Pi LVDT and LVDT Amplifier APPLICATION NOTE

Transcription

1 Pi LVDT and LVDT Amplifier APPLICATION NOTE

2 Disclaimer Pi Research makes no representation or warranties of any kind whatsoever with respect to the contents hereof and specifically disclaims any implied warranties of merchantability or fitness for any particular purpose. Pi Research shall not be liable for any errors contained herein or for incidental or consequential damages in connection with the furnishing, performance or use of the software, associated hardware, or this written material. Pi Research reserves the right to revise this publication from time to time, and to make changes in the content hereof without obligation to notify any person of such revision or changes. A copy of the Pi Research Terms and Conditions of Sale is available on request, and includes a declaration of the warranty and limitation of liability which apply to all Pi Research products and services. Health and Safety information Under the terms of European and UK Health and Safety Legislation, Pi Research is required to classify any hazardous materials in the products it supplies and to provide relevant safety information to users. Any hazardous materials in Pi products are clearly marked with appropriate symbols. Product Safety Data Sheets relating to these materials are available on request. 2 LVDT Application Note

3 Contents Introduction... 4 LVDT Construction and Theory of Operation... 6 LVDT Types... 7 Pi LVDT Amplifier... 9 Connecting the LVDT and Amplifier Installation Notes LVDTs LVDT Amplifier Calibration and Testing Pi LVDT and Amplifier Error States Interchanging Pi LVDTs and LVDT amplifiers Pi LVDTs Specifications LVDT Dimensions Parts Numbering Frequently Asked Questions Glossary Appendix A: Fault Finding Contact information

4 Introduction LVDT is an acronym of Linear Variable Differential Transformer (LVDT), which as the name implies is a transformer with variable secondary coupling dependent upon the core position. The Pi LVDT range and LVDT Amplifier are designed to make using an LVDT as simple as using a linear potentiometer, with all the advantages associated with using LVDTs of extreme ruggedness, low friction, infinite resolution and high temperature operation. LVDTs are used to directly measure linear or rotary (RVDT) displacement or indirectly to measure parameters such as force or pressure. LVDTs are available in wide variety of ranges from a few millimetres to a few metres in stroke length. The LVDT like any transformer must be driven with an AC voltage which produces an AC voltage at the secondaries, the relationship of this can be used to determine the core position displacement. The complex drive (modulation) and decode (demodulation) circuitry required by the LVDT is all provided by the Pi LVDT amplifier. Configuring the modulator/demodulator circuitry for different LVDTs can be complex and specialised. The Pi LVDT and Pi LVDT Amplifier together simplify this problem by producing the same output voltage range, independent of stroke length. This makes using the Pi LVDT and Amplifier as simple as using a linear potentiometer; with the amplifier producing a DC voltage proportional to position. The added advantage is that unlike a potentiometer, a well regulated supply is not required. 4 LVDT Application Note

5 In particular the Pi LVDT and LVDT Amplifier combination produce an output voltage range of 0.5V to 4.5V (where the centre position is 2.5V) irrespective of stroke length. This simplifies changing Pi sensors and amplifiers as only a change in the software calibration is required for a change in LVDT stroke length. = = OUTPUT VOLTAGE 4,500 2,500 0,500 RETRACT TOTAL ELECTRICAL STROKE EXTEND LVDT Output Voltage Range 5

6 LVDT Construction and Theory of Operation An LVDT consists of a primary winding, two secondary windings and a core. The primary and secondary windings are housed in a metal barrel, with a moveable magnetic core rod which goes inside the barrel. The Pi LVDT has an outer sleeve which provides mechanical protection and low friction bearing surfaces. Primary Barrel Moveable magnetic core Secondaries Cross Sectional Diagram of an LVDT When the primary is driven with an AC voltage a corresponding AC voltage is induced in the two secondary windings, the magnitude of which is dependent upon the core position and hence displacement. SEC+ PR1+ VsecA Vpr SEC CENTRE PR1 VsecB SEC AC Voltage in the Two Secondary Windings 6 LVDT Application Note

