Final Report: Arachnae II

 Mechanical Design


The design of robotics legs has been well studied. The types vary in complexity from real easy ones to extremely complex and hard to build leg designs. Due to our limitations in mechanical engineering experiences and, what is much more critical, the lack of proper machinery, led to a relatively simple construction. Nevertheless they fulfill the requirements concerning stability, weight and maneuverability.



The most important considerations in the design of the body were:


Overall Size

The length of the frame is mainly determined by the size of he servos and the maximum stepwidth of the legs (see figure 5.1).

Figure 5.1: Fundamental structure of the robot's body frame


Maintenance and Repairability

This requirement is obtained by the 'openess' of the frame. Add-on's like electronics and sensory can be mounted on almost any place on the construction.
Have a look at the circular shape at the left: first of all it is closed so only the outer surface is accesible for mounting. and, due to the lack of plane surfaces, it is also hard to mount any hardware on it. Compared to this the squared shape is something better but the inner surfaces are still not accessible. Only the H-shaped body gives a sufficient large surface to work on. See figure 5.2 for details.

Figure 5.2: Several shapes for body frame


From the kinematically point of view the very left shape in figure 5.2 has the best virtues with regard to rigidity but it's rather difficult to mount legs and electronics on it. Though the squared shape is also rigid, it has a relatively small surface compared to that at the right. This H-shape is the form we finally chose.



Important design constraints for the legs were:


Figure 5.3 shows a draft of the chosen leg design. The fundamental structure is derivated from that of insects as shown in figure 5.2.

Figure 5.3: Structure of insect legs


Figure 5.4: Structure of Arachnae's leg

The Body-Coxa joint in an insect body is a 3-degree-of-freedom joint 'in one place'. We were not able to imitate this so we ended up in a design as shown above.
The swing servo actuates the coxa and propels the robot forward by moving the leg backward and then swings it forward for the next step.
The lift servo both supports the weight of the robot and lifts and drops the whole robot body if necessary (e.g the robot must slip through a low passage or so).
The Knee servo lifts and drops the tibia. This may be necessary when the leg collides with an obstacle in its trajectory or if the robot climbs over obstacles.

The servos must be carefully selected concerning the torque and weight requirements. The most critical ones are the lift servos which must support the whole weight of the robot. A rule of thumb says, one motor shall be able to support half the weight of the robot. For safety purposes we strongly recommend to take into account the total weight of the robot.. For Arachane II this is assumed to be around 5 kg.


Calculation of torques

Swing Servo

The swing servo is mounted in the inner of the body frame, so no torque calculation was made. The Motor plus its gear has a torque of 180 Ncm which is supposed to be strong enough to move the robot forward.

Lift Servo

The most torque will be required when the leg is fully extended and supports the weight of the robot. Following the figure 5.5 below

Figure 5.5: Determining the required torque in the lift servo

the moment arm and required torque for lifting are


The chosen servo has a maximum torque of 180 Ncm. In consideration of the applied safety factor (see above) this seems to be enough even for continoous operation.


Knee Servo

The knee servos operates only when the leg is lifted so they need not to be as strong as the lift servos. The chosen servos are standard RC motors which have a maximum torque of 30 Ncm.



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Last updated: 26.6.1999