Poco - Quadruped Walking Robot
|[click to blow-up pictures]|
--- PRELIMINARY INFORMATION ---
[11.11.10 - this project was put in storage for a couple of years, now active again,
major changes were made in March/April 2011].
Poco is a simple quadruped walking robot with 3 active-DOF in each front leg, being shoulder-rotate,
shoulder fore-aft, and elbow joints, and 2 active-DOF in each rear leg, being hip and knee joints.
[04.2011] Spring-feet were added.
In Poco, the front legs have vertical-axis rotation, similar to that in human legs.
This is an experimental arrangement, meant to aid in turning movements.
This is being used instead of a front-rear horizontal axis 3rd-DOF
Aramies and others),
since we expect to be able to get a similar effect on turning as the front-rear axis without
having to use large torque servos at the uppermost shoulder joints.
In humanoid biped robots, the hip joint [sideways axis] servos experience the largest torques.
In other words, the idea is to try and simulate a 3-DOF leg without the weight and complication of adding a
full 3rd bendable leg joint.
Poco was built from
parts scavanged from
a short-lived biped
Like other of our walkers, Poco's frame is made from single-sided copper blank pcb material [FR4 fiberglass].
This material is lightweight and easy to drill. It is difficult to cut [best not to use a motorized saw due
to flying dust], but luckily it's available precut in different sizes, and it's easy to build frames of different
sizes and shapes. The Poco frame was pieced together with 4 precut pieces.
Furthermore, the pcb material is somewhat flexible, which can help compensate for slight misalignments and
servo positioning matters.
- dimensions: approx 10" (254-mm) long, 9" (228-mm) wide in front, 6" (152-mm) wide in rear.
- leg extension below frame: approx 7" (178-mm) in front, 6" (152-mm) in rear,
plus extra 1.25" [32-mm] for the new spring-feet.
- weight: total = 52-oz (1470-gm), frame+controller = 11-oz (311gm), 12/ea servos+SES = 30-oz (840-gm),
6/ea AA-Cells = 6-oz (170-gm).
- frame material: copper-coated pcb.
- servos: 8/ea Hitec HS-322HD (44 oz-in), plus 2/ea GWS S03TXF-2BB on rear-leg hips (76 oz-in).
- leg brackets:
- [added 04.11] new servo pan'n'tilt sensor mount, with GWS S03N standard and Cirrus CS-25BB micro servos.
Originally, Poco was controlled by a dual-processor controller board containing a WMC12 walking machine chip plus
24-pin Stamp-compatible module, such as the BS2, ARMexpress, or BX-24. [04.2011] the design was changed to the
new PIC24 controller, so software could be written for better direct-control of the legs, and to provide more
The software is described elsewhere as Conductor-MBS.
- controller board:
- --- [new 03.11] custom-designed 3.5" x 2.5" pcb with 28-pin PIC24 microcontroller chip. About half the pcb area
is proto-area, and a MSCC20 servo-controller chip was wired in there for driving servos.
- --- [original] OricomTech Bot-Cop Dual-Processor Controller -
containing a 28-pin PIC Walking Machine Controller chip, plus socket for 24-pin BasicStamp-compatible module.
- batteries: 6/ea rechargeable NiMH AA-cells, 1.2v @ 2200-mAhr each - powers both electronics and servos.
- [added 04.11] XBee Pro module for wireless remote-monitoring and control - 2.4-Ghz 80-100 mW transmitter.
- battery voltage.
- [added 04.11] QRD1114 photoreflectors on each foot, for foot touch-down and release.
- [added 04.11] Sharp GP2D12 IR Ranger on pan'n'tilt - usable range 10-80 cm [4-32 inches].
- [added 04.11] Maxsonar ultrasonic ranger on pan'n'tilt - usable range to 254-inches [630-cm].
- [added 04.11] Memsic x-y tilt/accelerometer.
- [future] servo current sensors to provide leg-torque information.
The next page discusses the design of Poco's legs,
as well as that of the new spring-feet.
The legs of Poco were designed using aluminum 2-point servo mounts in order to produce stronger leg-joints, compared
to having the limbs simply tied to the servo horns. This has made the legs approx 2X as heavy as they otherwise
might be, and this has meant that proper balance is a more critical matter in Poco than in other quadruped robots
we've built in the past.
In short, to get good stability in the walking gaits, it's been necessary to make individual leg steps more
complex by adding additional movements and more critical timing when the feet are off the ground and
Bad timing means the frame will pitch over forwards and crash into the ground when the [heavy] front legs are
lifted and extended to take a step.
Proper coordination will allow the frame to move stably from a backward-facing "tripod" to a forward-facing tripod,
as decribed on the first link below.
In essence, quadruped walking involves making a well-timed dynamic transition from one statically-stable
tripod to another.
Learning to Walk
Ideas for Poco
© www.robotrambler.com, orig Jan 2008, updated Apr 2011