7 The cores are normally wound in series opposition so the two secondary windings are 180 out of phase with respect to the centre tap. The example above shows the electrical zero position where both voltages are equal magnitude but 180 out of phase with respect to the centre tap. The electrical and mechanical centre of travel is the same for many LVDTs. The electrical range of the LVDT is the range over which the electrical specifications hold true. The mechanical range of the LVDT is usually longer than the electrical range and it is not necessarily true that it will remain within its electrical specification, as the accuracy or linearity may suffer. The Pi range of LVDTs have three grooves etched in the barrel to denote the nominal retract, centre and extend positions. LVDT Types LVDTs are available with either four wires or five wires, where the extra wire is the centre tap. The Pi LVDT amplifier only functions with five wire LVDTs as it is a phase insensitive design and requires the centre tap as a reference voltage. The relationship of the output voltage to the mechanical position is specified by LVDT manufacturers in two ways: 1. As a sum and difference ratio at full scale with respect to the centre tap position i.e. R = (VsecA VsecB)/(VsecA+VsecB). Common ratios are 0.3, 0.4 and

8 = = R = RETRACT EXTEND Total Electrical Stroke LVDT Types 2. As a sensitivity ratio dependent upon the primary excitation and core position. This is usually specified as Output Volts per Volt Primary Drive per mm Stroke Length, where the units are in the form mv/v/mm. For example, if a 25mm LVDT with sensitivity of 16mV per Volt In per mm is driven by 2V RMS at +10mm the output would be 2 x 0.016x10=0.320V RMS measured across the windings. The equation of Mode 1 has the advantage of being independent of primary excitation level, thus not affected by loading and temperature drift problems. Many LVDTs, although specified in Mode 2 terms, can actually be used successfully in Mode 1. The LVDT s electrical performance is specified at a spot frequency with 2.5 khz and 7.5 khz being common. The lower frequency usually gives better linearity but requires more drive current. Unfortunately, the sensitivity of the LVDT varies with frequency; if an LVDT is being used at a frequency which is far removed from the specified sensitivity, the new sensitivity should be measured. 8 LVDT Application Note

9 Pi LVDT Amplifier In order to derive the position from an LVDT, a modulator is required to generate the primary AC Voltage and a demodulator is required to translate the AC secondary voltage to a DC voltage proportional to position. All this circuitry is contained within the Pi LVDT amplifier. Vin+ Vin Modulator Core Demodulator LVDT LVDT Modulator/Demodulator Circuitry The modulator has to be capable of generating different frequencies dependent upon the LVDT requirements. The demodulator requires different gains and operating mode dependent upon the LVDT type and scaling. The Pi LVDT amplifier supports both LVDTs specified as Mode 1 (sum and difference ratio) and Mode 2 (sensitivity mv/v/mm). Mode 1 is the preferred operating mode for the Pi LVDT amplifier as this gives the highest accuracy with the additional benefit of indicating some errors such as broken LVDT primary wiring. Many demodulator circuits are phase sensitive. Phase errors from primary to secondary may also introduce errors. The Pi LVDT amplifier circuit is phase insensitive so does not suffer from this problem, but it will not function with 4-wire LVDTs as it requires the centre tap as a reference. 9

