Electrical Design

The original Vantage design had a 230V phase-phase working voltage with a delta configuration. In New Zealand, however, the standard three phase is 230V phase-neutral or 415V phase-phase. This means in order for the printer to operate, a heavy-duty transformer on a separate trolley is required for the supply voltage to be converted to a suitable level.

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3.2.1.1.

Power supply

Due to the inconvenience of the required power supply and size of the transformer, the power supply is redesigned and the machine is converted to a ‘star’ configuration. This would allow the printer to be plugged into any 3-phase power supply in New Zealand. The conversion to a ‘star’ configuration requires a connector change from 4-core (3P&E) to 5-core (3P, N&E) as well as an additional terminal block connecting the neutral to the filter. A redesign of the PCB is also required in order to switch phase-neutral rather than phase-phase. The power supply PCB is redesigned and split into two separate PCB’s: A Power Supply Control Board (PSCB) and Power Supply Board (PSB).

The design philosophy follows that of the original equipment manufacturer (OEM). Fig. 3.3 shows a photograph of the redesigned power supply. The incoming supply through the filter is connected to the contactor and L3 connected to the back panel main switch controlling the supply to the PSCB. A Crydom MCX240D5 solid-state relay (SSR) is used to power the UPS from the PSCB as well as a small Point of Load (PoL) 24VDC power supply for the contactor. The PSCB provides the signal to the contactor and the gate voltage to a FET for switching the contactor. The stepping motors are powered from the fused side of the contactor return side. Heater supply loads are distributed across the three phases and switched using a Crydom D2425 SSRs.

Figure 3.3Power supply redesign showing main components, connections, and control board.

Transistor driver circuits Heater power

Crydom SSR 3-Phase

power supply

Power supply control board Arduino

Thermistor connection E-stop RS-232

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The secondary coils of a power transformer found in the bottom of the printer is used in the development of the High Voltage Direct Current (HVDC) supplies for the extruder head using GBU806-HB bridge rectifiers and smoothing capacitors for rectification of the power supply. The resulting PSB and PSCB are designed using DesignSpark PCB software. These PCBs are manufactured using a LPKF PCB mill. The digital control board design is based around an Arduino Mega board which is powered by a redundant 12V supply. The primary supply is connected to L3 from the PSB and the secondary to the UPS return from the PSB with the two supplies tied together through a pair of 1N4001 rectifier diodes and connected to the input of a 4N35 opto-insulator. The output of the opto-insulator is connected to the Arduino Mega along with an LED indicating the presence of power on each. L1 and L2 are connected to two Myrra 47154 PoL power bricks allowing the Arduino Mega to monitor the connection health of the 3 phases. The Arduino is also responsible for monitoring four DC supplies, the 24V supply to the motherboard and extruders as well as the three 60V supplies to the Stepping Motor Drives.

The Power Supply Board and the Power Supply Control Board have been designed so that the oven temperature could be monitored by the PSCB and the control signal provided to the Power Supply Board as it is used for controlling the oven heater SSR’s.

3.2.1.2.

Extruder head

The OEM used a single PCB in order to control both extruders with a PIC micro-controller and a Xilinx FPGA. The two 100-ohm heating elements with embedded thermocouples were connected in parallel and switched using IRF644 MOSFET’s. The same MOSFET setup is used in order to drive the support material extruder up and down when required with the extruders being driven by Faulhaber 2342L036, 36V DC micro-motors attached to a Faulhaber 23/1 planetary reduction gearbox. Each motor has a Faulhaber IE2-512 encoder providing 512 pulses per revolution. For the retrofitment of this printer the extruder control board is broken into two separate sister boards mounted to a motherboard on a back plate along with the HVDC components (Fig. 3.4). The new extruder boards utilize an ATMega328P microcontroller operating at 16MHz. The original extruder motors and DC motor controllers are used as they are perfectly fit for the extrusion process and only require the working voltage of the capacitors on the motor control side as well as the transient voltage suppressor attached across the motor outputs. Serial communication between the

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extruder and motherboard uses a RS-485 multi-drop bus allowing a single host (the motherboard) to communicate with multiple slaves (extruders and power supply).

Figure 3.4 (a) Controller board with extruder boards mounted. (b) PCB schematic of the solenoid voltage

switching/opto-isolator board.

3.2.1.3.

Motherboard

The motherboard design is based on the Makerbot v2.4 motherboard for the Makerbot Thing- O-Matic. Even though the original OEM positioned the I/O PCB in the bottom of the machine, this retrofitment positions the motherboard behind the front display panel as this would allow easy access to the SD card slot and minimise the amount of wiring needed between the motherboard and the FPC. Three PCB’s have been used for the FPC/Motherboard combination. A keypad interface PCB, the motherboard itself (Arduino Mega 2560) and a third board for the SD card slot that could be mounted just behind the front panel for SD card insertion. The original panel had two LED’s which are removed allowing for the implementation of an extra button to control the chamber lights inside. The original display also makes use of a 4x40 LCD display around which the front panel is designed where the MakerBot design uses a 4x16 LCD display. In order to use the larger Stratasys

Extruder control boards Communication channel from Motherboard

(a)

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FPC display, an equivalent LCD display is used based on the same Hitatchi HD44870 chipset used by the MakerBot system. The larger display requires an extra I/O pin in order to drive a second ‘Enable’ line.

A photograph of the motherboard is given in the Fig. 3.5a while Fig. 3.5b shows a schematic of the connections between Arduino Mega controller board and the units it controls for the printing process.

Figure 3.5Motherboard layout with Arduino Mega controller (a) and connection diagram (b).