Alex Pittman, Vincent Kertesz, and Lucas West, Dept of Mech
Engineering, University of Ottawa
Goal:
Construct a platform capable of remaining level
in the two axes parallel to the ground using digital sensors,
various electronic components and actuators.
Major Components:
·
Two linear actuators
·
Four tilt switches
·
Double H-bridge integrated circuit
·
Breadboard & DC power supply
Design Process:
When we began the design process we planned to have
a simply constructed platform and base connected by a ball and
socket joint to allow the necessary freedom of movement for the
platform. This was changed to a universal join because it allows
the platform to tilt with much less resistance. This universal
join was fitted to the upper platform and then screwed onto the
aluminum support protruding from the base. This allowed us to
move the join up and down along the support in order to fine
tune its position relative to the linear actuators. The
actuators were placed 25mm away from the central support, which
allowed their 20mm stroke length to be translated into ±45°
tilt. This device was constructed from inexpensive materials
found at any hardware store, with a few parts purchased at a
hobby store (such as the universal join). A power switch was
installed to ensure the device can be shut down without the need
to disconnect the power source.
Initially we planned to use an accelerometer to
measure the platforms inclination; this would involve using an
Arduino board to take those sensor signals and a program to
control the motors based on this data. We ended up choosing a
much more direct, while slightly less accurate method of
orientation sensing based on tilt switches.
These switches contain a ball that can move to
either end of a metal channel. When the ball rolls to one end it
will complete the electrical connection, if it rolls to the
other, this connection will be broken. To fully define an axis,
two tilt switches are required, one for each of the possible
orientation of tilt.
The H-bridge circuit used in this project was
originally meant to be built from basic electronics components,
but using individual transistors and an optical isolator chip
proved ineffective. The transistors would overhead and the
optical isolator was unfortunately burnt out by excessive
current. We ended up using an H-bridge integrated circuit which
significantly simplified our overall electrical design. The
platform was connected to the breadboard using 10 individual
wires, two for each motor, and two for each sensor. The motors
were run on a single 11V DC power supply while the H-bridge and
sensors ran at 5V DC. The DC power supply could easily be
replaced with batteries, but this an unnecessary expense in our
case.
When ordering parts online we chose mercury based
tilt switches, but ended up being ball tilt switches when we
received the order. The signals generated by the two opposing
sensors are processed by an H-Bridge circuit which regulates the
movement of the linear actuator. When the tilt switch adjacent
to the actuator is activated, a positive voltage is run through
the actuator, causing it to extend. When the opposite sensor is
triggered the voltage across the actuator is reversed, causing
it to retract. This system allows the platform to react to tilt,
and keep the platform level in each axis.
The linear actuators were the most
expensive part used in the platform, but they were very
necessary to its overall functionality. Due to another error in
our order we received actuators that ran at two different
voltages, one at 6V and one at 12V. This is apparent in their
movements. The 12V motor moved much more slowly than its 6V
counterpart at the voltage we used. But this minor complication
didn’t have any major repercussions on the overall functioning
of the platform.
Possible Improvements:
The tilt sensors that were purchased are not
especially sensitive and cause the platform to react sluggishly,
if at all. These could be replaced by either a different ball
tilt switch or a more expensive mercury tilt switch.
The actuators used were an expensive option that
simplified the design of the platform. These could be replaced
by a system using two servo motors and a micro-controller. This
change would lower the overall cost while increasing the
complexity of the project.
The actuators are connected using a pin type
connector that tends to be stressed when the other actuator
tilts the platform to either of its extreme positions. The
connection is loose enough to allow movement, but would be
susceptible to fatigue over a long period of time. This could be
fixed by either using a different type of connector, or by
simply making the connection lose enough to ensure no stress is
induced.
