Pair Hydraulic Automotive

Pair Hydraulic Automotive

In-depth Tutorial on Linear Position Sensors (LVDTs)

What is An LVDT?

 

The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several inches, but are also capable of measuring positions up to ±20 inches (±0.5 m).

The transformer's internal structure consists of a primary winding centered between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a one-piece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of the position sensor.

The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to move axially within the coil's hollow bore, and mechanically coupled to the object whose position is being measured. This bore is typically large enough to provide substantial radial clearance between the core and bore, with no physical contact between it and the coil.

In operation, the LVDT's primary winding is energized by alternating current of appropriate amplitude and frequency, known as the primary excitation. The LVDT's electrical output signal is the differential AC voltage between the two secondary windings, which varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic circuitry to high level DC voltage or current that is more convenient to use.

 

Why Use An LVDT?

 

LVDTs have certain significant features and benefits, most of which derive from its fundamental physical principles of operation or from the materials and techniques used in its construction.

Friction-Free Operation

One of the most important features of an LVDT is its friction-free operation. In normal use, there is no mechanical contact between the LVDT's core and coil assembly, so there is no rubbing, dragging or other source of friction. This feature is particularly useful in materials testing, vibration displacement measurements, and high resolution dimensional gaging systems.

Infinite Resolution

Since an LVDT operates on electromagnetic coupling principles in a friction-free structure, it can measure infinitesimally small changes in core position. This infinite resolution capability is limited only by the noise in an LVDT signal conditioner and the output display's resolution. These same factors also give an LVDT its outstanding repeatability.

Unlimited Mechanical Life

Because there is normally no contact between the LVDT's core and coil structure, no parts can rub together or wear out. This means that an LVDT features unlimited mechanical life. This factor is especially important in high reliability applications such as aircraft, satellites and space vehicles, and nuclear installations. It is also highly desirable in many industrial process control and factory automation systems.

Overtravel Damage Resistant

The internal bore of most LVDTs is open at both ends. In the event of unanticipated overtravel, the core is able to pass completely through the sensor coil assembly without causing damage. This invulnerability to position input overload makes an LVDT the ideal sensor for applications like extensometers that are attached to tensile test samples in destructive materials testing apparatus.

Single Axis Sensitivity

An LVDT responds to motion of the core along the coil's axis, but is generally insensitive to cross-axis motion of the core or to its radial position. Thus, an LVDT can usually function without adverse effect in applications involving misaligned or floating moving members, and in cases where the core doesn't travel in a precisely straight line.

Separable Coil And Core

Because the only interaction between an LVDT's core and coil is magnetic coupling, the coil assembly can be isolated from the core by inserting a non-magnetic tube between the core and the bore. By doing so, a pressurized fluid can be contained within the tube, in which the core is free to move, while the coil assembly is unpressurized. This feature is often utilized in LVDTs used for spool position feedback in hydraulic proportional and/or servo valves.

Environmentally Robust

The materials and construction techniques used in assembling an LVDT result in a rugged, durable sensor that is robust to a variety of environmental conditions. Bonding of the windings is followed by epoxy encapsulation into the case, resulting in superior moisture and humidity resistance, as well as the capability to take substantial shock loads and high vibration levels in all axes. And the internal high-permeability magnetic shield minimizes the effects of external AC fields.

Both the case and core are made of corrosion resistant metals, with the case also acting as a supplemental magnetic shield. And for those applications where the sensor must withstand exposure to flammable or corrosive vapors and liquids, or operate in pressurized fluid, the case and coil assembly can be hermetically sealed using a variety of welding processes.

Ordinary LVDTs can operate over a very wide temperature range, but, if required, they can be produced to operate down to cryogenic temperatures, or, using special materials, operate at the elevated temperatures and radiation levels found in many nuclear reactors.

Null Point Repeatability

The location of an LVDT's intrinsic null point is extremely stable and repeatable, even over its very wide operating temperature range. This makes an LVDT perform well as a null position sensor in closed-loop control systems and high-performance servo balance instruments.

Fast Dynamic Response

The absence of friction during ordinary operation permits an LVDT to respond very fast to changes in core position. The dynamic response of an LVDT sensor itself is limited only by the inertial effects of the core's slight mass. More often, the response of an LVDT sensing system is determined by characteristics of the signal conditioner.

Absolute Output

An LVDT is an absolute output device, as opposed to an incremental output device. This means that in the event of loss of power, the position data being sent from the LVDT will not be lost. When the measuring system is restarted, the LVDT's output value will be the same as it was before the power failure occurred.

