As an example of some target requirements to be achieved by the manipulator, it should be able to:

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  Reach any object at least 40 cm from the servomotor axis.

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  Achieve a precision of ±0.5 cm for repetitive movements.

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  Be able to lift at least 10% of its own weight, not considering power supplies.

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  Maximum time of 3 seconds to get from any position A(x,y,z) to any position B(x,y,z).

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  Variable speed control for the movements.

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  3DOF plus a 1 DOF gripper to lift the specified weight.

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  Manipulator should be designed to allow for servomotors and bearings to be disassembled for service or replacement.

These requirements are considered in all the design and manufacture process, taking them into account for negotiation if any change is to be made and verified during final acceptant test phase.

The main selected materials for the proposed platform are listed in Table 1 and shown in Figure 1:

Figure 1.

Main materials for the robotic station (A)Servomotors HD-3001HB, (B)Servomotor HD-1440A, (C)Bearings, (D)Turntable, (E)ARDUINO, (F)Foamed PVC.

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The robot is integrated by the following main sub-assemblies shown in Figure 2; dimensions are only shown as an example and they should be determined by the student during the mechanical design phase:

Figure 2.

Main subassemblies of robotic station.

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The main recommended tool to be used for the mechanical design is CAD software; by using it the dimensions of the different links can be easily tested and modified in order to meet the prototype specifications.

It is essential to have an iterative process between the 2D engineering drawings and the 3D model used to simulate the mechanic limits, advantages and disadvantages of the proposed prototype.

It is recommended to start the mechanical design with the arm of the robot, focusing on the dimensions of its links. Once the arm is designed the base should be planned to provide stability and support. There are some critical assemblies that must be taken care of during the design:

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  The axis of the 2 arm servomotors that will power the parallel links must be properly centered.

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  The bearings, couplings and joints of the arm and their relative tolerance.

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  The attrition of the parts or joints during movements.

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  Weight vs. stiffness in the arm links.

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  Stability vs. weight in the base.

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  Gripper design based on the desired application.

One of the most critical sections is the arm servomotors’ position and adjustment, as shown in Figure 3 the space between these two motors must be minimized and the axis must be centered in order to achieve a soft and precise movement of the arm. This will also help to reduce vibration while moving the arm or lifting weight.

Figure 3.

Position and adjustments of the arm servomotors.

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It is suggested to design a base to add stability to the arm while keeping its own weight at minimum; a design example is presented in the Figure 4, as shown the lateral faces of the polygon have been prolonged to give the base more supporting area while reducing its volume.

Figure 4.

Example design.

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It is essential that the prototype can be disassembled or repaired in case it is needed; mechanical design plays an important factor in this matter to resemble industrial criteria. A “plug – socket” configuration is one possibility for the union of different mechanic parts in the prototype, such as the motor base with the rotating bearing table or the lateral sections of the base with the top section, this will give an easy and precise assembly while allowing other parts to be disassembled and assembled any time it is needed.

Once the pieces were designed and tested in the 3D environment, 2D engineering descriptive drawings are created. The pieces can be hand cut, or machined depending on the facilities available and production processes selected, as an example here a laser cutting machine was used, so the procedure is therefore described. After the pieces are completed in AutoCAD it is necessary to change its format to a .dxf and save them in AutoCAD 2004 version, this format is later used by another software called RHINOCEROS; this program is used to define the final specifications to the laser cutting machine such as the size of the material to be cut, the colors of some lines to define the sequence of cutting area in the material and finally the coordinate origin in the material.

The cutting machine shown in Figure 5 has a positioning tolerance better than +/− 0.1mm, this let us create geometries with high precision and detail, CAD modeling allows to make the necessary changes to the geometry, thus obtaining a refined model.

Figure 5.

Utilized laser cutting machine.

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All the parts were manufactured by this laser cutting machine and the material used was foamed PVC, which is a light material (0.3 gr/cm2) and also rigid enough to meet the design specifications.

To manufacture the L shaped pieces, the sheets were hot bended by placing them half-minute into the bending machine, then these had to be molded according to the required geometry.

Several assembly processes can be used to put the pieces together like making holes to fix pieces were screws are required for the support components, as an example some motor base-rotating bearing tables are shown in Figure 6.

Figure 6.

Examples of motor base-rotating bearing table assemblies.

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To assemble the base, its components are attached by epoxy glue that allows having a good rigidity. In general, manufacturing processes are not very complex, this is accomplished if most of the efforts are focused in the design phase on doing a sound CAD design and prototyping work; the purpose is avoiding future problems in the assembly of the robot, thus saving time and money.

The utilized power supply was a D.C. 7.5 volts 2 Amp voltage source. The designed PCB includes a 5 Volts regulator to keep the power to the required level, providing a rectified signal that is essential to guarantee the correct operation of the motors and avoid unwanted misbehaviors.

The control circuit is fed by an independent source; in this case the digital voltage is obtained directly from the master control computer. It is basic to have a reliable power supply in order to guarantee the correct operation of the circuits.

The designed printed circuit board is necessary to hold the power supply lines; it also provides mechanical support for all the other electronic components: the control circuit, capacitors and the pin connectors for every motor. The PCB design was made in Livewire, this is specialized but light software for the development of printed circuit boards, and it contains the necessary libraries to use all the components for the designed circuit. The first step is to create a schematic circuit of the components with connections; the electric circuit is shown in the Figure 7.

