CHAPTER 1 GETTING STARTEDOhaus Corporation www.ohaus.com 1-8 Pioneer™ Balances Service Manual1.5.7 The Load CellFigure 1-4 illustrates a typical Magnetic Force Restoration (MFR) Load Cell.The force of a weight placed on the Weighing Pan moves the Hanger which is guided by theUpper and Lower Flexure Arms. The Load Flexure transfers the force vertically to the RatioBeam. The Ratio Beam Flexures form a pivot for the Ratio Beam. When the Ratio Beam ismoved, the Force Coils mounted on the Ratio Beam are moved. The Force Coils are located inthe magnetic field created by the Permanent Magnets. The magnetic forces are affected bytemperature. A Temperature Sensor is attached near the magnets to measure the temperatureand allow the electronics to compensate for the changes in temperature.OperationA mass placed on the Pan applies a downward force to the Load Flexure by means of theHanger. The Load Flexure moves one end of the Ratio Beam which pivots, moving the ForceCoil up into the air gap of the Magnet. The Force Coils are located in the field of the PermanentMagnets. The movement of the Vane (which is connected to the Ratio Beam) is detected by thePosition Sensor. The Position Sensor contains a sender and receiver. When the Vane moves,the amount of light sent to the receiver also changes. The change in the position of the SensorVane is measured and used along with the Temperature Sensor signal to adjust the current inthe Force Coils. The current in the Force Coils produces a magnetic field which restores theRatio Beam to its normal position.Figure 1-4. Load Cell Assembly.Ratio BeamFlexureVerticalFlexureTop BracketHangerBottom Flexure Arm FlexureMagnetPrinted CircuitBoardTemperatureSensorPositionSensorVane ForceCoilsRatioBeamCounterWeight Upper Flexure ArmPan
