (Fig.8)
Two motor mounts (Fig.9)
Two wheel mounts (Fig.10)
Fig.8. A Pololu micro metal gearmotor.
The prototype uses a 6V, 75:1, LP,
extended shaft (#2209).
T rack 1
W h eel 1B
LiP o
battery
Motor
d rive r
LiP o ch arger/
5V booster
P ower
j ump er
link
W h eel
mount
W h eel
mount
P ower
button
W h eel 1A / 2A : d rivi ng
W h eel 1B / 2B : auxi liary
W h eel 2B
Note that each building block falls into
one of three distinct categories, colour
coded as follows:
New elements associated with the
acrylic chassis are shown in red (mainly
the mechanics of the wheels)
New elements associated with the
daughterboard are shown in blue (all
the new electronics associated with
power circuit, and the motor-driver)
Existing modules from the MIWM series
are shown in grey.
Motor 2
T rack 2
Fig.6. The Adafruit PowerBoost
1000C module contains all the battery
management circuitry allowing a LiPo
battery to power our robot.
Motor 1
W h eel 2A
W h eel 1A
P iezo
US B
B oB
B luetooth
Mod ule
B luetooth
Mod ule
MK C
DM
T erminal
ap p
Fig.7. Block diagram of the Micromite Robot Buggy. The chassis and daughterboard
contain all the new parts, and also allows the MKC, and existing modules, to be plugged in.
Four toothed wheels and two tracks
Power button (Fig.12) (plus pull-down
(Fig.4)
The daughterboard.
resistor and power-link). These control
the EN pin on the Adafruit module
Motor-driver module (Fig.13) (an
H-bridge interface allowing motor
control from the Micromite I/O pins)
Pins and sockets (Fig.14) positioned in
a precise manner making it easy to plug
everything together, as detailed here:
Pins for the MKC and Bluetooth
module to be plugged in (from below)
Sockets for a MIWM module to be
plugged in from above (such as the
TFT module)
A pin and socket pair for each
motor (allows for easy assembly/
disassembly of the daughterboard
from the chassis)
Header pins for attaching the ‘SMD’
modules (Adafruit, motor driver,
USB BoB).
Incidentally, the first prototype of the
MRB had the motors and wheels attached
directly onto the stripboard (ie, mounted
onto the daughterboard). However, there
was too much flexing of the stripboard;
it just didn’t have the required rigidity to
keep the wheels in place and the tracks
suitably tensioned. Hence the all-in-one
chassis/daughterboard/stripboard idea was
abandoned and replaced by the ‘chassis
plus daughterboard’ concept.
The daughterboard is a piece of
stripboard (fixed to the chassis) with the
following items/modules attached:
LiPo booster/charger module (the
Adafruit Powerboost 1000C module)
(Fig.6)
Micro USB BoB (breakout board – see
Fig.11). This provides a conveniently
located USB socket from which to charge
the LiPo (via the Adafruit module)
Fig.9. (left) Micro metal gearmotor (extended) bracket kit (Pololu #1089) and Fig.10. (right)
Two M3 threaded mounting brackets are used to fix the ‘auxiliary’ wheels to the chassis.
Practical Electronics | May | 2020
Comp uter
All of the above will be discussed in more
detail, but you should be able to make
reasonable sense of the block diagram in
Fig.7. Your existing MKC is being upgraded
to having a set of wheels and a battery. The
wheels will need to use four low-power
I/O pins on the Micromite and these are
connected to the input of a motor driver
module. The motor driver then outputs
the higher power required to operate the
motors. The battery circuit also uses an
I/O pin and this is used to switch the robot
power off (with the pushbutton turning
power on). All power functionality is
managed by the LiPo booster/charger
module which simply ensures 5.2V
power is supplied to the robot from the
rechargeable LiPo battery whenever the
robot is turned on. To recharge the LiPo,
a 5V charger can be plugged into the
USB BoB.
55
Fig.11. (above) A USB
BoB is utilised in order to
conveniently locate the
LiPo charging socket.
Fig.12. (right) A miniature
push-to-break button is
used to turn on the LiPo booster and
hence deliver power to the robot.
