you will find this page in PDF-format in the movies/papers section
A robot, which
1. - can reach every freely accessible place in a building;
2. - is not longer than 50 cm;
3. - needs as few actuators and sensors as possible;
4. - can be controlled by simple controll-structures.
A robot which moves freely in a building has to be adapted to an environment made for humans. On its way it may encounter small obstacles ( up to 4 cm of hight, e.g. door steps, sills etc.) and stairs. These stairs often have nosings or ledges and sometimes open risers. The slope of indoor stairs can vary between 25° and 42°. Sometimes you will find in residental buildings even steeper stairs, especially spiral stairs. In public buildings stairs often have a rise s of 17cm and a run a of 29cm (slope approx. 30°)
a = run s = rise u = nosing/ledge a = limit of safe stairclimbing for humans
b = maximum slope for stairBOT
c = "ideal" stairs in public buildings
*) I can't find the correct english word. Some papers dealing with stair climbing robots use "ledges". On the other hand "STAIR SAFETY, A Review of the Literature and Data Concerning Stair Geometry and Other Characteristics" , a paper prepared for U.S. Department of Housing and Urban Development, did not use the word "ledge" at all, but you will find a lot about safety riscs caused by "nosings".
StairBOT can negotiate stairs with a run length a not smaller than 25cm and a maximum rise s of 20cm. Thanks to its big wheels are nosings and open risers no problem.
If you search the internet, you will find quite a number of stair climbing robots. There are the famous two-legged robots( e.g. Asimo, HRP2 ), six-legged robots ( e.g. RHex ), and tracked robots, mainly in military or law enforcement applications (e.g. Urbie, packBot). Looking for wheeled robots you will find only a small selection. Best known probably shrimp of the EPFL Lausanne. Helios V also climbs up and down stairs. There are also some hybrid designs with rotating legs, a mixture of a wheel and a 1-DOF-leg: whegs, whegsII and mini-whegs IV ( and RHex). Mini-whegs IV uses yet another concept: It jumps from step to step.
As impressive these robots are, according to the objectives mentioned above they also have some drawbacks. Either they are very complex and thus expensive, or they use tracks (not very appropriate to indoor environments) or some designs have problems with nosings and open risers (e.g. shrimp). Even RHex, (my favorite stair climber: more than 200 steps of Montmartre-stairs in Paris or fire escape stairs, slope 42°!) can only show its outstanding performance, if it appropriately hits the first step. At least downstairs RHex has to be placed manually in the right starting position and the stairs should not have large nosings and open risers. Actually you will find only few robots climbing downstairs.
In the amateur area are successful stair climbers too, for example the Lego robot P'titgneugneu. It climbs stairs in both directions, but it is not well suited for floors, due to its design especially developed for stair climbing.
This differentialdrive robot uses omniwheels as castors when driving on the floor. Climbing upstairs these omniwheels are the bearings for the linear guides whilst pushing up the wheels. They have to stay in their position against the horizontal reaction force. Brakes prevent the turning of the omniwheels, which should provide a sufficient wheel grip.
The adjustment of the wheel speed and the speed of the spindle drive of the linear guides has proven as crucial. To slow or to fast, always is the result that the friction between the omniwheels and the floor will not be sufficient enough to hold the wheels in position against the horizontal reaction forces, the robot falls down. For the build robot a working combination of the two speeds was found by trial and error.
To "drive" stairs safely the orientation to the step is another important issue. For most of the robots (e.g. small tracked robots) it is best to start with and to hold a perpendicular orientation to the step. To recognize its orientation to the edge of the step, the robot should have appropriate sensors in a symmetrical configuration.
To climb stairs the robot has to be:
- small enough, to fit into the length of a step,
- long enough, to span the distance of two steps.
To meet these conflicting requirements stairBOT was engineered as a differentialdrive robot of variable size.
Therefore it was build of two relocatable units:
- the wheel-unit with the main drive and the support,
- the linear-guides-unit with spindle drive, omniwheels and sensor head
mass is splitted pretty evenly to both units. The approximate position of
the centres of mass for both units is shown in the drawing. The prototype
of stairBOT has a total mass of approx. 6 kg. 20% of the total mass
are contributed by the batteries. The distribution of the batteries is the
easiest way to balance the robot.
With a wheel diameter of 255 mm the drive wheels are significantly larger than the wheels of robots of comparable size.
The robot uses two 6 Watt DC motors with built-in 16 cpr quadrature encoders and 84:1 planetary gearboxes. An extra gear reduction stage yields a total reduction of 224:1. The motors are controlled by PID-controllers via 3A H-Bridges.
A foldable support is mounted on the wheel unit to hold the robot on a step when the linear guides with the omniwheels are moved up or down. The wheel unit is mounted like the carriage of a linear motion system.
The wheel unit can be moved along the linear guides by a leading screw ( pitch 5mm, travel 290 mm). The drive motor is a DC -Motor ( 11Watt, 4.8 :1 planetary gearbox, encoder ). The motor is controlled by a PID-controller via a 3A H-Bridge. Additionally two limit switches are used for termination and calibration. With this mechanism the length of the robot is continuously adjustable between approx. 60cm and 30cm.
As castors two 60 mm omniwheels (TRAPO, polyurethane) are used.
To recognize the steps, its orientation to the step and the position on the step the robot is equipped with the following sensors:
climb the stairs only sensors # 2, 3 and 4 are necessary. To provide for
the perpendicular orientation of the robot to the step these sensors are symmetrically
mounted on both sides of the robot. If for example the left wheel reaches
the edge of a step the left driving motor is stopped while the right motor
still runs until the right wheel reaches the edge too. Thus the robot can
climb spiral stairs - if the run is long enough.
Sensors 1 are mounted on the tiltable sensor head. The sensor head preserves a given line of sight, because its adjusting servo gets a feed back of the actual spindle drive position.
addition to the two GP2D12s
the sensor head is equipped with a CMUcam2 and a SRF08 ultrasonic sensor. These
both sensors are not used for stair climbing.
click to enlarge
2 BrainStem GP 1.0 (acroname, USA)
2 Brainstem Moto 1.0 (acroname, USA)
2 GP2D12 range finders (front)
2 GP2D120 range finders (rear)
2 micro switches (wheel-bumper)
2 micro switches (linear guide bumper)
2 micro switches (limiting switches for the spindle drive)
11 SubC NiMH 3000Ah(13.2V) motor
5 SubC NiMH 3000Ah(6V) servos
5 SubC NiMH 3000Ah(6V) controllers
1 iPAQ LiIon-Battery
Type 2 wheel
differential drive (2 omniwheels with brakes as castors)
1 linear guides with leading screw
2 DC-motors 6W, 84:1 geartrain, encoder (differential drive)
1 DC-motor 11W, 4.8:1 geartrain, encoder (spindle drive)
1 servo (omniwheel-brake)
1 servo (support)
1 servo (sensor-head up and down)
fischertechnik construction kit parts, custom made aluminium, plastic
and plywood parts
depending on linear guides position
L x W x H:
short: 30cm x 36cm x 60cm
long: 65cm x 36cm x 27cm
about 6kg with batteries
|G. Wendel, Dezember 2004|