What's New
The original Zephyrus, now
called Zephyrus 1, was rebuilt using muscles as antagonistic pairs. This
was for greater air supply tolerance and ease of maintenance; it also
allows for higher pressure air and so a heavier load. Zephyrus 1 will
principally be used to test sources off board. Zephyrus 2 on the other
hand will become the live test vehicle.
The original Zephyrus is still used as a workshop demonstrator and has
a leg speed of 4.5 Hz.
General
overview
The perception is that pneumatic
robots are only useful in fixed applications or when tethered to an air
umbilical. Pneumatic robotics are often associated with large compressors
to maintain the steady 3 bar or more that is often needed. This project
aims to challenge that view.
Both stored gas and compressors will be tried. The best solution will
be the one that can give up to 6 bar pressure with 1kg. or less weight.
The consumption of the robot is about 80 cubic cm. per second at 3 bar,
but this can be reduced by simply allowing the robot to walk slower. The
other main consideration is cost. The target is about 900 UK pounds total
sale price which limits the robot to compressors currently on the market.
The current stumbling block is still a lightweight fixed air regulator
to get us from a 50 bar CO2 cylinder to the 6 bar region needed for the
robot operation.
Physical
construction
The frame is plastic construction
using the polymek ® construction kit. These materials are available
in the UK via Unilab®. Another robot, Zephyrus 2, is to be constructed
from metal for greater strength. This allows the use of more powerful
actuators and so extends the load bearing capacity.
The legs have no joints save that to the body which have two degrees of
freedom. The legs are 15mm plastic square beam. Taken together this results
in probably the simplest walker that can be created with all of the legs
active.
There is one area where we are having unforeseen problems. The robot now
goes so fast that vibration has become a problem. We are frantically adding
locking washers as different parts work themselves loose!
Actuators
The actuators used are Shadow
air muscles®, a development of the "McKibben" muscles.
Shadow Air Muscle®
(6mm)
These were chosen due to their
light weight, easy attachment, their ruggedness, high efficiency and high
power over short distances. The 150mm by 6mm muscles were used throughout
because of their much lower cost relative to the larger sizes. Twenty
four muscles are now used. These are arranged in antagonistic pairs, two
pairs to each leg. It is possible to run the legs from three such actuators,
but with less strength and with more complicated valve arrangements. Larger
diameter muscles (180 by 8) will be used in the next version of the robot,
due to the higher load, also eight legs instead of six will be used. The
aim is to achieve a 3kg. load. The actuators are mounted as a lever action
3cm. from the R2 pivot joint on the body. They are controlled
by five 2 way "Isonic"
5 V valves available in the UK. from the Engineering
research council. In order to simplify construction a single block
of five 2-way valves have be used. These are driven from ULN2803 drivers
which pull the valves low from a 12V rail. The 12V rail is tapped and
a regulator used to generate 6V for the other electronics. If you have
any information on similar performance valves, but drawing less current,
an e-mail would be greatly
appreciated. The antagonistic pair approach has several advantages to
the elastic band returns used earlier...
- Greater tolerance
to air pressure
- Ability to walk
backward
- Greater stability
is now achieved as muscles pull hardest when extended which was previously
the least stable point.
- Ability to use
higher pressures and thus increase speed (now 6 bar).
- Greater tolerance
to a muscle failure.
The five 2-way valves are connected
as follows...
1. Raises and lowers opposite
triangles.
2. Controls left side middle leg, forward and backward motion.
3. Controls right middle leg, forward and backward motion.
4.Moves forward and aft left legs forward and back.
5. Moves forward and aft right legs forward and back.
All valves exhaust directly
into the atmosphere.
The tip of the leg carves out a diamond with respect to the body. If the
up/down 2-way valve is replaced by a pair of 3/2 valves then a distorted
hexagon is produced. Much smoother. If a pair of 2-way valves are used,
then the ideal clipped triangle can be produced, greatly smoothing the
rather jerky motion. This will be one of the modifications in the next
version of the vehicle.
At present the step time is 110ms for a 12cm. step on one side. i.e. 220ms
for full step giving approx. 50cm/sec. or 1.2mph. The legs need to be
properly tensioned as they thrash about somewhat at present. A 15cm. Step
at a slightly higher speed is then possible for a speed of 1.6mph.
The next version will have several improvements.
The walking gait is a simple double quadruped. This has the advantage
over the tripods of grater stability. The stable area for the centre of
gravity to be in is twice the size of the tripod one. Also, the tripod
gait results in half of the total weight of the robot being placed on
the middle leg during a half cycle. This situation is prevented here.
