LVDT Use in Automotive SuspensionTrans-Tek, Inc. LVDT Application 800-828-3964For OEM applications, AC-AC LVDTs from the Series 270-280-290 provide cost-effective solutions for accurate positionfeedback. To fully support these transducers, only basicelectronics are required, including one of several commerciallyavailable Integrated Chips or I/Cs. Combining an AC-ACLVDT with the necessary electrical components will result ina relatively low-cost system with high-end performance.AimRite Holdings Corporation is a pioneer in the field ofcomputer controlled suspension technology for the automotiveindustry. Targeting mainly sports utility vehicles (SUV),trucks, buses and RVs, AimRites trademark product - theCOAST system - uses the Model 0283-0000 AC-AC LVDTfor continuous position feedback. This advanced suspensiondesign offers numerous benefits including: luxury car ridewith sports car performance, on-road and off-road application,replacement of sway bars and shock absorbers, simultaneouscontrol of all vehicle dynamics, compatibility with conventionalor air springs, reliability and flexibility at a minimal cost, androbust construction.The COAST system consists of four electronically adjustablehydraulic control units for a standard automobile or SUV, allconnected to a computer controller. Six units are required fora truck, bus or RV. Each control unit contains its own LVDTwhich reports the exact position of each wheel in hundreths ofan inch to the controller 200 times per second. The controllerthen determines the optimum force needed for every wheeland sends this compensation factor to each unit, also at a rateof 200 times a second. Each control unit in turn generates theproper force, ranging from over ±1000 pounds for a standardautomobile or SUV to over ±2500 pounds for a truck, bus orRV.The COAST constant force approach differs from conventionaldamping in that it uses pressure regulators rather than orificesto control fluid flow. The LVDT measures the fluid levelthrough the position of a float inside a valve. Housed in eachhydraulic control unit, the valve reacts to the up and downmotion of the wheel and the LVDT takes a precise reading ofthat movement. This design gives COAST a key technicaladvantage. An orifice must be constantly and very rapidlyadjusted as the wheel moves, whereas the COAST systemsupplies a signal representing the appropriate force based onthe fluid level change. The force value is largely independentof any movement at that instance of time.The COAST systemis totally automatic:continuously moni-toring and controllingthe vehicles ride per-formance to providesoft and stable ridecharacteristics. Itsparameters can beconfigured to anyspecific land surfacetransportation ve-hicle. COAST ad-justs and controls upto nine dynamic areasof a vehicles suspen-sion through its unique and patented adaptive suspension tech-nology. The position feedback signal from the LVDT is acritical source of data, processed by the computer controllerto obtain independent control responses for each degree of mo-tion.AC-AC LVDTs provide a cost effective solution to OEMapplications requiring a rugged sensor with accurate andreliable position feedback capabilities.Vehicle SpeedPowerPosition SignalCompression &Rebound SignalsControllerLVDT Applications23.1LVDT use in ATM to Sense Dollar BillsTrans-Tek, Inc. LVDT Application 800-828-3964bills are picked at the start of the dispense operation. Totalroller movement is equivalent to one bill pitch.Voltage readings aretaken alternately fromeach LVDT, measuringeach interrupt from themain timing disk. (Thistiming disk alsodetermines when onetotal revolution hasbeen completed.) The overall sample reading is obtained byintegrating varying LVDT outputs for one bill pitch. Thisvalue is stored electronically and is later subtracted from thereading of an actual bill passing through the roller wheels.The resultant number represents the cross-sectional area ofthe bill and is immediately compared to an existing table ofcross-sectional areas for single, double and triple bills. Thismathematical process eliminates the risk of passing anunexpected number of bills, while at the same time compensatesfor the buildup of debris or dirt on the roller surfaces.A common goal for most designers today is to incorporateprecision sensors with the smallest possible package. Withincreasing emphasis placed on size constraints, the Trans-TekSeries 230 transducers are a great alternative for positioncontrol in areas of limited space.Miniature transducers, such as the Series 230 AC-AC LVDT,offer high accuracy linear measurement in a compact design.Packaged in a 3/8" diameter housing with excellent stroke tolength ratio, these units are often utilized where size constraintsprohibit the use of standard LVDTs. In addition, the low-mass core is ideal for systems with low driving forces or highacceleration and, therefore, will not adversely influence thedelicate nature of these applications. Operating ranges areavailable from ±0.005" to ±1.00", divided into eightintermediate strokes.Despite their small size, the models from the Series 230 arerugged sensors, making them well suited for a wide variety ofindustrial applications. Where changes in position are a smallpercentage of the total stroke, the end-user benefits from thisitems infinite resolution and superb temperature stability. Ifany of the Series 230 units are overranged, system reversal isavoided with monotonicity maintained at 150% of the normalworking range. (Monotonicity is defined as continuallyincreasing or decreasing output signal.)One application requiring a miniature LVDT is the multiplebill detector of an automated teller machine, or ATM. Thismechanism is capable of detecting single, double and triplebills, as well as folded, taped, overlapping and soiled bills.The LVDT assembly is carried on a block fixture fastenedbetween the structures side frames. A shaft attached to theblock acts as a pivot point for two support arms. A smalldiameter metal wheel is mounted at the end of each arm.Springs apply constant pressure so that each wheel bears downon the top of a larger diameter wheel, which is attached to agear driven shaft in the transport. Bills passing between thetwo sets of wheels push up on the small wheels. This motionis transferred to the LVDT core element through an armaturestationed above the pivot arm, thus generating a changingoutput signal. As the