外文翻译--系统辨识与实时控制斗轮式装载机的液压系统土方设备

Modeling identification and real time control of bucket hydraulic system for a wheel type loader earth moving equipmentAbstractThe earth moving equipment industry is quickly gearing up to achieve great gains in efficiency, performance, safety, and operator comfort by the rapid deployment of recent digital control technology in its products.There are two major types of earth moving equipment operating in large numbers: excavators and wheel type loaders. Excavators have received much attention by the industry recently. The wheel type loader product studied in this paper is another example of a high volume versatile machine at the opposite end of the configuration spectrum.A state of the art electro-hydraulic open centered non-pressure compensated valve control system is studied to evaluate the potential gains by implementing digital velocity servo control. The control objectives are to meet operator perceived response requirements, meet operator perceived smoothness requirements, create a sub-system that could accept commands froman autonomous high level planning controller. Closed loop digital velocity control is successfully implemented in the racking motion of a wheel loader using a standard proportional-integral (PI) and a dynamic valve transform algorithm. The dynamic valve transform is a function of hydraulic flow rate which is a function of engine speed and rod end cylinder pressure. Robustness of performance was verified through extensive system modeling, validation, and hardware tests on a large Caterpillar wheel loader model. IntroductionThe automotive industry has made great gains in efficiency, performance, safetyand passenger comfort by the extensive and rapid deployment of the recent digital control technologies in its products. The earth moving industry is quickly gearing up to achieve similar gains in the short term. There are two major types of earth moving equipment: excavators and wheel type loader . The long term goal is to develop an autonomous product that no longer relies on the operator skill and endurance to maximize performance. The performance is measured in the form of tons/h of the material processed and minimizing the cost of the operation in the form of cost/ton of material processed. The goal is to develop controlled sub-systems that improve operator/machine performance, reduce operation cost, and that would serve as lower level control sub-system in the autonomous controller hierarchy.Wheel type loaders (WTL) come in many sizes. Operating weight ranges from 15000-350000ib and horsepower ranges from 100-1200 hp. The small to mid-size machines have the broadest spectrum of applications (e.g., construction and material handling applications) while the larger machines tend to be used mostly in mining applications. One common function WTL are utilized for is truck loading.The truck loading cycle is a repetitive four step process by which some type of material is transported from a stock pile to a truck. The process starts when the operator rams the stock pile and commands the linkage to lift load while at the same time the bucket is racked backwards (step 1: travel, to stock pile and dig).When the bucket has a full load, the operator shifts into reverse and travels backwards while steering to a position that allows him enough room to then shift into forward and travel to the truck. The operator continues to raise the load during this travel portion so that it clears the truck bed when he slows down and reaches the truck (step 2: travel to truck). The operator then commands the bucket to dump thus releasing the load to the truck bed (step 3: dump). Finally, the operator commands the bucket to rack back to its level position. At the same time, the operator shifts into reverse travel and commands the linkage to lower back to the ground for another dig cycle (step 4:travel to start position). This study focuses on the implementation of closed loop digital velocity control on the implement sub-system of a current state of the art wheel type loader earth moving equipment. The application of this technology is not limited to WTL and in fact has broad applications to a variety of earth moving equipment .The excavator hydraulic sub-system control problem has received much attention recently as have hydraulic sub-systems in general.Description of the wheel type loader sub-systemEarth moving equipment can be broken down into four sub-systems. (1) power-train,(2) brakes,(3) steering, and (4) hydraulic actuators. The power-train consists of a power source which is typically a diesel engine. Power is transmitted to a mechanical transmission via a torque converter which then connects to differentials, drives and finally tires. This is typically the case for WTL. Excavators have a hydro-static drive train (i.e., hydraulic pumps and motors) that connects to a track. Several engine power take-offs provide power via pumps to run the steering hydraulic system, the brake system which is typically hydraulic，and the hydraulic actuator system. The hydraulic actuation system contains the ground engaging tool that provides the force and motion to engage the soil or other material that needs to be processed.2.1. Vehicle degrees of freedomThe operator cab is considered to have six degrees of freedom: three linear (fore-aft, lateral, vertical) and three angular (yaw, pitch, roll). The bucket has two degrees of freedom. Steering is one additional degree of freedom. Therefore, the vehicle has nine degrees of freedom. To simplify the upcoming analysis, two constrains will be imposed on the system. First, the front frame motion will not be allowed to rotate relative to the rear frame. Second, the rear frame motion will be constrained to a plane. The First constraint eliminates one degree of freedom (steering) from the analysis. The