Also it increases the total carrying capacity by a third.
Electronics
The on board computer is a
Stamp II controller chip.
This processor has 16 input/output lines, 32 bytes of RAM and 2K of EEPROM.
Six of these are used to control the muscles, a further four will be used
to read simple right and left bumpers mounted at the front and rear. It
includes it's own regulator for it's own internal 5V operation.
The isonic valves require 1.3 Watts at 12V which is too much for the controller
to supply directly. The controller outputs are fed to ULN2803A drivers
that pull the 12V line low through the valve. Sensor inputs require no
additional circuitry beyond that recommended in the controller manual.
Sensors: Whisker or
bump sensors are due to be mounted front and rear. No other sensors are
planned as this is rather off topic. The Shadow project has a habit of
recycling old robots, however, so this may change. Switches may be added
to the feet to give feedback of the gait. The ideal would be electronic
stops, making the robots behaviour robust with respect to air supply fluctuations.
Energy
Sources
The energy source is the raison
d'être of the Zephyrus project. Several suggestions have been tried.
Here are some of the conclusions...
1.Air can results
2.Gas cylinders
3.Small compressors
4.Exotic suggestions
Air can results
The first attempt of gas supply
was a liquid air canister. This was rather unsatisfactory due to the tendency
of the can to freeze if a large continuous draw of air was made. This
prevents further evaporation. Using 15 bar "Air duster" style
cans, this can take as little as ten seconds when drawing 80cc per sec
at 3.5 bar. A similar try was air brush cans available from hobby shops.
Unfortunately these also have
a freezing tendency. Range is about 2 metres! If the air can is warmed
in a bucket of water, the robot will run for 10 minutes. at 0.6 m/sec.
Clearly the robot cannot carry around a bucket of water. Reducing the
flow rate is the core of this problem. With the greater carrying capacity
of Zephyrus 2 it is hoped to carry four such cans. Zephyrus 2 uses at
least twice the air of Zephyrus 1, but by lowering the speed some sort
of performance should be possible.
Gas cylinders
Gas cylinder options are many.
The gas can be stored as liquid or solid. Storing as a gas seems either
too clumsy due to the amount needed or too dangerous with a high pressure.
Candidates are...
Solid carbon dioxide
Acts as an insulator preventing further evaporation.
Liquid carbon dioxide
Requires a 50bar storage cylinder, but fast.
Clean air
Provides low cost, but needs a high pressure cylinder.
At the group we have an 1800
litre, 50 bar CO2 cylinder. It only costs 7 uk pounds to recharge from
BOC. The next experiment will be to continually run Zephyrus 1 until this
cylinder is useless. This crude test will give us our first real data
point for future designs. Note that liquid CO2 gives about five times
the capacity of the gaseous state even though the cylinder can only hold
about 40 percent of it's volume in the liquid state for safety reasons.
A paintball gun cylinder will be built into Zephyrus 2.
Small compressors
A small diaphragm compressor
and battery have been tested. This means that the ultimate energy source
is electrical, thus the problem becomes the old one of batteries. At present
Zephyrus 1 is too weak to carry a full load of batteries for the compressor.
Alterations in the design are being attempted as a stopgap whilst Zephyrus
2 is built. The existing design will then function as a measuring tool
for gas supplies.
Exotic options
I have received many curious
suggestions from the Internet. Unfortunately I have not logged the originators
of these ideas for which I apologise. These include...
Hydrogen peroxide - A bit dangerous
and not much gas produced.
Beer brewing kit - A bit too much of the by-product I think.
Thanks for these and keep 'em
coming. Please Please mail me
if you have any other suggestions or applications for this research.
Mapping
There is currently no map building
strategy. A simple wall following strategy will be implemented initially.
Other (more complex) strategies will be studied later, but I am still
open to any e-mailed ideas. Bear in mind that the Stamp II only has 32
bytes of RAM and 2k of EEPROM.
As the only sensors to be fitted so far planned are bumpers, the robot
will have to follow the wall by repeatedly ramming it.
Upon hitting the wall, the robot will turn and attempt to hit the wall
again. In failing to do so, it will be following the wall. After a timeout,
it will head toward the wall again. The turn angle and time-out depend
on the odomotry errors from the robot movements. If anyone has any better
strategies or references to some, please mail me. Any ideas on simple
sensors that could revolutionise this approach will also receive grovelling
thanks.
© The Shadow Robot
Company 1999 |
![[An icon saying Shadow Zephyrus]](../images/title-text-zephyrus.gif)
