HighVoltageSupplyDesignDocument.lyx

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\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 $V_{extraction}$
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 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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	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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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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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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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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\begin_layout Standard
The LCD panel is a cheap AliExpress item with built in I2C control option.
 
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\begin_layout Section
Software
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\begin_layout Standard
There are several LCD libraries written for the arduino that can communicate
 with this panel.
 I've used the liquidcrystal_i2c.h library from here: https://bitbucket.org/fmalpa
rtida/new-liquidcrystal/downloads.
\end_layout

\begin_layout Standard
The encoders work by firing interrupt routines.
 I did not want to poll the encoders all the time because this could have
 slowed down the user interface and missed events.
 For this to work properly I had to use the EnableInterrupt.h library from
 here: https://github.com/GreyGnome/EnableInterrupt.
 This exposes many more interrupts than the standard Arduino setup does.
 Since the user interface uses five rotary encoders, ten interrupts are
 used to ensure no events are missed no matter which way the encoder is
 rotated.
 I wrote a library called Enc.h that can be used to assign interrupt pins
 to an Encoder class and interprete whether the encoder has been turned.
 It then counts the number of turns since it was last checked.
 Since the encoders trigger interrupt code, the encoder counts are updated
 even if the user interface is blocked by serial code between the Arduino
 and PC or MPS modules.
 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.
 
\end_layout

\begin_layout Standard
I've also written SimpleTimer.h class which allows timers to be used for
 triggering events at correct intervals.
 See the code for use examples.
 Finally, I've written MPS.h to handle communication to and from the MPS
 modules.
 This file parses code from the PC and MPS and can pass the relevant information
 between them.
 It also interpretes messages from the PC or front panel and updates the
 MPS modules as necessary.
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\begin_layout Subsection
Program flow
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The flow of the program works as follows:
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\begin_layout Enumerate
Setup front panel controls including LCD, interrupts for encoders and digital
 pins for the two three way switches.
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\begin_layout Enumerate
Setup some timers to make sure code is executed as often as necessary in
 the main control loops.
\end_layout

\begin_layout Enumerate
Setup the serial communications baud rates etc in the setup() function.
\end_layout

\begin_layout Enumerate
Read the currently set voltages from the MPS modules and update the PC and
 front panel with this information.
 Also report the actual voltages and currents back to the PC.
\end_layout

\begin_layout Enumerate
The main loop() function checks the status of the front panel controls and
 updates the MPS modules as necessary.
 It also parses commands from the PC and sets or reads parameters from the
 MPS modules.
 The actual voltage and current values are requested and sent to the PC
 every few seconds (interval defined by the longInterval SimpleTimer).
\end_layout

\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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