RoboClock, A Stepper Motor Driven
AVR Controlled Clock
This
is my third AVR based clock. It's not quite finished
yet and still half on the breadboard. It is a digital clock with a true
analogue readout. The hands are driven by two individual stepper motors
which allow me to place them anywhere around the face of the clock with 0.5
degree precision. At the push of a button this clock will show on the same
face what the current room temperature is and also by pushing another button,
what the humidity is. It does this by changing the numbers around the face
to range from 0 to 9 starting with zero at the twelve o'clock position and
blanking the 10 o'clock display. It then moves the hands
accordingly. Say for example it is 18 degrees, it will move the hour
hand to the 1 o'clock position and the minute hand to the 8
o'clock. The 11 o'clock display changes to a 'C symbol. The same is
true for displaying humidity, except the 11 o'clock display changes to 'rh' to
indicate relative humidity which is read off the hands in a
percentage.
I got
the idea for this clock after seeing a Tissot t-touch watch that does
pretty much the same thing with its hands.
I thought it would make an interesting project to have a play with
driving stepper motors and also positioning them accurately whilst also having to build some
mechanical parts and needing sensors for feedback. An ideal starting point for
getting into robotics, this clock realizes some of the challenges that
await me in that field.
The Clock
consists of three main parts:
1) The mechanics for moving the hands. 2) The main control electronics for
time keeping, measuring the environment, and driving the motors. 3) The electronics for driving the 7
segment displays behind the smoked plexiglass face.
The
first thing I started work on was the drive mechanism.
The Drive Mechanism
I found
some excellent small stepper
motors to use. They are small 18 degree per step motors
out of a laser printer laser unit. They came
with a little brass worm drive crimped on the output shaft which I filed off and replaced with a
gear. The first prototype drive I made was pretty much a single reduction
direct drive. using a 3:1 reduction with gearing I could get a 6 degree per step
resolution. 6 degrees is how much a hand must move on a clock face to
shift 1 minute. The shafts
are made of brass tubing which fits nicely into itself.
These shafts are made by
K&S
Engineering of Chicago. I bought them at the local hobby shop. I got three sizes,
the smallest for the minute hand, next size up for the hour hand, and
next one up for bushings and spacers. Fitted together co-axially it makes a
nice drive for the hands. However as is generally the case with prototypes,
design flaws are highlighted and one major flaw I had was that,
because my clock hands were made of brass strip, the clock worked quite well lying flat,
but as soon as I held it up, the little steppers didn't have enough lock on
their own to hold the hands in place and both hands drooped to the 6 o'clock
position. This could be counter acted by applying current to the motors at
all times and therefore locking the motor armatures in place, but this created heat
in the motors and drive electronics, plus required constant current draw. So
the search went on to create a better mechanism.
This search
didn't last long as I found what I needed in the form of a worm drive gear. The
gears and worm drives also came out of a copier. (handy that, working in the
copier industry) I replaced the gears on the motors with the worm gears and mounted the motors so that these
worms gears are engaged with gears on two co-axially mounted shafts. The ratio of the worm drive
to the gear is 35:1. This works out very nicely as it gives me 0.5
degrees per step resolution which means I need to step 12 times for one minutes
worth of hand movement (6 degrees) and every minute
worth of movement the hour hand needs to move 0.5 of a degree! The steppers
are mounted one on either side of the central shaft. Having a worm drive
meant that the load of the hands was no longer an issue, so now power is only
needed to shift the
hands. This is only a short burst every minute
Upon power up
the controller has no means of knowing where the hands are, so I fitted two
positioning flags that pass through photointerupters. These can be seen in
the above picture just below and above the right motor. The
photointerupters are standard Omron equivalent types. The flag breaks the
light beam as it passes through. The first task the processor has after
initializing the display is to move the hands so they are definitely clear
of the sensors. It then rotates the minute hands around the clock
until its sensor is activated, followed by the hour hand. The
processor now knows where the hands are and they are moved to the 12
o'clock position and the 'time adjust' routine is entered.
The
Display Controller
The display
controller is run by a AT902313. Its job is to display the correct
information on the 15 seven segment LED displays. I originally started
with a system that basically had a separate memory for each display by using a
data latch (74LS373) and having it controlled by the main processor. I had
it configured in a addressable bus system. The same data port on the main
processor was used to drive the motors and displays and a separate port for
addressing. The problem with this again was current consumption as each
display was on at all times and each one had its own data latch. A much
better way was to multiplex the displays as most displays are. I stumbled
across the new 'Charlieplexing' method on the web but after reading the
articles about that I decided that that can be a future project when I am a bit
more clued up :-). I decided to use a separate controller for the
multiplexing of the displays so that there wouldn't be a flicker when the main
controller is doing motor or ADC tasks. The two controllers talk to each
other using a logic level two wire Rs232 link. Each time I want
something new on the display I just send a string out through the serial
connection and the display controller handles the rest. All segments on
the displays are individually addressable, as in, I am not limited to
numbers. See the schematic for the display board here - DisplaySchematic.jpg
I was keen to see what the whole thing looked like
mounted up behind the smoked plexiglass face so I etched my first ever double
sided board. The diagrams and board layout were done with
Eagle. Many attempts were made to try and fit it on a single sided board
but in the end we settled for a double sided board. The track layout was
printed onto the board using "Press and Peel"
film fed through a laser printer and ironed onto
the copper clad board. Lining the two sides up was a bit of a challenge
but I managed to do it and etched my first ever computer routed
PCB. The board with the "Press n Peel" tracks
applied and
the board in the etching tank
The Main Controller
The main controller
consists of a ATmega16 running of its own internal 8MHz clock, It has connected
to its ADC ports the temperature sensor (LM35) and a humidity sensor (A TDK
CHS-MSS). There's 3 buttons for controlling the clock, and the inputs for
the hand sensors. The Motor drive is done through a M54585 driver chip
from Mitsubishi. It has its own dedicated port and both motors run off the
same IC, the low 4 bits for the minutes and the high 4 bits for the hour.
Connected as well is the 32.768 kHz crystal for timekeeping. I still
haven't decided what I am going to use for control of the clock, ideas so far
have been infrared, sound, touch, proximity. I am also contemplating
having a pendulum and maybe an hourly chime.