How does an LVDT work?

The LVDT's primary winding, P, is energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1 and S2 . If the core is located midway between S1 and S2 , equal flux is coupled to each secondary so the voltages, E1 and E2 , induced in windings S1 and S2 respectively, are equal. At this reference midway core position, known as the null point, the differential voltage output, (E1 - E2 ), is essentially zero.

If the core is moved closer to S1 than to S2 , more flux is coupled to S1 and less to S2 , so the induced voltage E1 is increased while E2 is decreased, resulting in the differential voltage (E1 - E2). Conversely, if the core is moved closer to S2 , more flux is coupled to S2 and less to S1 , so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1).

The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180 degrees, as shown in the middle graph.

This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry. This is shown in the bottom graph, where the polarity of the output signal represents the core's positional relationship to the null point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor can be used over an extended range with some reduction in output linearity. The output characteristics of an LVDT vary with different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.

LVDTs and their support electronics

Although an LVDT is an electrical transformer, it requires AC power of an amplitude and frequency quite different from ordinary power lines to operate properly (typically 3 V rms at 3 kHz). Supplying this excitation power for an LVDT is one of several functions of LVDT support electronics, which is also sometimes known as LVDT signal conditioning equipment.

Other functions include converting the LVDT's low level AC voltage output into high level DC signals that are more convenient to use, decoding directional information from the 180 degree output phase shift as an LVDT's core moves through the null point, and providing an electrically adjustable output zero level.

A variety of LVDT signal conditioning electronics is available, including chip-level and board-level products for OEM applications as well as modules and complete laboratory instruments for users.

The support electronics can also be self-contained, as in the DC-LVDT shown here. These easy-to-use position transducers offer practically all of the LVDT's benefits with the simplicity of DC-in, DC-out operation. Of course, LVDTs with integral electronics may not be suitable for some applications, or might not be packaged appropriately for some installation environments.

Macro Sensors offers an extensive line of LVDTs including AC- and DC-operated versions, linear and rotary sensors, free core and spring-loaded technology, hermetically sealed and high temperature resistance units as well as custom products. Macro Sensors’ extensive line of LVDT-based linear and rotary sensors are used for linear position measurement and feedback in a variety of industrial applications including factory automation, motion control systems, metal fabricating, automotive assembly as well as power plants, gas/steam turbines. For a catalog of Macro Sensors complete line of LVDTS, refer to the web site at: http://www.macrosensors.com

 

 

 

 

 

About the Author

Macro Sensors offers an extensive line of LVDTs including AC- and DC-operated versions, linear and rotary sensors, free core and spring-loaded technology, hermetically sealed and high temperature resistance units as well as custom products. Macro Sensors’ extensive line of LVDT-based linear and rotary sensors are used for linear position measurement and feedback in a variety of industrial applications including factory automation, motion control systems, metal fabricating, automotive assembly as well as power plants, gas/steam turbines. For a catalog of Macro Sensors complete line of LVDTS, refer to the web site at: http://www.macrosensors.com

Alaska - A Pair of Giant Hydraulic Placers Searching for Gold $19.99 Alaska - A Pair of Giant Hydraulic Placers Searching for Gold - Premium Poster

Hydraulic Assembly for Jeep by Crown Automotive $209.99 1994-95 Jeep YJ Wrangler only;4-cylinder and 4.0L 6-cylinder engine;Includes hose;Unit contains fluid and is pre-assembled at factory JEEP WRANGLER (87-95YJ) RENEGADE RIO GRANDE S SAHARA SE 1994 1995 2.5L (150) 4L Power Tech (242) GAS Hydraulic Assembly for Jeep by Crown Automotive

Hydraulic Clutch Master Cylinder by Crown Automotive for Jeep $87.99 Clutch Master Cylinder JEEP CJ5 CJ7 SCRAMBLER 1980 1981 1982 1983 1984 1985 1986 4.2L (258) 5L (304) GAS Hydraulic Clutch Master Cylinder by Crown Automotive for Jeep

Hydraulic Clutch Release Bearing by Crown Automotive for Jeep $134.99 1993 Jeep XJ Cherokee;4 and 6-cylinder engine JEEP CHEROKEE (XJ) WRANGLER (87-95YJ) COUNTRY RENEGADE S SAHARA SPORT 1993 2.5L (150) 4L Power Tech (242) GAS Hydraulic Clutch Release Bearing by Crown Automotive for Jeep