Motors, wheels and tracks
In Fig.7, you can see a pair of wheels,
a silicone track and a single motor,
positioned on each side of the robot.
The two motors used in our robot are
referred to as ‘micro metal gearmotors’
and as the name implies, they contain a
gearbox (fixed ratio) made from metal –
as shown in Fig.8. The gearbox gives the
motor more torque than a standard motor
– something that is useful in any robot.
The motors operate from 6V DC, applied
to a pair of contacts located on the nongearbox end of the motor (see Fig.15). As
with any standard DC motor, reversing
the voltage will reverse the spin on the
motor. Note that in our design, we are
driving the motor at approximately 5.2V,
which is still enough to provide a decent
amount of torque. Each motor is fixed to
the chassis by means of a motor mount,
as shown in Fig.16. One wheel in each
pair is attached directly to the motor’s
JP1
JP2
JS1
JS2
JP3
JP9
JP4
shaft and this is referred to as the ‘driving’
wheel. The other wheel is referred to as
the ‘auxiliary’ wheel and is free to spin
on a spindle (Fig.17). The spindle simply
screws into a wheel mount (Fig.10), which
in turn is fixed to the chassis. A silicone
track is positioned around the pair of
wheels with enough tension so that when
power is applied to the motor, it turns the
driving wheel, which in turn rotates the
track around the auxiliary wheel (Fig.4).
This setup is repeated on the other side
of the robot to provide the traction for
movement. Driving both motors in the
same direction will result in the robot
moving in one direction, and reversing
the voltage to both motors will move the
robot in the opposite direction. To turn
(or rather, to ‘spin’) the robot, one motor
is driven in one direction, and the other
motor in the opposite direction. Reversing
the voltages will cause the robot to spin
in the opposite direction.
Motor choice
A point to bear in mind is that the quality
of the motor is very important when used
in a project such as this. If you do an online
search for ‘micro metal gearmotor’, you will
see that there are many variants to choose
from, and prices will range from around
£5 per motor, to approximately £25 per
motor. A really good reference guide to
these motors can be found on the Pololu
website, so can I recommend that you
take a look: http://bit.ly/pe-may20-pololu1
Choosing the wrong motor type will
cause problems, so I will cover four highlevel points to help you choose right one:
1. These motors are available in two voltage
options, 6V and 12V. Ignore the 12V
ones – we need the 6V version.
2. Two shaft lengths are available, ‘normal’
(single shaft), and ‘extended’ (dual
shaft). The extended type (see Fig.15)
allow for a shaft encoder disc to be
mounted on the back of the extended
motor shaft (visit: http://bit.ly/pe-may20pololu2). The encoder disc (magnetic or
optical) can then be used with additional
JP5
JS3
JP6
JP7
JP8
Parts list and next month
JS5
56
hardware (a sensor) to provide a stream
of output pulses for each revolution of
the motor. By counting the pulses, the
number of revolutions can be counted
and hence the speed of the motor can
be inferred. This feature is not currently
implemented in our design; however,
we may add it at a later date. There is
only a small price difference between
the normal and extended shaft types,
so we would recommend opting for the
extended type. That said, the normal
type is absolutely fine to use.
3. Four power variants are available: low
(LP), medium (MP), high (HP), and high
with carbon brushes (HPCB). This is
the one parameter that really affects the
price of the motor. We used the cheapest
LP motors in our prototype and they
worked just fine. However, the more
power you can afford, then the more
torque your robot will have. The carbonbrush option is not a requirement.
4. The gear ratio is the final (but most
important) parameter to consider. You
will see figures quoted between 5:1 and
1000:1. The lower the ratio, the faster
your robot will move – however, its
driver shafts will deliver less torque
(or put another way, it won’t be able to
move as much weight). The ideal ratio
for our robot is somewhere between
50:1 and 100:1. In our prototypes we
have typically used 75:1.
The above has given you an idea as to
what can be used, but if you’re a little
overwhelmed with the number of choices,
then I suggest using two: 6V, 75:1, LP or
HP, extended (dual) shaft motors. LP or
HP depends on your budget and/or the
supplier’s availability.
