Martian Sand Crab

Sand Crab Legs

The Sand Crab uses a set of StrandBeest legs divided into two halves. These are driven
by two stripped-down servomotors and geartrains.

Stripping the Servos

The servos have to be stripped down and converted to continuous rotation.
This requires soldering skills and experience of precision work with hand tools.

Removing the Circuit Board

The three potentiometer pins can be unsoldered using a solder sucker. Wiggle the
pins to free them. The board can then be removed by applying medium force whilst
unsoldering the two motor pins.

Removing the pot

To remove the pot, solder a wire to the pins and apply medium force whilst releasing
the plastic latches.

Servo Geartrain

Remove the cover on the output end of the servo to expose the geartrain. Arrange some containment
to catch any errant gears. Clean off the old grease and reassemble with the metal shafts in the
main body and the gears arranged as shown.

Final Gear with Tang

The tang has to be removed to permit continuous rotatation. First nibble it away using
wire-clippers, then clean away residue using needle files. Assemble geartrain and
apply a small voltage to the motor to check for free continuous rotation.
When running free reassemble and add fresh grease to the geartrain.

Components for Assembly

These are the essential components of the Sand Crab, two sets of legs, two geared motors, and a speaker.

Sketch of Sand Crab Structure

The Sand Crab structure consists of a main box holding motors and speaker, and outer boxes to attach the legsets.
Material is balsa wood, chosen for its light weight, strength and ease of working in a domestic environment.
Strengthening in key areas spreads the mechanical loads.
A carapace will be added over the top to make it look like a crab.

Sketch of Controller Wiring

This sketch shows the wiring within the controller. There are three AA cells, giving a choice
of 1.5, 3 or 4.5 volts, selected by two-pole six way switches to give each motor three forward
speeds, stop, and two reverse speeds. The voice chip is controlled by a push-button.


The battery holders and voice chip are soldered onto a strip-board which in turn is mounted on
four pillars for ease of assembly.

Controller Test

The controller is working okay, with three forward and two reverse speeds for each motor. The sound chip language
is Standard Galactic, as spoken by a certain small movie star.

Distribution Board

The distribution board contains a bus which distributes controller inputs via headers to motors, leds and speaker.
The use of headers and sockets makes final assembly solder-free and permits polarities to reversed if necessary.

Distribution Board Test

The distribution board worked as expected and coupled the controller inputs to motors and speaker.
The speaker voltage was insufficient to light a led (not shown). The left hand motor had leds for
forward and reverse. These lit at medium and fast speeds but not at slow, which is only 1.5 volts.
Right hand motor leds not fitted yet.

Improved Six Pin Connector - Controller End Of Cable

The audio jacks had high insertion force and there was a risk of them damaging the soft balsa wood.
This arrangement uses quad-pair cable in conjunction with six pin circuit board headers and sockets.
The plug is connected to the cable with shrinkable sleeving. A ring of insulated wire forms an adequate
cable clamp, and the sleeving holds firmly onto the socket without needing to cut notches.

Improved Six Pin Connector - Crab End of Cable

The same connector design was used on the Sand Crab end of the cable. In theory the header board could
be merged with the distribution board but in practice the header board has vertical tracks and the
distribution board has horizontal tracks. The motor mounting screws have load-spreading plates, to
prevent the soft balsa wood being crushed by screw loads.

Drawing of Leg Plate

The leg plates link the legs to the body. Each is connected to the legs by long screwed rods.
The rod tension is spread out by 'finishing' washers, in this case the M2 threaded rods have
M2 nylon washers and M3 finishing washers. The leg plates were redesigned to accommodate the
large 15mm diameter washers. The drive couplings are protected by coupling shrouds which
connect the leg plates to the body.

Drawing of Body Side Panel

The body side panels line up with the leg support plates by inserting temporary rods to align them during bonding.

Drawing of Body Viewed from Top

The body is a simple box. The shelf is exactly square to hold the box square during bonding.
Holes for speaker, distribution board and cable connector are non-critical.

Body Parts

The body parts are cut from thick balsa sheet and will be glued together with balsa cement
to form the centre body structure carrying the mechanical parts.

Assembly of Body

The body was assembled and glued. First the rear panel was pinned to the assembly base and the shelf
was added. Then the side panels and front panel were added, taking care to observe the markings and
keeping the alignment holes clear.

Trial Assembly of Body and Legs

The body and legs are held together by the alignment rods to see how things fit. The rods will be used to
align leg plates and body during bonding to ensure concentricity of motors and leg cranks, and then
will be discarded.

Drive Coupling

The motor and leg crank shafts are within 0.5mm of concentricity so the drive couplings need only a little
compliance to produce a smooth drive. The method chosen is to make each coupling from a servo spider mating
with rubber grommets in a disc on the leg shaft. Ideally the disc might be made of metal with three equispaced
grommets. Alternatives are a metal plate with two grommets (non-preferred) or a trangle of easy-to-work paxolin
(srbp) with three grommets.

Drive Coupling Test

A drive coupling was made out of Vero stripboard and is being used to test the drive coupling concept.
The test was successful, and I am going to try to fab the actual drive couplings out of sheet ally

Metal Drive Couplings Assembled to Motors

The metal drive couplings were made from 32 thou (0.8mm) aluminium sheet, were drilled using a paper template,
and were cropped to size using masking tape and a nibbler.

Leg Plates

The leg plates interface the legs to the body. Space is allowed for motors to be assembled to body.

Trial Assembly to Measure Required Spacing of Leg Plates to Body Ends

The spacing was checked to ensure the spacing members would permit drive coupling engagement.

Trial Assembly using Alignment Rods

This trial assembly using the alignment rods shows how the SandCrab will fit together.

Using Two Rods to Align Leg Plates to Body During Bonding

The two alignment rods are being used for their primary purpose of aligning the leg plates to the body
during bonding. This assures good concentricity of motor shafts and leg cranks. Any residual error is
taken out by the radial compliance built into the drive couplings.

Sand Crab Body Completed

After withdrawal of the alignment rods the body shape can be seen. This is the shape I have been
carrying in my head whilst doing the drawings and cutting the parts. Nice to have that part of my
brain cleared and free for general use once more.

Top View of Wiring inside Body

The power inputs come up from the control connector and into the distribution board. Motors, speaker,
and eyes are plugged in and take their power from the distribution board bus.

Front View of Completed Sand Crab Chassis

The speaker can be seen behind to the front panel.

Rear View

The control connector can be seen on the rear panel.

Under View

The Drive Couplings can be seen either side of the body.

The Crab Walks

Drawing of Shell Structure

The shell is based on two balsa beams which are a snug fit on the body. Most of the surface is skinned
with thin balsa sheet. Some front sections are skinned with tissue to provide double curvature whilst keeping
the forward mass to a minimum.

Shell Structure before Covering

The shell structure was glued together and sanded ready for covering with balsa sheet and tissue.

Eye Mounting Board

The eyes protrude through the upper side of the shell and are mounted via a board underneath.
Wiring to two dil free sockets connects them to the distribution board in the Crab body.

Sand Crab Shell Mounted on Chassis

The Shell was coated in thin balsa sheet and tissue paper, and painted with Humbrol matt enamel.

Sand Crab - the movie

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