10 The standard operating mode for the LVDT amplifier is in Mode 1, with 0.5 sum and difference ratio and output range of 0.5V to 4.5V, with electrical zero corresponding to 2.5V. This allows errors such as broken LVDT drive and system looms to be detected by the logging system. If the LVDT amplifier detects the loss of primary excitation in Mode 1 it will flag this by going to between 7V and 8V. Connecting the LVDT and Amplifier A Pi LVDT and Amplifier require a nominal 12V supply and consume a maximum of 30mA. It generates an output voltage between 0.5V and 4.5V which is proportional to distance, of which the amplifier typically requires 20mA. Other LVDTs may need more current than the Pi LVDT and thus may require connection to a high power (100mA) channel. When operating in Mode 1, many LVDT wiring errors such as a broken LVDT loom are indicated by the analogue output going between 7V and 8V. If the amplifier loses power or the output voltage is lost, the logging system shows the analogue input at 0V. A negative output sense wire is provided to eliminate the effect of voltage drop along the 0V wire supplying power to the LVDT amplifier. This connection will give the highest performance but is optional. Connection of the LVDT amplifier to a standard 4-pin Lemo connector used on Pi junction boxes is shown below: FGG.OB AS PB-HE Connection of the LVDT Amplifier to a 4-pin Lemo Connector 10 LVDT Application Note

11 Grounding The LVDT amplifier body and connectors are joined to the system ground connection back to the logger and provide an effective RF shield. This means that care must be taken to avoid ground loops. The LVDT screen is connected at the LVDT Amplifier connector but not at the LVDT. To avoid a ground loop the LVDT amplifier must be mounted on industrial Velcro or Anti- Vibration (AV) mounts. When using other manufacturers LVDTs take care that the LVDT screen and LVDT amplifier LVDT connector screen are not connected otherwise an earth loop is formed. The looms to the LVDT Amplifier from the system and LVDT to the amplifier should each not exceed 5m. Indicates Electrical Connection Logging System LVDT Amp Chassis AV Mount (Electrically Isolated) LVDT No Connection Optionally Electrically Connected LVDT Amplifier Body 11

12 Installation Notes LVDTs Select a position where the sensor will not be in contact with water, fuel or oil. Make sure nothing is pushing the sensor barrel to one side. The grooves on the LVDT sleeve indicate the middle and ends of the stroke, use these as a first attempt at a mechanical datum and to check the mechanical movement range. Do not use the sensor as a physical end stop. Make sure that the sensor can move freely over its range without interfering with other parts of the vehicle. Use the 3mm rod end bearings to mount the sensor. Keep the temperature below 120 C. Try not to place the sensor near sources of electrical interference such as ignition coils, plug leads, alternators and telemetry antennas. Ensure that the movement range being measured does not exceed the mechanical stroke of the LVDT. LVDT Amplifier Select a position where the amplifier will not be in constant contact with water, fuel or oil. Make sure that the amplifier will not be affected by heat soak. Isolate the amplifier chassis from the vehicle chassis using AV mounts or industrial Velcro. Check that none of the LVDT amplifier metal parts including connectors will be in contact with the vehicle chassis. Try not to place the amplifier near sources of electrical interference i.e. ignition coils, plug leads, alternators and telemetry antennas. 12 LVDT Application Note

13 Calibration and Testing Pi LVDT and Amplifier The LVDT amplifier is designed to have low zero offset and gain error to ease interchangeability between amplifiers. The standard output range for the LVDT and Amplifier combination is 0.5V to 4.5V with the mechanical/electrical zero point being at 2.5V with the LVDT excitation frequency being nominally 2.5 khz. Displacement Output (10 bit ADC) Output (12 bit ADC) Full Scale In Centre Full Scale Out Error States For a Pi LVDT, if the LVDT position is in the stroke range and not as above within a few counts, there is a fault either in the installation, the wiring, the LVDT or the Amplifier. The 0.5V to 4.5V range is chosen so that a disconnected system loom or LVDT loom can be detected. The LVDT amplifier output when used in Mode 1 will go to around 7.5V if certain faults occur on the LVDT loom, such as a broken primary excitation. Loss of power to the LVDT or broken output wiring will result in the analogue output dropping to 0V. For further information on fault finding see Appendix A. 13

