7.1 Introduction 7.1.1 Force (1)What is the force? Force is one of the most important physical parameters .Various mechanical motion is virtually the transmission of force or torque. Force possesses the capability in both static and dynamic phenomenon: it can change the body’s motion status or cause body’s deformation.
(2)unit of force N(Newton): is the force which when applied to a mass of one kilogram gives it an acceleration of 1 m/s2. (3)Weight: is the force exerted on a body of mass m due to earth gravity. G=mg; g is the local gravity acceleration, typically about 9.8m/s2. Weight measurement is usually specified in terms of the mass.
(4) Torque (力矩、扭矩、转矩） Torque is sometimes called moment. It is the product of a force and its arm. For a rotating shaft of radius r, the torque T applied to the shaft to cause it to rotate is F×r, where F is the tangential force at the radius r. When a shaft is twisted as a result of the application of a torque, shear stresses and strain are produced in the shaft material.
(5) Pressure A force F perpendicular to an area A, the pressure P is P=F/A unit : Pascal(Pa) 1Pa=1N/m2 Atmospheric pressure: the atmosphere surrounding the earth exerts a force on the unit earth surface, that is the pressure caused by the weight of the atmosphere. It varies with weather condition, the height above sea level , latitude.
Absolute pressure: used for the pressure measured relative to zero pressure. Gauge pressure: the pressure measured relative to the atmospheric pressure. The differential pressure between the absolute pressure and the atmospheric pressure. Positive pressure: the gauge pressure higher than the atmospheric pressure.
Negative pressure: the gauge pressure lower than the atmospheric pressure. Vacuum: the absolute pressure lower than atmosphere.
7.2 Measurement methods 7.2.1 Force measurement methods (1) According to static or dynamic effect Static effect----- the static force causes mechanical deformation. By measuring the deformation or using some inner stress or force related physical effects. Dynamic effect---- according to the Newton 2nd law, the force is associated with acceleration. Knowing mass and acceleration measured, the force can be defined: F=ma
Force measurement (2) according to the methods using different structure & instruments • Lever balance methods Based on the principle of moments i.e. at static equilibrium the algebraic sum of the clockwise moments about an axis equals the anticlockwise moments.The moment of a force about an axis is the product of the force and the perpendicular distance from its line of action to the axis.This method is typically used for weights up to 1000Kg. 2. Force balance methods 3. Elastic element methods 4. Pressure methods
一种伺服式测力系统。无外力作用时，系统处于初始平衡位置，光线全部被遮住，光敏元件无电流输出，力发生器不产生力矩。当被测力F作用在杠杆上时，系统失去平衡，杠杆发生偏转，窗口打开相应的缝隙。光线通过缝隙，照射到光敏元件上，光敏元件输出与光照成比例的电信号，经放大后加到磁电力矩发生器的旋转线圈上。载流线圈与磁场相互作用而产生电磁力矩，用于平衡被测力F与配重(标准质量m)力的力矩之差；使杠杆重新处于平衡状态。当杠杆处于新的平衡位置时，其转角与被测力F成正比，放大器输出电信号在采样电阻R上的电压U0与被测力F成比例。一种伺服式测力系统。无外力作用时，系统处于初始平衡位置，光线全部被遮住，光敏元件无电流输出，力发生器不产生力矩。当被测力F作用在杠杆上时，系统失去平衡，杠杆发生偏转，窗口打开相应的缝隙。光线通过缝隙，照射到光敏元件上，光敏元件输出与光照成比例的电信号，经放大后加到磁电力矩发生器的旋转线圈上。载流线圈与磁场相互作用而产生电磁力矩，用于平衡被测力F与配重(标准质量m)力的力矩之差；使杠杆重新处于平衡状态。当杠杆处于新的平衡位置时，其转角与被测力F成正比，放大器输出电信号在采样电阻R上的电压U0与被测力F成比例。 磁电式力平衡测力系统 Force balance methods
Elastic element methods Sensors that are used for measurement of force, torque or pressure often contain an elastic element that converts the mechanical quantity into a deflection or strain which can then be transformed using another sensor into an electrical signal. Electrical resistance strain gauges are widely used in this capacity.
spring damping In general, elastic elements used for the measurement of force can be considered to represented by the model shown in Fig. When a force f(t) acts on the mass of element, a displacement is produced which varies with time before obtaining a steady-state value.The displacement variation with time is described by a second order differential equation. In practice, often it is not the displacement itself to be measured but the strain experienced by a strain gauge attached to what is equivalent of the spring.
