Initial Designs

Many different concepts were considered throughout the design and researching stage. A majority of these designs have been discarded and it was decided that the overall structure would incorporate a dual actuated ankle to increase power and to allow for desirable actuator placement. The overall proportions of the humanoid are based on scaled down, ideal human proportions.

3.4.1 Designs using DC motors and harmonic drives

The first two designs, (Figure 3-3), incorporated DC motors with harmonic and planetary drives. To produce a dual acting ankle, DC motors were placed in the knee with linear roller ball screws to act as a linear displacement. To actuate the knee a similar linear ball screw is proposed with the motor placement in the hip. The pelvis area of the body, including the hip area, at this stage is based on classical design where the motors are situated directly at the axis and act as a bearing.

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Figure 3-3: CAD designs of concept 1 & 2 using DC motors and harmonic drives

There are many issues with these particular designs. Firstly and most importantly, the DC harmonic drive motors that were found to be suitable in size and torque are far too expensive, at $1500 each. If this was disregarded there are still several significant issues. For example, each axis would need its own angular encoder, and each motor would need a dedicated motor driver with associated pulse width modulation. This greatly increases the complexity of the system both in manufacturability and controllability.

3.4.2 Designs using WCK and AX servos

Several other iterations similar to designs 1 and 2 occurred before Serial Controlled actuators were discovered. Benefits and specifications of these servos are discussed further on in this thesis. The servos used in designs 4 and 5 are the WCK1111 and the RX-28 which at the time were assumed to have similar network protocols. (Figure 3-4) on the left shows a RX-28 as the knee actuator. The reason for this is because initially it appeared that the knee would require the largest torque and therefore the RX-28, which produces 28kg/cm of torque, was proposed. After further research however, it became apparent that the largest torque is situated at the hip joint [34]. After further investigation of the network protocols it was found that the WCK servos are a full-duplex protocol whereas the RX (Dynamix) is only half-duplex which would require additional circuitry. The solution to this was simple, 2 WCK servos were used at the knee.

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This particular design allows lower specification servos to be used due to the novel dual pushrod actuation implemented in both the ankle and the hip. Having two servos actuate both roll and pitch of the ankle and hip ensures that both servos support the load of every movement, producing a joint with twice as much torque potential as a classical single actuator design. Also with the addition of pushrods we are able to move the servos further up in the body to reduce inertia. These two unique design characteristics are what have made the development of this low cost, high agility humanoid possible.

Figure 3-4: CAD designs of concept 4 & 5 respectively

3.4.3 Designs using WCK serial servo modules

(Figure 3-5) on the left shows design number 6. This has the beginnings of the implementation of a toe, the addition of pelvic pitch, roll, and also the hip yaw servos have been positioned closer to the centre of the body with pushrods in order to increase how compact the pelvic area is.

(Figure 3-5) on the right illustrates design number 7 which is the first design resembling the final layout of the system. This has a carbon fibre tubular leg section which makes for an extremely lightweight, simple to build, structure. Most importantly, however, is that these sections allow for the possibility of reducing or increasing the height of the humanoid with

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ease. This design has also moved the hip actuators above the pelvis yaw pivot in order to increase how compact the pelvis area is.[15] discusses the importance of weight distribution in a humanoid with respect to leg design.

Figure 3-5: CAD designs of concept 6 & 7 respectively

This is not the final WCK module design. The final design in described and discussed in greater depth in section 4. Through many iterations it became clear what the most efficient servo positioning and orientation was. The roll and pitch servos of the hip are positioned above the pelvis, the hip yaw servos are positioned in the center/bottom of the pelvis. This results in a total of 3 servos per hip. The knee uses two servo modules in parallel purely to increase total available torque. The ankle is actuated in a similar manner to the hip except with now yaw actuation. The ankle servos are position up high right by the knee in order to reduce overall rotational torque which is reviewed and described in the following section. There are a total of 8 servos modules in each leg allowing for 7 degrees of freedom. There is then the pelvis rotation and twist resulting in a total of 18 servo modules in the lower body.

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