## 3 EvoBot Implementation
### Mechanics  
In order to create a high quality, low cost, and easy to build platform, we built the robot from off-the-shelf components and, where it was possible, components used by the open-source 3D printer community and therefore readily available. Another key principle was to favour parts which can be produced by laser cutting or 2D milling large sheets and 3D printed parts over 3D milling or injection moulding because these techniques are usually available in small workshops. Another key principle was to favour laser-cut parts over 3D printing. The reasons being lower production time and high-quality of the produced parts in comparison to the elements produced through 3D-printing. However, we did use 3D printed parts outside the core mechanical structure primarily inside the syringe module (to be described later) due to geometrical flexibility afforded by 3D printers. Additionally, most parts have brass threaded inserts to avoid nuts and make the assembly easier. Few other techniques, like drilling, countersinking, or tapping, are used rarely. The frame consists of two vertical and rectangular sides on which layers can be mounted horizontally. It measures \(600\times 400\times 600\) mm, but its size can be easily extended up to 1500 mm, which is the maximum length of commercially available aluminium profiles. Aluminium profiles and Polyoxymethylene (POM) laser cut parts are used to build both the frame and the layers. To simplify the design, the aluminium profiles are used as linear guides. These profiles have a V slot where wheels with bearings can roll [15]. The carriages and the head are assembled by using laser cut sheets and off-the-shelf components. Movement is achieved by stepper motors with a pulley on the carriages and a fixed timing belt on the profiles. This adds extra weight but reduces the number of components, simplifies the design and makes it possible to have multiple carriages on the same rail. The mechanical connection between the modules and the head is made by two grooves on the head that match with two hooks of the modules. Two countersunk holes in the head with matching screws on the module aid the insertion of a module; see Figure 4.  
The syringe module has two degrees of freedom. Its main mechanical components are shown in Figure 5a. One Nema 8 stepper motor with a pinion-rack mechanism moves the syringe up and down and a linear non-captive stepper motor (25 NCLA-B01) moves the plunger of the syringe. In order to save space inside the module, a linear plain bearing and a rail (Drylin N17, Igus GmbH, Koln, North Rhine-Westphalia, Germany) are used to guide the movement of the syringe. The plunger movement does not need a linear guide for the small syring as the plunger in the syringe and the linear stepper motor provide enough guidance. In the 20 mL syringe, the friction force of the plunger combined with the longer distance between the motor and plunger can cause problems in some occasions. Therefore, a linear guide was designed with aluminium rods and plain bearings. Figure 4: Head of the actuation layer. (**a**) Mechanism to plug a module on the head, which automatically produces an electrical connection between the electronics of the module and the main controller. (**b**) Sockets of the head, which can have eleven modules connected at the same time. Figure 5: Main mechanical components of two modules: (**a**) Syringe module. (**b**) Heavy payload module. The dispensing module has only one degree of freedom and uses the same Nema 8 stepper motor, pinion-rack mechanism, and linear bearing as in the syringe module to move a connector up and down. Liquids are dispensed by using off-the-shelf peristaltic pumps. The pumps are placed on the side of the robot and controlled directly with the electronics of the robot; see Section 3.2, Electronics. Tubes transport the liquids from the pumps to the connector in the dispensing module, where there are four nozzles to dispense the liquids.