Various forms of elastic members are used. The simplest is just a spring to make a device called spring balance. The extension of the spring represents the force applied. Load cells, i.e. elastic members which transform force into displacement or strains, can take many forms .
The structure of typical elastic element and its designing calculation as follows: Columnar load cell
Bending beam Applied force Proving ring 测力环
The relative extension of strain gauge in elastic element : Δl-------Total extension of strain gauge l--------the original length of strain gauge F------applied force A------working area of elastic element E------Yang model of elastic element σ------stress of elastic element
Sensitivity : ----poison constant of elastic element Total strain of elastic element: Output of electrical bridge: In general, select k=2, Voltage sensitivity(mv/v):
Other principle of force measurement (1) Piezoelectric dynamometry 压电式测力仪利用压电材料(石英晶体、压电陶瓷）的压电效应，将被测力经弹性元件转换为与其成正比的电荷量输出，通过测量电路测出输出电荷，从而实现对力值的测量。弹性元件感受力F时压电材料产生电荷Q输出 dij: Piezoelectric constant 适用于动态力的测量
Piezomagnetic dynamometry 由工业纯铁、硅钢等铁磁材料制成的铁心在机械力的作用下磁导率发生变化称为压磁效应，或称磁弹性效应。如图a所示，在铁心上安置一对线圈一励磁线圈和测量线圈。两线圈的平面相互垂直, 若无外力作用，励磁线圈中心交流电流所建立的磁场对测量线圈没有输出，见图b。若外力作用在铁心上，铁心磁导率改变，则测量线圈被励磁线圈中的磁场交链而输出比例于外力大小的信号，见图c。
Torque measurement • Torque measurement are used for rotating shafts to determine the power transmitted and also to monitor against failure as a result of shear stress. The measurement of torque is always linked with the power and rotating speed. • The work done is the product of the force and the distance traveled in the direction of the force. Work done per second=F×2rn= (Power transmitted by a rotating shaft=2nT=T) F: Tangential force at a radius r T: Torque applied to a shaft to cause it to rotate.
Basic methods of torque measurement： （1）Transmission method (扭轴法） （2）Force balance method （3）Power transformation method
(1) When a torque applied to a n elastic shaft, as a result , the shaft is twisted and shear stress and strains are produced in the shaft material. G: shear model of material；: Torsion angle IP :polar second moment (极惯性矩)
When the torsion angle measured: When the shear stress of shaft measured: When the strain of shaft measured: 45、135: the strains along the direction of 45o、135o related to axis of the shaft.
1.电阻应变式转矩仪 在转轴上或直接在被测轴上，沿轴线的45。或135。方向将应变片粘贴上，当转轴受转矩M作用时，应变片产生应变，其应变量 与转矩M成线性关系。 • 对空心圆柱形轴： 对方形截面轴： G为转轴的弹性模量
When the torque is measured for a rotating shaft, it is necessary to obtain the output signal from the bridge connection of the strain gauges which are on the rotating shaft and provide the d.c. supply voltage for the bridge. One method of doing this is via slip rings. A problem with such an arrangement is the noise generated by variations in contact resistance between the rings and the brushes.
An alternative method is to mount the entire bridge, together with its voltage supply, on the rotating shaft and use the bridge output to modulate a radio signal which is then transmitted to nearby, stationary, equipment for display or recording.
