Safety Board Last update: 2019-08-13
The Safety Board monitors and controls
most of the system.
The board contains an Arduino MEGA2560 which communicates with the
Pi via an Ethernet Shield using the UDP-protocol.
Communicaion with the Winch Controller (WCB) uses a serial port.
The board is built on a standard EuroCard (100 x 160 mm) format
experimenting board. The hand wiring was done with insulated
copper wire, the type where the insulation disappears at soldering
schematic in PDF.
See the Layout
in PDF. The board was not routed. The layout was used
for placing components an hand-wiring the board.
See the document with the Pin-use
and the Messages exchanged.
To make the connctions to this board we had some very nice
connectors available, ripped from some other board in our storage.
These connectors allowed lots of GND terminals, so we could avoid
to have many wires in one terminal.
Also the terminals were consecutively numbered which made it easy
to find the right terminal.
Most digital inputs have explicit pull-up resistors, the arduino's
internal pull-ups are a bit weak, around 40 kOhm. We do use the
internal pull-up's, because some tests were to be done on an
almost bare Arduino Mega, and there you want to have the inputs
Most inputs were also filtered to make them more immune to HF
Most outputs go throug some kind of driver, 7 go through a ULN2003
which has open collector outputs allowing up to 50 Volts and 500
Outputs which switch mains voltage go through Solid-State
switches. See the Crydom Board.
We have a quite accurate voltage reference in the LM317. It gives
2.5 Volts. We use it to determine the supply voltage of the
arduino, which is the reference for other voltage measurements.
To measure the PWM percentage of the laser power input on the High
Voltage PSU we filter with a second-order low-pass filter.
However, our measurement relates to the Arduino's 5-Volt supply,
not the 5-V on the HVPSU. For that reason we also measure the
5-Volt from the HVPSU, and do some calculations.
The Laser Tube Voltage is measured with the help of a resistor
chain, consisting of 70 resistors of 10 MegOhm. With 100 kOhm as
foot-resistor and a ADC range of 5 Volt we can measure up to 35
kV. The resistor chain is housed in a silicone tube, with an extra
insulating tube on the high-voltage end.
The Laser Tube Current is measured using a shunt resistor of 140
Ohm, giving a range of 35 mA at 5V ADC range.
Measuring the conductivity of the coolant is described here. Note that the sensing input
T69 is kept on average half Arduino-VCC. The squarewave drive
signal on T71 also is half-VCC on average, so the sensor module
carries no DC.
Laser Mode switching:
On the GUI one can choose between Power mode By-Job, Manual or No
In the By-Job mode the PWM signal from the SMB is routed to the
HVPSU and measured via the T75 input. For the Manual mode a relay
is engaged which switches to a local PWM signal, in stead of that
from the SMB. In both cases the %Power indicator on the GUI shows
the actual value going to the HVPSU. In the No-Laser Power mode
the Enable_Laser signal on T1 is not activated.
See also the Laser Interface
Temperature measurements are done with the well known DS1820 Ic's
using the One-Wire library. We do supply the ICś with +5 Volt
because I found earlier that phantom supply did not always work
The ID button is also read by the OneWire library, but on a
different Arduino pin. The ID button does not allow other devices
on the bus.
Ethernet communication is done with a separate ETH module
connected to the SPI-bus. Initially a cheap W5500 module was used,
because using a regular ETH shield conflicted with the board
layout. However we experienced a serious EMC problem with this
module, switching noise on a mains cable often disabled the
communication. In the end we did manage to connect a "official"
Ethernet Shield and from then on these EMC problems were gone.
A very helpfull tool for EMC testing is an ordinary piezo-electric
gas lighter, preferably one without gas. The sparks (do not spark
on the electronic circuits themselves, only in the vicinity) are
strong enough to disrupt the functioning of equipment when it is
In many EMC-labs the "gas lighter test" is used to fast determine
if equipment is likely to pass the official regulatory tests or
Safety Board Connections. Download as .PDF