Hydraulic Throwout Bearing by Crown Automotive for Jeep $143.99 1987-89 Jeep YJ Wrangler;1987-89 Jeep XJ Cherokee;1987-89 Jeep MJ Comanche;With BA 10/5 Transmission;1991-92 Jeep YJ Wrangler with 4.0L 6-cylinder engine;1991-92 Jeep XJ Cherokee with 4.0L 6-cylinder engine JEEP CHEROKEE (XJ) WRANGLER (87-95YJ) CHIEF ISLANDER LAREDO LIMITED PIONEER S SAHARA SPORT 1987 1988 1989 4.2L (258) 4L (242) GAS Hydraulic Throwout Bearing by Crown Automotive for Jeep

Hydraulic Clutch Slave Cylinder by Crown Automotive for Jeep $60.99 1997-01 Jeep TJ Wrangler;2.5L 4-cylinder and 4.0L 6-cylinder engine JEEP WRANGLER (97-06LJ) WRANGLER (97-06TJ) 65TH ANNIVERSARY EDITION RUBICON SAHARA SE SPORT UNLIMITED UNLIMITED RUBICON X 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2.4L Power-Tech (148) 2.5L (150) 4L Power Tech (242) GAS Hydraulic Clutch Slave Cylinder by Crown Automotive for Jeep

Porto-Power Hydraulic Ram - 10 Ton, 10In. Stroke $406.95 ? For use with hydraulic collision repair setsBy Blackhawk Automotive.

Blackhawk Automotive BLAB65428 PortoPower Air/Hydraulic Foot Pump 10 Ton $753.61 Includes hydraulic hose mounting base plate. 10 000 PSI. Manufactured to the highest quality available. Satisfaction ensured.

Hydraulic Throwout Bearing by Crown Automotive $162.99 1987-92 YJ Wrangler;1987-92 XJ Cherokee;1987-92 MJ Comanche;Fits 2.5L 4-cylinder engine

Hydraulic Clutch Master Cylinder Hose by Crown Automotive for Jeep $41.99 1987-90 Jeep YJ Wrangler;4-cylinder engine JEEP WRANGLER (87-95YJ) ISLANDER LAREDO S SAHARA SPORT 1987 1988 1989 1990 2.5L (150) GAS Hydraulic Clutch Master Cylinder Hose by Crown Automotive for Jeep

Porto-Power Air/Hydraulic Foot Pump - 10 Ton $546.95 ? Includes hydraulic hose, mounting base plate ? 10,000 PSIBy Blackhawk Automotive.

Hydraulic Clutch Master and Slave Cylinder kit by Crown Automotive $198.99 1997-01 Jeep TJ Wrangler;2.5L 4-cylinder and 4.0L 6-cylinder engine JEEP WRANGLER (97-06LJ) WRANGLER (97-06TJ) RUBICON SAHARA SE SPORT UNLIMITED X 1997 1998 1999 2000 2001 2002 2003 2004 2.4L Power-Tech (148) 2.5L (150) 4L Power Tech (242) GAS Hydraulic Clutch Master and Slave Cylinder kit by Crown Automotive

Hydraulic Clutch Release Bearing by Crown Automotive $134.99 1993 XJ Cherokee;4 and 6-cylinder engine

Hydraulic Clutch Master Cylinder Hose by Crown Automotive $41.99 1987-90 YJ Wrangler;4-cylinder engine

Hydraulic Clutch Master Cylinder by Crown Automotive $71.99 1982-86 CJ with diesel engine and right hand drive;1987-90 YJ Wrangler;4 and 6-cylinder engines

Hydraulic Clutch Slave Cylinder by Crown Automotive $60.99 1997-01 TJ Wrangler;2.5L 4-cylinder and 4.0L 6-cylinder engine

Hydraulic Clutch Master Cylinder Assembly by Crown Automotive $209.99 1994-95 YJ Wrangler only;4-cylinder and 4.0L 6-cylinder engine;Includes hose;Unit contains fluid and is pre-assembled at factory

Hydraulic Servo-Systems: Modelling, Identification and Control $105.11 Hydraulic Servo-systems details the basic concepts of many recent developments of nonlinear identification and nonlinear control and their application to hydraulic servo-systems: developments such as feedback linearisation and fuzzy control. The principles, benefits and limitations associated with standard control design approaches such as linear state feedback control, feedforward control and compensation for static nonlinearities are also reviewed, because of their continued practical importance. Featuring: a theoretical (physically based) modelling of hydraulic servo-systems; a experimental modelling (system identification); a control strategies for hydraulic servo-systems; a case studies and experimental results. Appendices outline the most important fundamentals of (nonlinear) differential geometry and fuzzy control. The book is very application-oriented and provides the reader with detailed working procedures and hints for implementation routines and software tools. Hydraulic Servo-systems will interest scientists and qualified engineers involved in the analysis and design of hydraulic control systems, especially in advanced hydraulic industries, the aeronautical and space, and automotive industries.