14 Interchanging Pi LVDTs and LVDT amplifiers When interchanging LVDT amplifiers it should not be necessary to change calibration. When interchanging Pi LVDTs of the same stroke length it may be necessary to adjust the offset due to slight variations in position in the mechanical zero. When interchanging LVDTs of differing stroke length additionally the software calibration will require modification for the change in stroke length. Pi LVDTs Pi LVDTs are five wire LVDTs specified as Mode 1 with a constant sum and difference ratio at full scale, irrespective of stroke length. This means that changing between different Pi LVDTs requires only a software calibration change. The mechanical and electrical zero are the same throughout the Pi LVDT range, the excitation frequency being 2.5 khz. The Pi LVDT amplifier is configured as standard for operation with Pi LVDTs and produces an output voltage of 0.5 to 4.5V across the stroke, the centre being at 2.5V. Using other LVDTs means that the output range will differ in span but still be centred around 2.5V at electrical zero. When using LVDTs from other manufacturers, contact Pi Support to discuss your requirements. This is particularly important as many LVDTs require a higher excitation frequency than the standard 2.5 khz, resulting in the amplifier not functioning correctly. 14 LVDT Application Note

15 Specifications Dimensions M3 clear (2 posns) SYS LVDT LVDT Amplifier Description Value Input Voltage 8.5V 18V DC Supply Current Typical Output Voltage 0.5V 4.5 V LVDT disconnected Output goes to 7.5V Frequency Response 0 200Hz Zero Accuracy ±5mV Typical Zero Drift 20uV/ C Typical Gain Accuracy ±0.5% of Full Scale Typical Gain Drift 85ppm/ C Typical Vibration 50g Max Temperature Range 0 C to 70 C Environmental IP65 Weight 75 grams 15

16 LVDT Amplifier Connectors 1 1 System LVDT Description Amplifier Connector Mating Connector System AS206-05SB-HE AS606-05PB-HE LVDT AS206-05SN-HE AS606-05PN-HE LVDT Amplifier Connector Details Pin System Connector Pin LVDT Connector 1 +12V 1 Primary Excitation +ve 2 GND 2 Primary Excitation ve 3 GND Sense (Optional) 3 Secondary +ve 4 Analogue Output 4 Secondary ve 5 Case 5 Secondary Centre Scr Connected Scr Isolated at LVDT 16 LVDT Application Note

17 LVDT Dimensions 12.0 typ C ±0.5mm retracted Electrical stroke Ø3.05 Ø13.0 max E lectrical Stroke mm 25 (±12.5) 50 (±25) 75 (±37.5) 100 (±50) 125 (±62.5) 150 (±75) Dimension C Weight grams

18 The following specification is common throughout the LVDT Range. Description Value Sensitivity R 0.5 Excitation Voltage 1.0V 10V RMS Excitation Frequency 400Hz to 12.5 khz Sinewave Input Impedance >300 khz Load Resistance >50K ohms Frequency Response 0 200Hz Accuracy ±1.0% of Full Scale Linearity ±0.5% of Full Scale Gain Drift ±10ppm/ C Shock 20g Temperature Range 20 C to +125 C Environmental IP66 Mechanical Stroke Electrical Stroke +1mm LVDT Connectors Description Sensor Connector Mating Connector LVDT AS606-05PN-HE AS SN-HE LVDT Connector Details Pin LVDT Connector 1 Primary Excitation +ve 2 Primary Excitation ve 3 Secondary +ve 4 Secondary ve 5 Secondary Centre Scr Isolated at LVDT 18 LVDT Application Note

19 Parts Numbering Product Part Number LVDT 25mm Total Stroke 01B LVDT 50mm Total Stroke 01B LVDT 75mm Total Stroke 01B LVDT 100mm Total Stroke 01B LVDT 125mm Total Stroke 01B LVDT 150mm Total Stroke 01B LVDT Amplifier 01B System Connector Loom 03E LVDT Extension Loom 03L

