Instrumentation

1. Transducers with Charge Output

Transducers with charge output have some special properties which require particular attention in order to obtain precise measuring results:

• Always use special low noise cables.
• The cable length must not exceed 10 meters.
• The cable should not be moved during measurement.
• All connector nuts must be tightened.

Preferably charge amplifiers should be used. It is also possible to use AC voltage amplifiers with high impedance input. Both principles are described below.

1.1 Charge Amplifiers

Accelerometers with charge output generate an output signal in the range of some picocoulombs (1 pC = 1000 fC) with a very high impedance. To process this signal by standard AC measuring equipment it needs to be transformed into a low impedance voltage signal.

Preferably, charge amplifiers are used for this purpose. The input stage of a charge amplifier features a capacitive feedback circuit which balances the effect of the applied charge input signal. The feedback signal is then a measure of input charge. Figure 10 shows a typical charge input stage.

Figure 10: Charge amplifier

The input charge q_in is applied to the summing point (inverting input) of the amplifier. It is distributed to the cable capacitance C_c, the amplifier input capacitance C_inp and the feedback capacitor C_f. The node equation of the input is therefore:

Using the electrostatic equation:

and substituting q_c, q_inp and q_f :

Since the voltage difference between the inverting and the non-inverting input of a differential amplifier becomes zero under normal operating conditions, we can assume that the input voltage of the charge amplifier uinp will be equal to GND potential. With u_inp = 0 we may simplify the equation:

and solving for the output voltage u_out:

The result clearly shows that the output voltage of a charge amplifier depends only on the charge input and the feedback capacitance. Input and cable capacitances have no influence on the output signal. This is a significant fact when measuring with different cable lengths and types.

Referring to Figure 10, the feedback resistor Rf has the function to provide DC stability to the circuit and to define the lower frequency limit of the amplifier. The circuit in Figure 10 represents only the input stage of a charge amplifier. Other stages like voltage amplifiers, buffers filters and integrators are not shown.

Typical charge amplifiers are, for example, the M68 series Signal Conditioners and the IEPE100 Remote Charge Converters made by Metra.

1.2 High-Impedance AC Voltage Amplifiers

Instead of charge amplifiers, high impedance voltage amplifiers can be used with charge mode transducers. In this case, however, the capacitances of sensor, cable, and amplifier input must be considered (Figure 11).

Figure 11: Charge mode sensor at voltage amplifier

The voltage sensitivity of an accelerometer with known charge sensitivity B_qa and inner capacitance C_i is calculated to:

B_qa and C_i can be found in the sensor data sheet.

Taking into account the capacitance of the sensor cable C_c and the input capacitance C_inp of the voltage amplifier, the resulting voltage sensitivity B´_ua will become lower than B_ua:

A typical 1.5 m low noise cable Model 009 has a capacitance of approximately 135 pF.

The lower frequency limit fl will also be influenced by C_c, C_inp and R_inp:

The lower frequency limit increases with decreasing input resistance.

Example: A charge mode accelerometer Model KS56 with inner capacitance C_i = 370 pF is connected to a typical scope input with R_inp = 10 MOhms and C_inp = 20 pF. The sensor cable has a capacitance of 135 pF.

Result: The lower frequency limit will be at about 30 Hz.

2. IEPE Transducers

A special feature of IEPE compatible transducers is that power supply and measuring signal are transmitted via the same cable. So, an IEPE compatible transducer requires, like a transducer with charge output, only one single-ended shielded cable.

Figure 12 shows the principle circuit diagram.

Figure 12: IEPE principle

The integrated sensor electronics is powered with constant current in the range between 2 and 20 mA. A typical value is 4 mA. Some battery powered instruments even work at 1 mA. The constant current Iconst is fed into the signal cable of the sensor. The supply current and the length of the cable may influence the upper frequency limit.

The de-coupling capacitor C_c keeps DC components away from the signal conditioning circuit. The combination of C_c and R_inp acts as a high pass filter. Its time constant should be sufficiently high to let all relevant low frequency components of the sensor signal pass.

Important:

• Do not apply a voltage source without constant current regulation s to an IEPE transducer.
• False polarization of the sensor cable may immediately destroy the built-in electronics.