Figure 7.

Schematic representation of the electric circuit.

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The schematic circuit is later imported to another program called PCB Wizard, where circuit tracks were created.

Additionally the program creates other schematics that show the final distribution of some components as shown in Figure 8.

Figure 8.

Resulting printed circuit board.

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The wiring is used to transport the power from the source to the motors, which are in the motor base and the gripper of the robot. Wiring should be rigid enough to accompany the movement of joints and avoid interference to the movement of the links. Wire-wrap was used for the wiring; this is a 30 AWG cable. Wires were attached using epoxy glue, which provides a good support to the cables and reduction for vibrations.

The motors’ female pin connectors and the Arduino are fixed to the PCB with header pin connectors that are welded to the PCB.

The cable attachment allows free movement of the joints of the robot, the location of these cables is important to avoid some failure caused by the movement of the robot. All the movement routines were tested successfully, with no interference of the cables.

Two programs were developed to control the system, an Arduino code and a Central Control code. The Arduino code, is intended to generate the basic signals to control the servomotors position, while Central Control code could perform more complex task that involves heavy processing in case of requiring a difficult to compute trajectory that might consume considerable processor resources. Any microcontroller might be used for the position control; the microcontroller program receives the position of the different motors through commands from the Central Control program. This Central Control program also works as a User Interface (UI). This kind of distributed processing was tested for the following reasons:

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  To increase modularity of the software architecture.

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  Reduce microcontroller processing requirements

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  Allow for better User Interface possibilities.

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  :If any microcontroller (besides Arduino) is selected, it avoids the repetitive use of specialized hardware (programmers) in case reprogramming of the robot behavior is needed given that microcontroller is just programmed once. Microcontroller plus robot can be seen as one big block, having to deal with the programming just in the host PC, thus decreasing development time.

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  Possibility of further application development: allowing easier peripheral implementation such as with video cameras or voice recognition that require more powerful processing, and that also have protocols and libraries totally documented for traditional operative systems.

The communication between the robot “brain” and the servomotor controller was implemented on the well-known RS-232 protocol. An open source library called LnxCOMM, which is authorized under GNU license by Fernando Pujaico, compiled and executed on Windows. The serial communication is configured to 115 kbps, 8 bits, one stop bit, and no flow control.

The command is sent with the following structure: P,PM01,PM02,PM03,PM04,

As an example, this is a position command for the servomotors: P,0750,1230,0100,0050,

The motor positions are send on an [0,1500] interval, position that is read and interpreted by the motor controller positioning the robot as the master control orders.

The maximum refresh frequency of the motors is 25Hz because of the limitations of the control system that standard servomotors have implemented inside. Every character send is a representation in ASCII code.

The code for the control master was developed in C to increase the portability; the only additional library besides the standard ones is the already mentioned LnxCOMM.

The Arduino code for the motor controller has only one function, to modulate the PWM signal to change its duty cycle, thus moving the servo motors to the desired angle.

The positions are commanded by a central control, sent via RS-232. This function is achieved by using a timer in the microcontroller.

In the central controller several functions and routines were implemented, the main are described below:

Send command function: It will read the motors’ positions from an array or a pointer and will convert them into ASCII to send the command, by using LnxCOMM library it sends the complete command in the specified format (using delimiters between each motor position) via RS-232. If the microcontroller is programmed to send any message back to confirm the data reception, the data should be read from the port buffer, otherwise an overflow and execution time error might occur. This routine is the base to build other functions, such as:

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  Go home: Send fix coordinates to the robot; it goes to a basic known position that is usually called “home

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  Joint coordinate motion: By pressing keys, such as ‘Q’ and ‘W’, the user is able to move each joint of the robot an assigned step size. It is easily accomplished by changing the motors’ position in the array or pointer assigned.

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  Send servos position: It allows the user to send a value to each joint of the motor, it is especially useful if a previous sequence’s point wants to be proved in a new sequence.

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  Set speed: Changes the step size of joint coordinate motor, and the speed of execution of a sequence.

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  Save points: It is based on joint coordinate motion function. By moving the motors to the desired position, the robot interacts with its environment, and then a sequence of points is saved to be played later. It works very similarly to an industrial robot teach pendant.

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  Play sequence: It plays the saved points sequence from command 5 at a given speed in command 4.

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  Play sample sequence: A fixed routine can be directly written to the main program as a macro. It is especially useful if you need to demonstrate your robot’s skills in a predetermined way.

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  Print points: Useful for debugging purposes

This section is very important for an accurate positioning of the robot. On the datasheets of standard servomotors, the digital PWM controls are specified. Its maximum frequency is 25 Hz, and the duty cycle is modulated linearly according to the set point position. Just to say an example a duty cycle of 0% will take the motor to an angular position of 0° and a duty cycle of 100% will take the motor to a 180°. This is the theory, however, because of the diversity of methods and manufacturers, the variability of the real results of the control have a significant variance, for example to get the limit positions or refresh frequencies. This implies that for a better precision real life testing should be done, registering the real positions according to the PWM control signal as accurately as possible to test the real range, its effective resolution, and its maximum and minimum refresh frequency. This procedure was followed to ensure the real life accuracy of the motors once they are mounted on the robot.