MMF M72A1, M72A3, M72B3, M72B1 Instruction Manual
Instruction Manual
Manfred Weber
Metra Mess- und Frequenztechnik in Radebeul e.K.
Meißner Str. 58 - D-01445 Radebeul
Phone +49-351-836 2191 Fax +49-351-836 2940
Email: Info@MMF.de Internet: www.MMF.de
Charge / IEPE
Conditioners
M72 Series
The latest version of this document can be downloaded from:
http://www.mmf.de/product_literature.htm
© Manfred Weber
Metra Mess- und Frequenztechnik in Radebeul e.K.
Jan/ 17
Contents
1. Application...............................................................................................................................3
2. Function and Operation............................................................................................................4
3. Power Supply and Mass...........................................................................................................5
3.1. Grounding Concept...........................................................................................................5
3.2. External Power Supply......................................................................................................5
3.3. Sources of Errors...............................................................................................................5
3.3.1. Avoiding Ground Loops..............................................................................................5
3.3.2. Low-Frequency Noise.................................................................................................6
4. Inputs.......................................................................................................................................6
4.1. Charge Input...................................................................................................................... 6
4.2. IEPE Input.........................................................................................................................7
4.2.1. Switching Off the IEPE Power Supply.......................................................................8
5. Amplifier..................................................................................................................................8
6. Transducer Sensitivity (M72Ax / M72S1)................................................................................9
7. Level Indicators...................................................................................................................... 10
8. Filters and Integrators............................................................................................................. 10
8.1. Low Pass Filter................................................................................................................10
8.2. High Pass Filter...............................................................................................................11
8.3. Integrators........................................................................................................................11
9. Keypad Lock..........................................................................................................................13
10. Function Check and Calibration...........................................................................................13
11. Control via USB (M72R1 / M72S1).....................................................................................14
11.1. Rack M72R8 and M72S8..............................................................................................14
11.2. USB Interface................................................................................................................15
11.3. Interface Commands......................................................................................................16
11.4. Setup Tool for M72R8 / M72S8 Rack Cases.................................................................18
12. Technical Data...................................................................................................................... 20
Appendix: Warranty
Declaration of Conformity
1
The letter “x” in this manual stands for the channel number of the versions M72A 1, M72B1,
M72A3 and M72B3.
2
Figure 1: M72A1 front panel
Figure 2: M72A1 rear panel
1. Application
The M72 Series Signal Conditioners are intended for connection of piezoelectric acceleration,
force or pressure transducers. The input is suitable for sensors with charge output as well as for
IEPE compatible transducers and microphones.
The M72 enables optimal adjustment of the sensor signal to existing measuring equipment or
PC-based data acquisition systems. The Signal Conditioners provide the following functions:
• Adaptation of the sensor signal and sensor supply
• Amplification
• High- and low-pass filtering (for example anti-aliasing filter)
• Integration of the sensor signal, for instance, to measure velocity or displacement.
The instruments all have a robust aluminum housing making them suitable for work in the labo ratory as well as under field conditions.
The following model variants are available:
Type Channels Sensitivity Standardization
TEDS
1
Level Meter
M72A1 1 yes yes yes
M72A3 3 yes yes yes
M72B1 1 no no no
M72B3 3 no no no
1 Transducer Electronic Data Sheet
3
2. Function and Operation
Figure 3 illustrates the most important functional groups of M72 in a block diagram.
Figure 3: Block diagram (*these elements only with M72Ax and M72S1)
All functions are controlled by a microcontroller.
Depending on the position of the input switch, the input signal passes the impedance converter
Q1 or Q2. If IEPE operation has been selected, the signal is transmitted to the amplifier via a
buffer stage. During IEPE operation a constant current is fed into the input socket to supply the
sensor electronics. With the M72Ax and M72S1 instruments the stored electronic data (TEDS)
of selected IEPE sensors can be read out. If an AC voltage supply is to be connected to the input, the constant current supply can be switched off.
The input circuit is followed by the first amplifier stage, low pass and high pass filters. The high
pass filter can be switched between 0.1 Hz and 3 Hz. The low pass filter has 4 frequency limits
to choose from, including 0.1 / 1 / 10 / 50 kHz.
The sensor sensitivity of M72Ax and M72S1 can be entered to achieve finely tuned amplifica tion. To aid this there is an adjustable damping stage after the filters.
The single and double integrators can be switched on to measure velocity and displacement, re spectively.
A second amplification level follows subsequently. Due to the distributed amplification before
and after the filter there is an overload reserve which is also applied to signal components out side of the filter or the integrator pass band. Simultaneously a high signal-to-noise ratio is
achieved.
Before the output socket there is a driving stage with low pass filters to reduce disturbance. The
output is DC coupled.
The overload LED lights up when the output signal is over 90% of the full modulation. It also
signalizes the occurrence of overload before and after the filters and integrators.
The M72Ax and M72S1 instruments have a digital display which shows the modulation at the
output as a percentage of the maximum value.
All settings are configured via the buttons. Five seconds after the last press of a button the in strument saves the current settings. This means that the last operating status remains even after
the instrument has been switched off and on again.
The units with display M72Ax and M72S1 show the hardware version and the software version
for one second each, after switching them on.
