HighVoltageSupplyDesignDocument.lyx~

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\begin_body

\begin_layout Title
Computer controlled 5
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kV power supply system
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\begin_layout Author
Matthew Harvey
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\begin_layout Abstract
This document describes the development of a computer controlled, high voltage
 power supply system to be used in the new cold atom electron source experiment
 based in the Photon Science Institute at the University of Manchester.
 The experiment requires five computer controllable high voltage power supplies
 variable between 0
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V and 5
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kV.
 
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\begin_layout Section
Introduction
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\begin_layout Standard
The cold atom electron source (CAES) experiment creates cold electrons by
 photo-ionisation of a cloud of cold atoms, cooled and confined by a magneto-opt
ical trap (MOT).
 The electrons are then extracted from the cloud with a set of extraction
 electrodes before being focussed on to an imaging multi-channel plate and
 phosphor (MCP).
 Figure 
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reference "fig:ElectrodeDiagram"

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 shows the arrangement of the electrodes.
 The atom cloud sits between two extraction plates which are set to 
\begin_inset Formula $\mp V_{extraction}$
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 in order to accelerate the electrons towards the MCP.
 The third plate has a negative voltage, 
\begin_inset Formula $V_{focus}$
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, applied to it in order to focus the beam on to the MCP.
 The back plate of the MCP needs about 
\begin_inset Formula $V_{back}=1$
\end_inset


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kV applied to it and the phosphor needs 
\begin_inset Formula $V_{phos}=5$
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kV applied to it.
 
\begin_inset Formula $V_{extraction}$
\end_inset

 and 
\begin_inset Formula $V_{focus}$
\end_inset

 need to be varied between 0
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V and 5
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kV by computer control.
 In this way, the electron beam energy and focus can be optimised.
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LatexCommand label
name "fig:ElectrodeDiagram"

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Diagram showing the extraction electrodes and MCP used to image cold electrons.
 The numbers 1-5 show the elements where the high voltage applied must be
 computer controlled.
 Element 1 must be between 0
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V and -5
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kV.
 All other elements must be between 0
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V and +5
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kV
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\begin_layout Section
Hardware
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\begin_layout Standard
The cheapest quote for buying five dedicated dual polarity, 5
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kV programmable power supplies can in at >£9000.
 As such, I decided to buy five cheap power supply modules and build them
 in to a rack mount control box.
 The requirements were:
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\begin_layout Itemize
Five 5
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kV remote controlled supplies required.
 One negative, four positive polarity.
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\begin_layout Itemize
Must be controllable via the front panel.
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\begin_layout Itemize
Must have a switch or switches to select front panel control or computer
 control.
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\begin_layout Itemize
Front panel needs a display to show the applied voltages.
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\begin_layout Standard
A mock up of the front and back panels can be seen in figure 
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reference "fig:PanelMock"

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.
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	filename Figures/BackPanel.jpg
	lyxscale 15
	scale 12

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Mock up of front panel and back panel for power supply control box.
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\begin_layout Standard
The cheapest power supplies with serial data input were Spellman MPS5 supply
 modules with option DCC4 which has the addition of 16
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bit digital to analogue converters set by RS485 serial comms.
 For five, the total price including VAT is £1800.
 With hindsight, I should have purchased the modules without computer control
 and added that myself.
 This would have saved £600 and simplified the design.
 When the serial input option is specified, the ability to set the output
 voltage via an external control voltage is lost.
 The upside is that the output voltages are computer controllable to the
 nearest 76
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mV over the full output range.
 It took two weeks to iron out the bugs in getting the serial communications
 to work correctly but at least I'll know the subtleties for next time.
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Figure 
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reference "fig:PSRackPics"

