切削力传感器.doc

电阻应变传感器测量系统在数控车床切削力测量中的应用为了便于测量和研究数控车床切削力, 适应生产中设计和使用数控机床和刀具的需要, 一般都把总切削力Fr分解成三个互相垂直方向的力Fz 、Fy 、Fx 三个分力来测量分析。该系统切削力的检测装置, 我们采用电阻应变片传感器设计组成的八角环测力仪,作为测定 X 、Y 、Z 三个方向切削力的传感器。其中的八角环是弹性元件,在环的内外壁相应的应变节点上分别粘贴四片电阻应变片, 以克服测试过程中的交叉干扰,把四片电阻应变片按全桥方式联结分别构成三个测量电桥, 以提高测试灵敏度。在数控车床车削时,切削力经工件转动传递于车刀上,再由车刀刀杆传递到八角环, 八角环的变形使紧贴在其上的电阻应变片也随之变形,电阻值R就会随之发生变化( R ? $R ) 。当应变片受拉伸时,电阻丝直径变细, 电阻值增大( R+$R) , 应变片受压缩变形时, 电阻丝直径变粗, 电阻值变小( R-$R) , 电桥会输出与切削力成正比例电信号。由于电阻应变片的电阻变化很小, 为了适合单片机控制系统进行相应的数据处理,必须将信号放大到 0 5V 后才能输入单片机。电阻应变片组成的测量电桥电路如图 2 所示。图 2 电阻应变片组成的测量电桥F ig ur e. 2 T he measuring electr ic br idge co mpo sed of the r esistance strain g aug es这是由四个电阻应变片作为电桥桥臂所组成 的全桥测量电路, R1 、R2 、R3 、R4 分别为四个桥臂 的电阻。当 A 、C 端加以一定的电压 U 时, 则 B、D 端的输出电压 $U 表示为:$U =R 1 R3 - R 2 R4U( R1 + R2 ) ( R3 +R4 )由上式可知, 当 R1 R3 = R2 R4时, 电桥的输出电压 $U = 0, 即电桥处于平衡状态。在进行切 削力测量前, 还须对电桥进行调节, 使其处于平衡缺点：不宜在外界环境变化比较大的地方使用，对于大应变有较大的非线性、输出信号较弱。优点：精度高，测量范围广寿命长,结构简单，频响特性好，能在恶劣条件下工作，易于实现小型化、整体化和品种多样化。轮辐式切削力传感器轮辐式传感是利用轮辐受载后的变形检取应变,通过敏感元件(如电阻应变片) 来实现力电信号的转换。这种传载器根据轮辐的横截面形式分为变辐式和等辐式两种,本文仅论述便于加工的等辐式传感器。如图1示,该传感器的形状恰似一车轮,轮毅和轮缘由对称的四根辐条连接, 组成以轮缘为固定支座的交叉梁。三向切削力可由轮毅传至轮辐。这种传感器是基于剪辐式压力传感器的设想提出的川。为保证传感器的性能可靠,其中轮毅和轮缘的刚度应适当取大c 轮辐的截面为矩形, 既保持梁的特性,又不致使传感器横向尺寸过大。为分析方便, 首先讨论传感器在径向切削力F : 单独作用下的情况。根据对称结构, 取传感器在同一直线上由两轮辐及轮毅、轮缘组成的一跨, 可简化成图Za) 示超静定梁, 载荷及剪力图。如不计中间轮毅高度影响, 得到Zb ) 的原形梁, 并作出相应的弯矩由超静定协调条件得到:通过改变在轮辐上贴片的位置, 可分别以弹性辐体的弯曲、剪切或拉压应变作为传感器的输入信号。而这几类轮辐传感器的工作原理是不同的。一般的轮辐传感器主要用于单向重载荷的压力检测, 为撼高其刚度多利用纯剪状态下轮辐截面应力分布规律, 在与传感器轴线45”方向布片(图2a) , 即所谓的剪辐式荷重传感器川, 这种形式传感器的特点是传感器的灵敏度只与筋板抗剪截面积吞X 孔有关, 因此可缩短轮辐体长度, 进而减小传感器的体积, 同时也大大提高了传感器的刚度。显然, 这种设计方案对单向的重载检测是适用的。但切削力传感器的情况则复杂得多, 由干切剥力的方向未知, 通常要同时测出其在三个既定方向的切削分力Fx 、Fz 、FY。而径向切削力FY一般小于500kgf ，如仍采用上述剪辐式原理设计, 势必使轮辐截而积过小, 以至不能满足其它二向分力和贴片的要求。因此采用图1 (b ) 的布片形式, 即用轮辐的拉压变形分别测定Fz 、FY二向切削分力, FY采用辐板的端面布片, 还过轮辐的弯曲变形来测定。考虑到主切削分力 Fz 、FY, 而通常弯曲形与相同结构的拉压形传感器比较, 前者的灵敏度较高,所以采用图工(b)的设计方案可使传感器在 Fz 、FY分力作用下的输出差距缩小, 便于二次仪表的选配。同时, 这种方案也使传感器具有较好的抗干扰载荷能力, 可通过桥路自动补偿各向切削分力间的相互千扰及偏心载荷的影响。 用薄壁圆筒式切削力传感器测定传感器中部为空心薄壁圆筒, 外表面粘贴有两组电阻应变片。传感器的两端有法兰盘, 以此用螺钉联接安装在试材夹具与制材跑车搁凳之间。电阻应变片R1和R2纵向粘贴在圆筒表面Z方向的位置上, 相互错开180, 接成半桥。应变片R 3 、R 4 、R5 、R 6 与轴线交叉倾斜45角,周向均匀分布, 接成全桥。锯切时, 带锯条对木材切削力的切向分量Fx 和法向分量Fy分别在薄壁圆筒上形成弯矩M 和扭矩Mx。测Fx的电桥输出反映弯矩M 的大小, 与F x成正比。测Fy的电桥输出反映扭矩Mk的值,与Fy成正比。为便于数据处理,切削试验时,保持力臂a 不变。在锯切过程中, 切削分力Fx 和Fy的作用点是不断变化的, 但弯矩M 和扭矩Mk不受力点变化的影响, 所以电桥的输出也不受力点变化的影响。这是在木材切削力传感器的设计和安装中必须满足的一个条件。与之相反,薄壁圆筒上Z 向弯矩因受Fx作用位置前后变化的影响, 所以不能用来测Fy力。由于R3 、R4、R5、R6贴片位置的对称性, 切向分力Fx在测Fy的电桥中理论上无输出。因为应变片R1和R2 的中心位于通过圆筒中心线平行于z 轴的平面内, 所以Fy产生的z 向弯矩在测Fx的电桥中理论上也无输出。各电桥输出信号的单一性是多分量切削力传感器又一个必须满足的条件。因为Z 向力在两个测力电桥中都产生输出, 所以锯切时不允许有Z 向力存在。一般地, 薄壁圆筒式传感器测切削力两个正交分量时, 第三方向的切削力分量必须为零, 否则将干扰两向分力的测定结果。电桥系统框图如图2a 所示。木材切削力的两个分量Fx 二和Fy ,通过薄壁圆筒切削力传感器变为两组电桥的输出, 经动态电阻应变仪放大后, 输人光线示波器, 记录在示波纸上。切削力分量的记录曲线如图2b 所示。根据记录曲线的相对高度hx 和hy, 算得切削力分量Fx和Fy的数值。Design, development and testing of a four-component milling dynamometer for the measurement of cutting force and torque参考文献：Mechanical Systems And Signal Processing作者：Frank Unsacar ， Haci Saglam ，Hakan Lsik优点：具有很高的线性度和较低的误差，它已制定和提供必要的数据采集系统由硬件和软件。测功机可以衡量三个垂直切割力和扭矩期间同时铣削和模拟测量值可以存储在计算机数据采集系统。这是旨在衡量高达5000的最大力量和灵敏度的系统5 N。A three-force component analogue dynamometer capable of measuring cutting forces during milling was designed, developed and tested. A computer connection for data acquisition was also made and calibrated. The analogue data can be evaluated numerically on a computer and when required can be converted back to analogue. The schematic representation of the cutting force measurement system is capable of measuring feed force (Ff), thrust force (Ft) and main cutting force (Fc) which occurs during milling operations as seen in Fig. 1. This dynamometer consists of four elastic octagonal rings on which strain gauges were mounted and necessary connections were made to form measuring the Wheatstone bridgesOn-line and real-time information of the cutting force data are automatically read and stored