second constraint eliminates three degrees of freedom: operator lateral linear motion, yaw angular motion and roll angular motion. Thus, we consider only five degrees of freedom in our model: lift and tilt implement motion of the bucket and operator fore-aft and vertical linear motion and pitch angular motion.2.2. LinkageThere are several types of WTL implement linkages currently in use. A very common example, called the Z-bar linkage. It is a two degrees of freedom linkage consisting of four bodies (lift arm, lever, link, bucket) and two asymmetric hydraulic cylinders (lift and tilt), all connected together by nine revolute pin joints.2.3. Main hydraulicsA common hydraulic implement system often used to control the flow to the lift and tilt cylinders of a WTL uses the open center non-pressure compensated spool type valve. This system contains a pump/relief valve which sends fluid to a main directional valve which in turn partitions the flow to hydraulic cylinders and tank.The components and their operation are best described by following the hydraulic fluid through the circuit under various operating conditions. The simplest condition is when there is no command current from the ECM amplifiers. The lift and tilt spools will be centered since the E/H valve solenoids will not be activated. In this case, the pump sends flow across a load check valve (set at maximum operating pressure) to the tilt spool. Since this spool is centered, the flow proceeds on to the lift spool, which is also centered, and back to tank for recirculation. The name open center comes from the fact that when valve is in neutral position, fluid circulates from the pump through the valve to the tank.If the tilt spool is centered and the lift spool is shifted to the right, an orifice to tank area called the pump to tank area (P-T) becomes restricted. The pump pressure builds up and overcomes the load check valve sending flow to the head end (HE) of the lift cylinder across another orifice called the pump to cylinder area (P-C). At the same time, fluid from the rod end (RE) of the lift cylinder flows across another orifice called the cylinder to tank area (C-T) and on to tank for recirculation. As a result, the lift cylinder rod extends raising a load that may be contained in the bucket. If the lift spool is shifted to the left the opposite happens. Fluid from the HE flows across the C-T to tank, fluid from the pump flows across the P-C to the RE as well as across the P-T if the spool is not shifted enough to completely close the P-T. As a result, the lift cylinder rod retracts lowering the bucket to the ground. If the lift spool is centered and the tilt spool is activated, the tilt cylinder will behave similar to the lift cylinder.In WTL vehicles, the lift spool is piped in series with the tilt spool. This configuration is called tilt priority, since the flow requirement of tilt circuit can over-ride and shut off the lift circuit. Additionally, cylinder relief valves may be added to each RE and HE if the maximum operating pressures for each section varies from main relief for structural life or safety concerns. Make-up valves are often added to these systems as well. These check valves provide tank flow to the cylinder RE or HE in the event that a vacuum is created during operation. In this way, cavitation can be significantly reduced. This is a problem for gravity driven functions such as lift arm lower or bucket dump in which the C-T area has been designed to provide restrictions that yield fast cylinder velocities. In this case, the pump flow to the cylinder can not match the flow from cylinder to tank creating low pressures and cavitation. This is highly undesirable in closed loop control since controllability of the hydraulic system is effectively lost during cavitation.2.4. Electro Hydraulic pilot valveA pilot pump supplies flow to a pressure regulation valve that maintains a constant supply pressure to an E/H valve which is also connected to tank. A driver in the ECM sends a current to the solenoid which moves a control spool. As this spool moves, a meter-in orifice connected to the supply pressure port and a meter-out orifice connected to tank proportionally open or close to maintain a control pressure. This pressure acts on the main spool causingit to shift and open the main orifice area (i.e., C-T, P-T, P-C). In some cases, position feedback of the main spool is used to provide closed loop position control of the spool.2.5. Digital control systemBasic low cost components will be chosen for this study to be consistent with the current practice in this industry. Typically, 7-bit microprocessors are used in which assembly coding is required to achieve loop times of 20 ms. Rotary potentiometers are used for sensor feedback as well as reference input signals.Dynamic model: tilt circuit and vehicle planar dynamicsDynamic model of the tilt circuit is developed from an input-output relations point of view. The input is the spool position of the tilt circuit valve. The outputs are (1)bucket angular velocity due to tilt cylinder displacement, and (2) planar motion (x, y, ）of the vehicle.The following assumptions and approximations are made for the model:(1) Lift circuit is stationary at a nominal lift angle. Therefore, we expect to obtain different tilt circuit models for different nominal lift circuit positions.(2) Vehicle is modeled as a mass-spring-damper system in 2-D space which has three degrees of freedom: x, y, u. This is a fairly good approximation since vehicle body and tires act like a mass-spring-damper system.(3) Hose volume and hydraulic losses are included in the hydraulic model.(4) Electro-hydraulic valve dynamics is included as a second order filter with 0.707 damping ratio. This is consistent with the behavior of the actual E/H valves used in WTL vehicles.