(3)能量转换法 这是按能量守恒定律来测量力矩的仪器。它是通过测量其它与转矩有关的能量系数(如电能系数)来确定被测力矩大小的。 根据能量转换法制作的测力矩仪一般多用于测量各种电机的转矩。因其影响因素较多测量误差大，所以只有测量电参数的电机转矩测量仪应用较多。
Pressure measurement • Together with temperature and flow, pressure is the most important parameters in industrial process control • The unit of pressure is the Pascal (Pa) with 1Pa being 1N/m2 • At the surface of the earth, the atmospheric pressure is generally about 100KPa. This is sometimes referred to as a pressure of 1bar.
1.Manometers（液体式压力计） • U-tube manometer • The cistern manometer • The inclined tube manometer 2.Diaphragms(薄膜，弹性式压力计） • Reluctance diaphragm gauge • Capacitance diaphragm gauge • Strain gauge diaphragm gauge • Force-balance diaphragm gauge • Piezoelectric diaphragm gauge 3. Bourdon tubes
Dead-Weight Tester Schematic Calibration of the pressure gauges in the region of 20Pa to 2000kPa is generally by means of the Dead-weight tester. Pressure is produced by winding in a piston. The pressure is determined by adding weights to the platform so that it remains at a constant height.
Manometers The basic manometer consists of a U-tube containing a liquid.A pressure difference between the gases above the liquid in the two limbs produces a difference h in vertical heights of the liquid in the two limbs. If one of the limbs is open to the atmosphere then the pressure difference is the gauge pressure. P2 P1
Water, alcohol and mercury are commonly used manometric liquids. U-tube manometers are simple and cheap and can be used for pressure differences in the range 20 Pa to 140KPa. The accuracy is typically about 1%. Temperature affect---------liquid expansion
Thus the pressure when measured by a U-tube manometer at a temperature , when the manometer liquid density at 0°C is known, is given by:
P2 P1 A1 d h H A2 Cistern manometer An industrial form of the U-tube manometer is cistern manometer. It has one of the limbs with a much greater cross-sectional area than the other.A difference in pressure between the two limbs causes a difference in liquid level with liquid flowing from one limb to the other.
This form of manometer thus only require the level of liquid in one limb to be measured from a fixed point.
The inclined tube manometer The inclined tube manometer is a U-tube manometer with one limb having a larger cross-section than the other and the narrower limb being inclined at some angle to the horizontal. It is generally used for the measurement of small pressure differences and gives greater accuracy than the conventional U-tube manometer.
P2 P1 d x H h Initial zero level with no pressure difference Since A2 is much greater than A1, the equation approximates to:
The bourdon tube may be in the form of a “C”, a flat spiral, a helical spiral. In all forms, an increase in the pressure in the tube causes the tube to straighten out to an extent which depends on the pressure. This displacement may be monitored in a variety of ways, for example, to directly move a pointer across a scale, to move a slider of a potentiometer, to move the core of an LVDT.
Diaphragms With diaphragm pressure gauges, a difference in pressure between two sides of a diaphragm results in it blowing out to one side or the other. If the fluid for which the pressure is required is admitted to one side of the diaphragm and the other side is open to the atmosphere, the diaphragm gauge gives the gauge pressure. If fluids at different pressures are admitted to the two sides of the diaphragm, the gauge gives the pressure difference.
1 Reluctance diaphragm gauge(磁阻隔膜压力计） The displacement of the central part of the diaphragm increases the reluctance of the coil on one side of the diaphragm and decreases it on the other. With the two coils connected in opposite arms of an a.c. bridge, the out of balance voltage is related to the pressure difference causing the diaphragm displacement
Capacitance pressure transducers were originally developed for use in low vacuum research. This capacitance change results from the movement of a diaphragm element. The diaphragm is usually metal or metal-coated quartz and is exposed to the process pressure on one side and to the reference pressure on the other. Depending on the type of pressure, the capacitive transducer can be either an absolute, gauge, or differential pressure transducer.
The diaphragm is between two fixed plates and its movement thus increases the capacitance with respect to one fixed plate and decreases it with respect to the other. As shown in Figure ，the deflection of the diaphragm causes a change in capacitance that is detected by a bridge circuit. The capacitor can also form part of the tuning circuit of a frequency modulated oscillator and so give an electrical output related to the pressure difference across the diaphragm.