Broken Filament Detector
Born back in 1993 the original BFD proved a massive success in
the task of detecting broken filaments. The original development
of the BFD was a collaboration between meiners-del and a world leading
To date the BFD and BFD2 monitor thousands of thread lines world
The original BFD was upgraded some years back and became the BFD2,
whilst maintaining compatability with the original the BFD2 gained
additional communication modes.
The introduction of the BFD2 brings together both the BFD 'Type
A' Single Output model and the 'Type B' Dual Output, into one configurable
Unit. As well as combining the existing two models of the BFD the
BFD2 benefits from the inclusion of RS232 and RS485 communication
thus making for a more adaptable System that vastly reduces the
Whilst the Sensors and analogue electronic circuit that conditions
the broken filament signals remains primarily unchanged the digital
processing of the conditioned signal is now handled by a single
micro controller. The introduction of the micro controller enables
the BFD2 to be configured to any one of four possible Modes of operation.
1. Type A:- Single Output Mode via Opto-Isloator
2. Type B:- Dual Output Mode via Opto-Isolator
3. Type C:- RS232 Communication Mode
4. Type N:- RS484 Network Communication Mode
Compatibility with existing installations has been maintained to
ensure that the BFD2 can be used as a direct replacement for both
Type A and Type B, but bringing the addition of improvements made
to the BFD2's overall operation common to all operating modes.
BFD2 - New Features
1. Type C Operating Mode: RS232 Communication
When configured to this mode the BFD2 connects to a PC Standard
Communications port. The Broken Filament Count and Alarm data are
all transmitted in a single data string when a 'Read' command data
string is transmitted from the PC. A calibration check on the sensors
can also be commanded by transmitting another data string from the
PC instructing the BFD2 control unit to perform the Calibration
Function and return a data string containing the data for the calibration
just performed. The BFD2 retains the total broken filament count
in memory until a 'Reset' command has been received from the PC.
Development of a Data Logger software package to record and display
charts of broken filament counts during a test brings a new dimension
to the BFD2. Now the BFD2 can be used to monitor those problem threadlines
causing quality issues and the software will record the filament
count along the total yarn length tested. This information is then
displayed in a graph showing the distribution of broken filaments.
2. Type N Operating Mode: RS485 Communication
When configured to this mode a maximum of 30 x BFD2 Units can be
connected on a Two-Wire Multi-Drop Data Network connected to a standard
485 port. The communication transfer of data and commands uses the
same format as the RS232 but with the advantage of linking the BFD2
units with a single 'twisted pair' cable.
1. Power Supply
The linear voltage regulators have been replaced with a DC to DC
converter to improve overall isolation between the incoming supply
and the BFD2 electronics.
2. Electrical Noise Immunity
The inclusion of a micro controller in the BFD2 has enabled some
additional electrical noise rejection. The power supply is now monitored
for electrical noise and if noise is seen on the supply it is indicated
to the micro controller. Because the micro controller is responsible
for processing the conditioned broken filament signals, either outputting
or counting the broken filaments and receives indication of electrical
noise on the supply allows distinction between electrical noise
and broken filaments, thus rejecting signals generated by electrical
noise on the supply.
3. Output Length (time) Setting
The way in which the output switching time is set has been changed.
To set the desired output length to match the data collection system
being used a simple switch arrangement is now used. Two rotary switches
located on the Digital Processing circuit board are used to set
the output length from between 10mS to 990mS. The requirement for
an oscilloscope to measure and set the output time is removed.
4. Calibration Amplitude Signal level Setting
The method of adjustment to set the calibration amplitude to 90mV
has been improved. A 10-turn trimmer is now used in place of the
single turn making it easier to set. The location of the trimmer
has also been changed to make it more accessible.
5. Sensitivity Threshold
Adjustment to the sensitivity threshold has been added. The factory
setting is 50mV and suits the vast majority of cases. The adjustment
will allow settings of between 30mV and 0.5V therefore it is possible
to accommodate for any situation that would require a different
sensitivity threshold level.
6. Calibration 'Pulse'
The 'Pulse' that operates the Analogue Switches in order to perform
the calibration function is now under the control of the micro controller,
this has resulted in the removal of the circuitry previously used
to generate the 'Pulse'.