20 Frequently Asked Questions How can I check that my LVDT is correctly connected? This can be achieved before connection by using a DVM to measure the resistance between windings using an LVDT break-out loom. Measure the resistance between the Primary +ve and the Primary ve; this should be <1K ohms. Measure the resistance between the Sec +ve and SecCentre; this should be <1K ohms. Measure the resistance between Sec ve and SecCentre; this should be <1K ohms and similar to the previous value. Finally measure between Sec ve and Sec +ve; this should be <4K ohms and approximately twice the value previously measured. An LVDT simulator which plugs directly into the LVDT loom connections can be obtained from Pi for checking the LVDT loom and amplifier operation. Why do you specify an output range of 0.5V to 4.5V? This range is specified to give reasonable resolution in the 0V 5V range but also allow errors such as a broken system loom or LVDT to be flagged to the logger. A broken system loom is seen as 0V and a broken LVDT is seen as 5V by a Pi datalogger. Why do I get different outputs from the amplifier with different LVDTs? Different types of LVDTs have different sensitivities. When amplified through the LVDT amplifier this results in a different output range. If different LVDTs of the same sensing range produce varying output, this is usually a direct indication that the accuracy between LVDTs is poor. The Pi range of LVDTs simplifies this problem by having the same sensitivity throughout the range. 20 LVDT Application Note

21 How long can the cables be? The cables to the LVDT Amplifier from the system and from the Amplifier to the LVDT can each be up to a maximum of 5m. What is the advantage of the standalone amplifier over the built-in amplifiers in Pi Loggers? The standalone amplifier is optimised for use with the new range of LVDTs. It also typically offers better offset accuracy, gain accuracy and lower temperature drift compared to the built in amplifiers. In addition, it can also signal broken LVDT connections in sum and difference ratio mode. 21

22 Glossary Electrical Range Electrical Zero Mechanical Range Mechanical Zero Sensitivity Sum and Difference Ratio The electrical range of the LVDT is the range over which the electrical specifications hold true. The point where both voltages appearing on the secondary are the same magnitude. The mechanical range of the LVDT is usually longer than the electrical range but it is not necessarily true that it will remain in electrical specification. This is the centre of the mechanical range. The electrical and mechanical zero for many LVDTs including the Pi LVDT is the same. This is used by some manufacturers to specify the output range and is in units mv Out per Volt In per mm. Where Volt In is the primary excitation in V RMS and mm is the stroke position. Other LVDT manufacturers specify their LVDTs as a Sum and Difference Ratio i.e. (VsecA VsecB)/(VsecA+VsecB) where Vsec A and Vsec B are the LVDT secondary voltages. 22 LVDT Application Note

23 Appendix A: Fault Finding Below is a non-exhaustive fault finding diagram. If you experience any further problems please contact Pi Research. Move LVDT to Centre Position Faulty Check System Loom < 0.2V LVDT Amp > 7V Reading Check LVDT Wiring Faulty OK < 8.5 > 20V Reversed Check Power Supply Move LVDT to Retract Position OK Replace LVDT Amplifier or LVDT OK GAIN FAIL 0.2 < or < 2.5V Box A LVDT Amp Reading > 2.5V PASS <> Move LVDT to Extend Position Functional Incorrect Gain GAIN FAIL 2.5 < or > 7 Box B LVDT Amp Reading < 2.5 PASS < LVDT & Amplifier Functioning Correctly LVDT Fault Finding Diagram The LVDT Simulator which plugs directly into the LVDT loom can be obtained from Pi to help the fault finding process. Alternatively the LVDT connections resistance can be used to check the wiring. 23

24 Contact information For more information about Pi products and details of worldwide authorised agents, please contact: Pi Research Brookfield Motorsports Centre Twentypence Road Cottenham CAMBRIDGE UK Customer Support Tel +44 (0) CB4 8PS Fax +44 (0) Pi Research, Inc Haverstick Suite #275 Indianapolis IN Tel +1 (317) USA Fax +1 (317) Research Part Number: 29B E Issue 3.0 September 1999 Pi and the Pi logo are trademarks of Pi Group Limited Pi Research LVDT Application Note