4
U
Q/10
Q
U
Q/10
Q
Q1
Q2
IEPE
supply
IEPE on/off
L.P. frequency
0,1Hz
3Hz
V2
1. Int. 2. Int.
V1
Gain
a
v
d
Gain
Input
Overload
LED
Output
+
1
TEDS*
Read TEDS
1
10
100
1
10
0,1
1
10
50
kHz
100 kHz
Sensitivity*
0,1...1
Level
indication*
High pass Low pass
µC
3. Power Supply and Mass
3.1. Grounding Concept
The inputs and outputs of the signal conditioners are single ended, i.e. asymmetrical. The signal
ground is connected to the outer conductor of the input and output sockets.
The instrument housing is internally connected to ground.
The power supply is signal-referenced, i.e. the negative terminal is connected to ground.
3.2. External Power Supply
The M72 Signal Conditioners are powered by an external DC voltage
A mains plug adapter for 115/230 V AC with 12 V output voltage is supplied with the Signal
Conditioners M72Ax and M72Bx. The power supply socket according to DIN 45323 is located
at the rear of the instruments. Any DC voltage source of 7 V to 28 V DC and around 250 mA
(for M72x1) or 750 mA (for M72x3) can be connected to this socket. The positive supply terminal is connected to the center pin (tip). The ON/OFF switch is located at the rear.
Models M72R1 and M72S1 are powered from the rack cases M72R8 or M72S8 via a 9 pin DSub connector at the rear side of the modules.
3.3. Sources of Errors
3.3.1. Avoiding Ground Loops
Earthing or ground loops are often the reason for measuring errors in multichannel measuring
systems. In most cases you will find a superimposed 50 (60) Hz or 100 (120) Hz voltage on the
measuring signal. One reason for this effect may be that the transducers are ground referenced
not only via their cable at the signal conditioner, but also at the measuring point through their
housing. Vibration transducers are often mounted at grounded machine parts. Within earthing
systems transient currents may appear. These transient currents cause a potential drop across the
earthing or grounding wires. Via the signal input of the amplifier they may result in a consider able measuring error.
To avoid this, insulated attachment of the transducers is recommended. Metra offers several in dustrial vibration transducers with an insulated mounting base and different insulating flanges
for non-insulated sensors.
A star-shaped grounding network is the ideal solution to avoid ground loops. Star-shaped means
that all grounding wires of the sensors and the amplifier outputs are tied to ground at the signal
conditioners, without any transverse connections. In many cases this is more difficult to realize
for the outputs than for the inputs, because the subsequent measuring equipment may have single-ended inputs. If you have the choice of using differential inputs, which can be found on
many data acquisition boards, you should preferably use them.
5
3.3.2. Low-Frequency Noise
When the M72 is used with high gain and with the lower frequency limit of 0.1 Hz (high pass
off) it may be sensitive to temperature transients. Even draft may result in output voltage fluctuation of some hundred millivolts at frequencies around 0.1 Hz. To reach thermal balance a
warm-up time of some minutes is recommended before measurement. For measurements outdoors it may be advantageous to isolate the device by foam rubber or similar materials.
Piezoelectric sensors have the so-called pyroelectric effect which may cause similar distur bances. This phenomenon is particularly strong with compression type and bending type (beam)
transducers. Here it is also important to maintain stable temperatures.
If lowest frequencies are not of interest the 3 Hz high pass should always be used. With the inte grators (see Chapter 8.3) the 3 Hz high pass is always on.
4. Inputs
The Signal Conditioners M72 are designed for both sensors with charge output and sensors with
integrated impedance converters to IEPE standards. Both input types use the same BNC input
socket. Select the input type by pressing the “Input” button repeatedly. The current status is indicated by a blue LED for charge input or white for IEPE. If neither of the LEDs are lit, the in strument is running as a voltage amplifier without IEPE power supply.
4.1. Charge Input
Capacitive signal sources, usually piezoelectric sensors with charge output, are connected to the
charge input. The input stage is a capacitive feedback amplifier, which transforms the sensor
charge signal into a voltage. The ratio of output charge (U) to input charge (Q) is determined by
the capacitance of the feedback amplifier (C).
U =
Q
C
The M72 Signal Conditioners have two charge converter stages. At the lowest gain range a
charge converter with a gain of 0.1 mV/pC is used. For the other gain ranges a charge converter
with 1mV/pC is active.
To ensure that the output charge reverts to zero after a specified period of time without a charge
signal, discharge resistors are located over the feedback capacities. The resistors are designed in
such a way that a lower frequency limit of 0.1 Hz is obtained. This makes manual reset no
longer necessary.
The advantage of charge measurement is that cable capacitance and insulation resistance have
almost no influence on the measuring results. For sensors with charge output it is strongly rec ommended to use special low-noise cables. Ordinary cable will cause a considerable measuring
error under mechanical stress, as a result of the so-called triboelectric effect. Cables with low insulation resistance, for example caused by humid connectors, reduce the accuracy of measure ment at lower frequencies. A desirable insulation resistance is higher than 10 G W. Cables longer
than 10 m are not recommended at the charge input. Proper shielding is also important.
6
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