by a system during metal cutting. Since the output from Wheatstone bridge circuits is very low due to the high stiffness requirement of the dynamometer, the analogue signals coming from dynamometer amplified by strain gauge input modules (Advantech ADAM 3016) are then converted to digital signals and captured by PCI-1712 data acquisition card installed in MS-Windows-based PC. The stored data can be retrieved and used for analysis when required. The data acquisition software is capable of averaging and graphical simulation of force signals in process. The lists of the experimental equipments used are shown in Table 1Table 1. Experimental equipments and their technical propertiesMachine toolUniversal milling: Taksan, FU-315 V/21250DynamometerStrain gauge-based four-component cutting force dynamometerStrain gaugeHBM: LY 41-10/350; effective gauge length 10 mm; Gauge factor 2.091%; gauge resistance 3500.3% ; transverse sensitivity of 0.3%Strain ringOctagonal in shape; made of AISI 4140 steel; b=30 mm; r=32 mm; t=8 mmStrain amplifierAdvantech: ADAM 3016Data acquisition cardAdvantech: A/D converter; PCI 1712, 16 single channels (8 differential), 110 MHzData recording softwareWritten in C; capable of recording, simulating and data processing.Vibration analyser packageCommtest Instrument vb3000: range 120.000 Hz, ISO 2372 and ISO10816 standard. Accelerometer: frequency range 0.515 kHz, dynamic range 50 gCoupler/power supplyKistler: 5118B2; bandwidth 0.03, 0.006 Hz; gain 1, 10, 100; output voltage 10 V; operated by internal battery (41.5 V) or external voltage 628 VDCUniversal testing machineLLOYD instrument T50 KThe thickness t, radius r, and width of the circular strain ring b are the three basic controllable parameters that affect the rigidity and sensitivity. Since there is no effect of ring width b and modulus of elasticity (E) on the strain per unit deflection, bmin can be taken as 30 mm to set up the rings securely 6.The deformation of circular ring under the effect of thrust force Ft and main cutting force Fc separately is shown in Fig. 2(b) and (c), respectively. As long as strain on A and B where the strain gauges are going to be fixed (Fig. 2(a) are within the elastic limits of the ring material, the strain and deflection due to the main cutting force should be considered for the purpose of the ring design for maximisation of sensitivity (c/Fc) and stiffness (Fc/c). The strain gauges should be placed where the stress concentration has maximum value. The experiments have shown that good results are obtained for octagonal rings when the inclined gauges are at points 45 from the vertical instead of 39.6 required by the circular ring theory. The strain per unit deflection can be expressed as 6(1)where t is the deflection in a radial direction and t is the strain due to thrust force Ft. It is clear that for maximum sensitivity and rigidity t/t should be as large as possible. This requires that r should be as small as possible and t as large as possible. But small r brings some difficulties in mounting the internal strain gauges accurately. Therefore, for a given size of r and b, t should be large enough to be consistent with the desired sensitivity. Ito et al. 7 performed a finite element analysis for the elastic behaviour of octagonal rings. They expressed that the octagonal ring is substantially stiffer than the circular ring when t/r less then or equals to 0.05, the difference in displacement of circular ring and octagonal ring is 10% if t/r greater then or equals 0.25. In order to be consistent with this expression, the ring thickness and ring radius were taken as 8 and 32 mm, respectively. Thus, the rate of t/r (8/32=0.25) provides corresponding sensitivity to stiffness ratio /(/r) for the octagonal ring.The cross-sensitivity can be expressed as strain measured on axes that is