Hydraulic Groove $8.99 Hydraulic Groove

Avid Elixir R 160 Hydraulic Disc Brakes $155.99 Avid Elixir R 160 Hydraulic Disc Brakes. The Avid Elixir R hydraulic disc brake from SRAM delivers pure, simple performance that you can count on every time you grip the levers. Like all of Avid's new hydraulic disc brakes, it features TaperBore Technology, tool-free reach adjustment, Power Reserve Geometry, Tri-Align Caliper Positioning System, and drip-free bleeding. Sure, it lacks the Elixir CR's carbon lever and adjustable banjo, but then again, it also costs less. Pair it up with the SRAM X7 family and you've got yourself a whole heap of priceless riding ahead of you.

Hydraulic Brake Systems: ASP-SS-KT-359-00 $253.99 For the automotive enthusiast a vehicle’s brake system rarely generates the same level of interest that the powertrain does. But the professional automotive technician knows that brake system servicing represents a large portion of shop work and, therefore, income. Aspire’s Hydraulic Brake Systems course will provide you with a thorough understanding about the role that each component plays in a standard hydraulic brake system. It is designed to teach you the fine points of brake system service as well as help you develop sound diagnostic techniques. While the information presented in this book is not designed to substitute for the appropriate service manual, it will help you interpret the meaning behind the vehicle manufacturer’s diagnostic procedures.

Hydraulic Brake Systems: ASP-SS-SK-359-00 $109.49 For the automotive enthusiast a vehicle’s ASP-PT-SK-299-00 brake system rarely generates the same level of interest that the powertrain does. But the professional automotive technician knows that brake system servicing represents a large portion of shop work and, therefore, income. Aspire’s Hydraulic Brake Systems course will provide you with a thorough understanding about the role that each component plays in a standard hydraulic brake system. It is designed to teach you the fine points of brake system service as well as help you develop sound diagnostic techniques. While the information presented in this book is not designed to substitute for the appropriate service manual, it will help you interpret the meaning behind the vehicle manufacturer’s diagnostic procedures.

The 2006-2011 World Outlook for Industrial Pumps Excluding Hydraulic Fluid Power Pumps, Automotive Circulating Pumps, and Measuring and Dispensing Pumps $795 The latent demand for industrial pumps excluding hydraulic fluid power pumps, automotive circulating pumps, and measuring and dispensing pumps is not actual or historic sales. Nor is latent demand future sales. In fact, latent demand can be lower either lower or higher than actual sales if a market is inefficient (i.e., not representative of relatively competitive levels). Inefficiencies arise from a number of factors, including the lack of international openness, cultural barriers to consumption, regulations, and cartel-like behavior on the part of firms. In general, however, latent demand is typically larger than actual sales in a country market.

Rubber Seals for Fluid and Hydraulic Systems $149 Rubber Seals for Fluid and Hydraulic Systems is a comprehensive guide to the manufacturing and applications of rubber seals, with essential coverage for industry sectors including aviation, oil drilling and the automotive industry. Fluid leakage costs industry millions of dollars every year. In addition to wasted money, unattended leaks can result in downtime, affect product quality, pollute the environment, and cause injury. Successful sealing involves containment of fluid within a system while excluding the contaminants; the resilience of rubber enables it to be used to achieve these two objectives and create a tight sealing effect. A sound understanding of the complex factors involved in successful fluid sealing is essential for engineers who specify, design, operate and maintain machinery and mechanical equipment. This book focuses on the characteristics of rubbers as seals, their manufacturing procedures, the implications of their physical and chemical characteristics for the sealing function in the fluid and hydraulic systems, how rubbers seal and prevent leaks, what properties are required for sealing function, and how they change before and after installation. The chapter on Manufacture of Seals and O'Rings includes approximately 25 workable starting point formulations based on different rubbers, with cure and property data of those formulations as guidelines for technologists and engineers. Emphasis on important areas such as applications of rubber as fluid seals in the nuclear, aviation, oil drilling and automotive industries Includes a chapter on Rubber Expansion Joints as the function of such expansion joints as pipe connectors is indirectly linked with leakage and prevention of fluid flow through the pipes The chapter on Manufacture of Seals and O'Rings includes approx. 25 workable starting point formulations based on different rubbers, with cure and property data of those formulations as guidelines for technologists and engineers