Overview of the BFD2
Broken Filament Detection
Each unit is supplied with either four or eight sensors. The sensors
are optical and use infra red emitters and receivers. They are supplied
with three metre cables that are hard wired to the sensor and have
a connector that plugs into the control unit at the other. Each
sensor must be calibrated to the position that it is plugged into,
this is to say, that if the sensor is plugged into channel one the
sensor must be calibrated to that channel. The BFD2 unit is supplied
with sensors that have been factory calibrated to each position
and must be plugged into the corresponding position number engraved
on the enclosure. The requirement to calibrate each sensor is to
ensure that the sensors are working at their optimum detection point,
however once calibrated to the unit the procedure not need be done
again, unless the sensor is changed or to correct for ageing of
the sensor. The procedure to calibrate the sensors to the unit is
covered in a later section. When mounting the sensors consideration
to their position is required, it is normal to position the sensors
where the yarn has the least amount of movement and where any broken
filaments will be easily detached from the yarn body. The recommendation
is to position the sensors above or below the yarn and where the
angle of the yarn changes, this utilises centrifugal forces. The
sensors are supplied individually and require a suitable mounting
bracket (not supplied). The mounting bracket MUST BE ELECTRICALLY
INSULATED from the machine because the bracket MUST be connected
to the BFD unit's green binding post. The reason for this procedure
is to avoid 'ground loops' and to 'screen' the BFD sensors and cables,
if it is NOT done this way then electrical noise can be injected
into the sensors and cable thus giving false outputs. The sensors
are aluminium and are black anodised. The overall dimensions are;
Width 27mm, Height 30mm and Depth 9.5mm, they are U shaped and the
gap is 6mm. During development of the BFD, extensive tests were
carried out at our factory and on site at a working plant. All the
information gained from the tests determined how the BFD2 is today.
This is to say that the beam current and calibration figures were
decided upon because of the results found during development.
The Broken Filament Signal
When a broken filament passes through the sensor head and breaks
the infra red detecting beam, a signal is generated in the receiver.
This signal is then sent to the BFD unit and is amplified. The amplified
signal is then sent to another amplifier. The signal is amplified
once more and then sent to the digital circuit. The second of the
amplifier has a threshold voltage on its input; the reason for this
is for noise immunity. It is known, from tests, that the smallest
broken filament will generate a 100mV signal, at the output of the
first amplifier so we can safely say that any signal below the threshold
voltage, is not a broken filament. The threshold voltage is adjustable
and is factory set to 50mV and would normally never be altered.
There may circumstances that require reduction of the threshold
level because of smaller filaments being produced in the future,
hence generating a smaller signal in the sensor, or increased because
of high levels of airborne electrical noise.
The Alarm Signal
The output of the first amplifier of the Broken Filament signal
has a D.C. voltage, this voltage is fed to a comparator circuit
that compares this voltage with a reference voltage. The sensors
can become contaminated, as this happens the D.C. voltage changes,
the more contaminated the bigger the change in the voltage. When
this voltage reaches the reference voltage of the comparator circuit
an output is generated from the comparator; this is the ALARM SIGNAL.
The Calibration Function
The BFD2 unit has a function that checks the performance of the
sensors. This is done by electronically simulating a broken filament.
An input to the BFD2 unit is required this will be in different
forms depending upon the operating mode the BFD2 is configured (see
configurations details). When the calibration input is given to
the unit an output or count for every channel, that has a sensor
plugged in, is generated, as long as the sensors are working correctly.
The simulation generates a signal in the head by dipping the infra
red emitter current, by a pre-set value. This value was again determined
by the tests during development. The dipping of the emitter current
is exactly what happens when a broken filament passes through the
sensor infra red beam. This signal is then processed through the
amplifier and digital circuits in the exact same way as if it were
a filament. If a sensor fails to generate a signal of sufficient
amplitude than there will be no output or count for that channel
therefore that sensor has failed calibration and requires attention.
Broken Filament Count/Output
When a Broken Filament is detected the BFD2 will either count if
configured for Type C or N, alternatively an output will generated
if configured for Type A or B. If an output is generated it will
be active for the pre-set time (see configurations for details).
This is either a count or an output that is generated by the BFD2
when the Calibration Input is given to the BFD2 unit (see configurations
for details). The input activates the Calibration function as described
Calibration Input Requirements
The required input to activate the calibration function will depend
on the configuration of the BFD2 (see configurations for details).
Sensors that are in the 'Alarm' state are indicated in one of three
ways depending upon the BFD2 configuration (see configurations for
details). The purpose of the Alarm is to give warning that sensors
are becoming contaminated and require cleaning or have failed and
need repair or replacement.
Broken Filament Detection Sensitivity Adjustment
BFD2 units are fitted with sensitivity adjustment for the Broken
Filament signal. The factory setting is 50mV and this means Broken
Filament signals less than 50mV will not generate a count/output.
There are three trimmers that set the sensitivity, one sets the
lower limit (factory set to 30mV), the second sets the upper limit
(factory set to 0.5V). The third is used to alter the threshold
between the upper and lower limits. To determine the correct setting
for the Broken Filament sensitivity threshold, should the factory
setting be unsuitable, the signal generated by a single filament
of the yarn being monitored needs to be known (at machine speeds).
To measure this signal an oscilloscope is required. The signal is
measured at the same point as the 90mV calibration signal (see diagram
BFD2ANA). Ensure that the channel selected to measure the signal
is the same channel that the BFD2 sensor is connected.