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 shows the finished result.
 The front panel has an LCD display which shows the voltages of the five
 power supplies, and whether the voltage step size of the front panel encoders
 is set to 0.1
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V, 1
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V or 10
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V.
 The voltage step size is set by a three way switch on the right hand side
 of the LCD.
 The switch on the left hand side controls whether the power supplies can
 be controlled solely by the PC, solely by the front panel, or both.
 The five rotary encoders set the voltages on the five power supply modules
 according to the voltage setting switch, except when the control mode is
 set to PC only.
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	width 8cm

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	filename Figures/IMG_20150609_174041.jpg
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Top left: The front control panel.
 Top right: The back panel showing USB connection and HV out.
 Bottom left: The modules mounted on the rack frame.
 Bottom right: The back of the front panel showing the (crude) wiring of
 the encoders, LCD, Arduino Mega2560 board and MAX485 breakout board.
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\begin_layout Standard
The USB feedthrough on the back panel requires a USB-A male to USV-A male
 cable.
 The USB connection emulates a serial port on the PC such that a simple
 serial communication interface can be used to read the voltages and currents
 on the modules and set the voltages from the PC.
 
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Figure 
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LatexCommand ref
reference "fig:HardwareSchematic"

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 shows the schematic for the control and interface hardware between the
 front panel, PC and MPS HV modules.
 An Arduino Mega2560 is used to interface between the PC and the MPS modules
 as well as display the voltages on the front panel LCD and accept voltage
 adjustments via the rotary encoder knobs.
 
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	width 17.5cm

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A schematic showing the hardware connections used to control the MPS HV
 modules.
 An Arduino Mega 2560 interacts with an I2C interfaced LCD, 5 rotary encoders,
 two three way switches and a MAX485 interface breakout board.
 The MAX485 board handles a small RS485 network used communicate with the
 MPS modules.
 Not shown is the USB PC serial connection since this is the standard USB
 connection made directly to the Arduino board.
 A 24
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V, 6.5
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A switched mode power supply powers the MPS modules and a linear 8
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V, 1
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A power supply powers the Arduino, MAX485 board and LCD panel.
 All share a common ground including the HV outputs.
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\end_inset


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\begin_layout Standard
The LCD panel is a cheap AliExpress item with built in I2C control option.
 There are several LCD libraries written for the arduino that can communicate
 with this panel.
 The 
\end_layout

\begin_layout Standard
The loss of external voltage control due to the DCC option means that the
 voltage dials on the front panel will now turn rotary encoders.
 I'll use an arduino to keep track of the voltages and handle a small RS485
 network to communicate with the MPS modules.
 The PC will communicate with the arduino via its USB serial interface.
\end_layout

\begin_layout Standard
The front panel now consists of a three way switch used to select PC only,
 both front panel and PC, and front panel only control.
 Five rotary encoders set the voltages on the individual power supplies.
 A second three way switch selects the voltage 
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\begin_layout Section
Hardware
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\begin_layout Standard
Five Spellman MPS5 kV power supply modules with option DCC4, RS485 comms.
 interfaces are housed in a 19
\begin_inset Quotes erd
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 rack mountable box.
 A 24V 6.5A switched mode power supply is used to power the modules.
 An Arduino Mega2560 is used to handle communications between a PC and the
 MPS modules via a MAX485 adapter module.
 The Arduino also handles inputs from the front panel and updates an I2C
 interfaced LCD display.
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\begin_layout Section
Software
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\begin_layout Standard
Required arduino libraries:
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\begin_layout Standard
Enc
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\begin_layout Standard
MPS
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\begin_layout Standard
EnableInterrupt
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LiquidCrystal
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\begin_layout Standard
The encoders use three pins: A, B and GND.
 The arduino is connected to A and B and these pins are set so that their
 internal pullup resistors pull the voltage level high.
 When the encoder is turned, the contacts on A and B are shorted to GND.
 
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\begin_layout Subsection
Serial protocol
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The modules communicate with an ascii encoded format.
 The 
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<STX>n5V1=xxxx.xC<LF>
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<STX>
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 is the ascii code 0x02 for 
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start of text
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 program control character.
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<LF>
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Valid commands to send from PC
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