JS4
Fig.14. A complete set of connectors
used in the robot buggy. Ensure you
modify the connectors to match them as
they are shown here.
Fig.13. The motor driver module is based
on the 2-channel DRV8833 H-Bridge IC.
Fig.15. Close-up of the modified 4-way
socket soldered to the motor contacts.
Note the motor contact marked with ‘+’
needs to be to the left when in the motor
mount. Here the ‘extended’ shaft can be
seen protruding out the back of the motor.
That just about wraps it up for this month.
Next month, we will look at assembly and
the electronics involved. In the mean time,
have a look at Table 1 to see a complete
list of all the parts required to build the
robot chassis module. Most of the items are
referenced to the various photos throughout
this article to assist with identification and
Practical Electronics | May | 2020
(a)
(b)
(c)
Fig.16. Fixing the motor and motor-mount bracket (shown here mounted on an off-cut chassis). Left to right: a) Motor gearbox sits into
slots; b) Place nuts into bracket ‘wings’; c) Screw onto chassis from the other side.
assembly. The majority have already been
mentioned already in the text; however,
there are a few other parts required that
will come up in the assembly guide. For
example, some nylon nuts and bolts used for
attaching the daughterboard to the acrylic
chassis. You may already have some of the
items in the list; if not, they should be easy
Questions? Please email Phil at:
contactus<at>micromite.org
enough to source through the Internet.
Online UK suppliers such as Cool
Components, PiHut, Pimoroni, Hobby
Components, and Rapid Electronics are
worth visiting for the items that may
be more specific (I have no affiliation
with any of these – but I am a regular
customer of them all thanks to the
useful product range they stock). If
you prefer a one-stop-shop, then as
usual, micromite.org will offer various
options including the custom laser-cut
acrylic chassis.
Table 1: Parts list for the robot chassis module. You will also need a length of wire
to complete all the all wire-links (1m is enough)
Qty
Fig
1
Part No
Fig.17. CAD drawing showing how to attach
the ‘auxiliary’ wheel with the supplied spindle,
washer, and nut. (Note: in our design, the
generic blue ‘robot chassis’ shown here is
actually the motor mount – see Fig.10.)
Description
Comments
Fig.2
Acrylic chassis
Laser-cut (from micromite.org)
1
Fig.3
30T track & wheel kit
Tracks, wheels, spindles, washers + nuts
(Pololu #3033)
1
Fig.5
LiPo battery
Minimum 2000mAh (with 2-way JST connector)
1
Fig.6
LiPo charger/booster
Adafruit PowerBoost 1000C
2
Fig.8
Micro metal gearmotor
various options - see text
2
Fig.9
Motor mount kit
Brackets, screws + nuts (Pololu #1089)
2
Fig.10
Wheel mount
M3, metal (from micromite.org)
MOD3
4
12mm M3 nylon screw
8
M3 nylon nut
1
Stripboard
50 holes x 36 tracks (typically 95mm x 127mm)
1
Fig.11
MOD2
micro USB BoB
(from micromite.org)
1
Fig.12
S1
Push-to-break button
Multicomp MC8MS8P1B06M7QES (from
micromite.org)
R1
10k
1
Fig.13
MOD1
Motor-driver module
1
Fig.14
JP8
2-way pin strip + jumper-link
2
Fig.14
JP1, JP2
4-way pin strip (modified)
For motors
1
Fig.14
JP7
5-way pin strip
For USB BoB
2
Fig.14
JP3, JP4
6-way pin strip
For motor driver module
1
Fig.14
JP9
8-way pin strip (modified)
For LiPo charger/booster module
1
Fig.14
JP5
13-way pin strip (modified)
For connecting MKC/BT module
1
Fig.14
JP6
14-way pin strip (modified)
For connecting MKC/BT module
2
Fig.14
JS1, JS2
4-way socket (modified)
For motors
2
Fig.14
JS3, JS4
6-way socket
For plugging in MIWM modules
1
Fig.14
JS5
14-way socket
For plugging in MIWM modules
1
Practical Electronics | May | 2020
pull-down resistor
DRV8833 (from micromite.org)
57