normal to the main axes. It is desired that dynamometers must not be completely insensitive to the cross-strain. It is possible to measure the cutting forces independently and accurately as long as the cross-sensitivity is small. The strain errors will be less if this effect is within an acceptable range. These errors can arise because the strain gauges are not fitted symmetrically to the ring axes and if the strain rings are not mounted in the direction of measured force axes. The average errors for cross-sensitivity in three axes were calculated in range of 0.61.7% as shown in Table 2(b).Table 2. The results of tests performed on the dynamometer(a) The results of linearity test AxesLoad (N)Output- (mV)Calibration value- (mV)Error (%)Ff2400128.3130.01.3Fc2400126.8125.01.4Ft5000134.2135.81.2(b) The results of cross sensitivity test AxesLoad (N)Output (mV/m) Average error (%) XYZXYZFf2400128.30.81.30.61Fc24001.2126.82.211.7Ft50001.61.2134.21.21.20.9(c) The results of eccentricity test AxesLoad (N)e=0 mm (mV)e=50 mm (mV)(%) Output errorFf100054.654.70.18Fc100053.853.90.18Ft100025.8625.530.13(d) The results of performance test Axes(mV)F(N)F(N)% Output errorX14.45255Accuracy=1000/1014.9=0.985Y13.502501014.9 NError=14.9/1000=0.015Z32.87950Error=0.15%In this study, strain gauge-based dynamometer has been designed and developed. It has been devised and connected with necessary data acquisition system consisting of hardware and software. Dynamometer can measure three perpendicular cutting force components and torque simultaneously during milling and the measured numerical values can be stored in computer by data acquisition system. This dynamometer was designed to measure up to 5000 N maximum force and the sensitivity of system is 5 N.The orientation of octagonal rings and strain gauge locations were determined to obtain maximum output of ring minimum cross-sensitivity under deformation. To measure the dynamic cutting force, an accelerometer was attached to the dynamometer in measurement direction and the dynamic cutting force calculation was also given. For data transfer between the dynamometer and PC, a proper experimental set-up was performed and suitable software was written. In order to determine accuracy, the dynamometer was calibrated statically and dynamically and subjected to the linearity test, cross-sensitivity test, eccentricity test and performance test.The static calibration curves for Ff, Fc and Ft forces have shown that it has very high linearity (in errors 1.3%, 1.4% and 1.2%) and low cross-sensitivity errors (in range of 0.61.7%). In face-milling operations, appropriate results were obtained in cutting force measurements. As a result, recorded cutting force data were presented for evaluation. Also the natural frequency of dynamometer in X-, Y- and Z-directions satisfies the necessary rigidity and dynamic range.The results obtained from the machining tests performed at different cutting parameters showed that the dynamometer could be used reliably to measure cutting forces not only in milling but also in other machining processes as turning, grinding and shaping.The signal recording and processing unit can be used, for example, to monitor or control processes. This type of measuring chain has proved successful for measuring force and torque.An overview of data acquisition system for cutting force measuring and optimization in milling参考文献：Journal of Materials Processing Technology作者：F.Cus，J.Balic优点：特别适合运用在刀具磨损检测的领域，并且能够在运作过程中监测刀具的磨损情况。