In Figure 13 can be seen that IEPE compatible transducers provide an intrinsic self-test feature. By means of the bias voltage at the input of the instrument the following operating conditions can be detected:

Figure 13: Dynamic range of IEPE compatible accelerometers

In Figure 13 can be seen that IEPE compatible transducers provide an intrinsic self-test feature. By means of the bias voltage at the input of the instrument the following operating conditions can be detected:

• UBIAS < 0.5 to 1 V: short-circuit or negative overload
• 1 V < UBIAS < 18 V: O.K., output within the proper range
• UBIAS > 18 V: positive overload or input open (cable broken or not connected)

A variety of instruments are equipped with a constant current sensor supply. Examples from Metra are the Signal Conditioners of M68 series, M208 and M32, the Vibration Monitor M12 or the Vibration Calibrating System VC110. The constant current source may also be a separate unit, for example Model M28.

3. Intelligent Accelerometers to IEEE 1451.4 (TEDS)

3.1 Introduction

The standard IEEE 1451complies with the increasing importance of digital data acquisition systems. IEEE 1451 mainly defines the protocol and network structure for sensors with fully digital output. Part IEEE 1451.4, however, deals with "Mixed Mode Sensors", which have a conventional IEPE compatible output, but contain in addition a memory for an "Electronic Data Sheet". This data storage is named "TEDS" (Transducer Electronic Data Sheet). The memory of 64 + 256 bits contains all important technical data which are of interest for the user. Due to the restrictions of memory size the data is packed in different coding formats.

• When measuring at many measuring points it will make it easier to identify the different sensors as belonging to a particular input. It is not necessary to mark and track the cable, which takes up a great deal of time.
• The measuring system reads the calibration data automatically. Till now it was necessary to have a data base with the technical specification of the used transducers, like serial number, measured quantity, sensitivity etc.
• The sensor self-identification allows to change a transducer with a minimum of time and work ("Plug & Play").
• The data sheet of a transducer is a document which often gets lost. The so called TEDS sensor contains all necessary technical specification. Therefore, you are able to execute the measurement, even if the data sheet is just not at hand.
• The standard IEEE 1451.4 is based on the IEPE standard. Therefore, TEDS transducers can be used like common IEPE transducers.

Figure 14 shows the principle of TEDS.

Figure 14: Accelerometer with TEDS to IEEE 1451.4

If a constant current source is applied, the sensor will act like a normal IEPE compatible sensor. Programming and reading the built-in non-volatile 64 + 256 Bit memory DS2430 is also done via the sensor cable. The communication uses Maxim’s 1-Wire^® protocol (www.ibutton.com). For data exchange TTL level with negative polarity is used. This makes it possible to separate analog and digital signals inside the sensor by two simple diodes.

Metra's 8-channel IEPE signal conditioner M208A provides full TEDS support with automatic transducer sensitivity normalization.

3.2 Sensor Data in TEDS Memory

3.2.1 Basic TEDS

A 64 bit portion of the memory is called application register. It includes the so-called Basic TEDS with general information to identify the sensor:

1. Model and version number: Metra stores in this location a coded model number. The actual model number, for example "KS78.100", can be decoded by means of a *.xdl file to  IEEE 1451.4 standard, the so-called "Manufacturer Model Enumeration File" which can be found in the download section of our web pages.
2. Serial number: This is the actual serial number of the sensor which can be found on its case.
   
3. Manufacturer code: A manufacturer-specific number assigned by IEEE. Metra's manufacturer number is 61. A complete list of manufacturer codes can be found here: http://standards.ieee.org/develop/regauth/manid/public.html
   

3.2.2 Template No. 25

1. Sensitivity in V/m/s²: Sensitivity value at reference conditions according to the supplied calibration chart
   
2. Calibration frequency of sensitivity in Hz
   
3. Lower frequency limit in Hz: Typical value according to sensor data sheet
   
4. Measuring direction: Relevant for triaxial accelerometers (0 = X; 1 = Y; 2 = Z; 3 = no data)
5. Sensor weight in grams
   
6. Polarity of output signal for positive acceleration: 0 = positive, 1 = negative
7. Low pass frequency in Hz (if the sensor includes a low pass filter)
8. Resonance frequency in Hz: Typical value according to sensor data sheet
9. Amplitude slope in percent per decade
   
10. Temperature coefficient in percent per Kelvin: Typical value according to sensor data sheet
11. Calibration date (DD.MM.YY)
12. Initials of calibrating person (3 capital letters)
13. Calibration interval in days: Recommended time until next calibration
    

1. Measurement point ID (1 to 2046)
2. User text: 13 characters

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