One of the most significant developments in the manufacturing environment is the increasing use of tool and process monitoring systems. Many different sensor types, coupled with signal processing technologies are now available, and many sophisticated signal and information processing techniques have been invented and presented in research papers. However, only a few have found their way to industrial application. The aim of this paper is to present the cutting force measurement system for the ball-end milling. The system is based on LabVIEW software, the data acquisition system and the measuring devices (sensors) for the cutting force measuring. The system collects the variables of the cutting process by means of sensors and makes transformation of those data into numerical values. Generally used measuring devices for cutting force measuring are piezoelectric dynamometer. Delivered signals are distorted due to their self-dynamic behaviour. Their dynamic characteristics are identified under normal machining operation. The proposed method is based on the interrupted cutting of a specially designed workpiece that provides a strong broadband excitation. The three components of the exciting force and the acceleration of the gravity centre of the dynamometer cover plate are measured simultaneously. The measured values are delivered to the computer program through the data acquisition system.The data obtained from the acquisition system, are a basis for the optimization of the machining processcutting parameters.Application of AE and cutting force signals in tool condition monitoring in micro-milling参考文献：CIRP Journal of Manufacturing Science and Technology作者：P.J.Arrazola 优点：灵敏度很高。Cutting forces and acoustic emission signals provide very useful information for tool condition monitoring in micro-milling. An acoustic emission signal is free from mechanical disturbances like resonance vibrations, which is very important in micromachining applications, where spindle speeds have to be very high due to the small tool diameter. Despite the small material removal rate in micromachining, the obtained AE signal was strong, easy to register, and showed a very short reaction time to the toolworkpiece contact, which makes it a very good means of detecting this contact and monitoring the integrity of the cutting process.The cutting force signals acquired in this study were severely disturbed by resonance vibrations in the dynamometer. In spite of this, the measurements still appeared to be very useful in tool wear monitoring.Signal feature integration in tool condition monitoring minimizes the diagnosis uncertainty, reducing the randomness in one SF and providing a more reliable tool condition estimation. The number of SFs should be as big as possible, preferably originating from different sensors. Very good results can be achieved using cutting forces and acoustic emission. TCM based on AE only, as an AE sensor is much less expensive and easier to install, is worse than that based on four signals, yet still provides acceptable results.Tool condition monitoring strategies should be tested on tool lives that are different from the tool life used to train the system. A good practice is to repeat the test for every available tool life to avoid selecting the best results, while ignoring the worst, less satisfactory results.Estimating cutting force from rotating and stationary feed motor currents on a milling machine参考文献：International Journal of Machine Tools and Manufacture作者：Young-Hun Jeong，Dong-Woo Cho优点：可使用的频域范围大。Automation and increased productivity have improved manufacturing systems since numerical control was introduced to the industry. However, accurately determining cutting conditions remains difficult, and experienced operators still produce better results than unmanned machines. This has hindered continued growth of manufacturing systems. To resolve these problems, some important machining process and control tasks have been studied, such as in-process monitoring and adaptive control. These tasks require reliable and industrially adaptable sensors that can provide informative signals about the state of the machining process.The cut