(5) E/H valve spool areas (called metering) are not linear functions of spool positions.The spool area geometry is accurately modeled as a nonlinear function of spool position.(6) Standard orifice equations are used to describe the relationship between the flow rate (Q) and the spool position (hence the metering area), and pressure differential, p(7) Flexibility due to the oil compressibility is taken into account as bulk modulus of the hydraulic fluid.The input-output relationship between spool position and bucket angular velocity is obtained in three stages of approximation:(1) Steady state input-output function between spool displacement and tilt cylinder velocity is called the tilt modulation. This model captures the nonlinear static relation including the geometric dead band and valve-cylinder gain. Notice that effective deadband and gain are functions of (1) flow rate (which is a function of engine speed in WTL case), and (2) external load. In other words, the DC gain of the transfer function is a nonlinear function of the flow rate, external load, and nominal lift circuit position.(2) The geometric relation between tilt cylinder linear velocity and bucket linear velocity is described by the linkage jacobian. This is represented as a series of curves (implemented in real time as interpolated table) for different lift positions.(3) Finally, we model the dynamic filtering effect of the hydraulic circuit from spool position to tilt cylinder velocity .The steady state gain information is captured in tilt modulation and tilt kinematic models. Therefore the d.c. gain of the tilt dynamic model will be approximately unity (0 dB). Furthermore, it will be function of lift position and external load. Therefore, all three block components of the model are evaluated for a full sweep of expected lift position and load values. E/H valve is an open-center type valve. As the spool command shifts the spool, the P-T (pump to tank) area starts to close, the P-C (pump to cylinder) area starts to open as does the C-T (cylinder to tank) area. The pump pressure starts to build up since the P-T is restricting its flow to tank. When the pump pressure exceeds the HE cylinder pressure, the load check valve pops allowing pump flow to enter the cylinder HE. This flow extends the cylinder which forces flow from the RE of cylinder across the C-T area to tank. As the external load varies, the minimum amount of spool command required to match the load via varying pump pressure varies as well. This results in effective deadband and gain changes. If the external load is reversed (i.e., over running load condition), the rate of cylinder extension will be primarily determined by the pressure drop across the C-T area unless the pressure drop is such that the C-T flow is less than the pump flow entering the HE of the cylinder. Then the rate of extension will be pump driven.(4) Closed loop control system performance objectivesThe hydraulic system components (pumps, cylinders, servo valves) were sized so that they can provide the necessary power levels (flow rate and pressure) to move the bucket for the range of loads the WTL is designed to handle. Here we will not repeat the hydraulic control system component sizing analysis, however, suffice it to say that they were sized to meet the power requirements for the following motions: lift circuit full raise in 10 s, lift circuit full lowering in 3.5 s, tilt circuit full rack in 4.0 s, tilt circuit full damp in 2.5s under maximum load condition. Maximum bucket load capacity is 1.2 MN.(5)Results: simulations and experimentsThe experiments were conducted on a Caterpillar model WTL 889 class vehicle. The control algorithm for the tilt circuit was implemented using a micro-controller which has 8-bit resolution A/D, D/A converters on board. Effective deadband is a function of load and engine speed. In real-time implementation different under-estimated deadband values were used in order to avoid overshoots. The effect of inaccurate deadband compensation on the delay of the response is shown in Fig.14.It is observed that the delay in response reduces to below 0.2 s when deadban compensation is within 1.0mm of its actual effective value. Figure 15 shows the rise time sensitivity to valve transform gains. It is observed that in order to meet the 0.5 s raise time requirement the gain must be fairly accurate. Figure 16shows a comparison step response for both experiment and analysis under the conditions of a full lift, full dump, no load, high idle rack. The performance criteria for this situation are met in the form of 0.5s rise time and 5%overshoot with less than 0.2s delay A concern regarding implementation of the dynamic valve transform is the level of RE pressure feedback filtering required for stable response. It was postulated that the pressure should be low pass filtered to a cutoff value at or below the bandwidth of the closed loop system. In this way the closed loop control could respond to the changes in gain and deadband as if they were disturbances on the system. Both experiment and analysis reflect this filtering. Though not shown, stability was maintained during all of the experiments conducted on WTL model 990.Therefore, the effective control of such an EH control system using a PI type closed loop control plus model based valve transform compensation requires very accurate models.ConclusionsA state of the art electro hydraulic open centered non-pressure compensated implement system was studied to evaluate the potential of implementing velocity servo control to meet both operator response and smoothness performance specifications, and to create a modular sub-system that would accept commands from an autonomous high level planning controller on a WTL vehicle. Closed loop velocity control was successfully implemented on the racking function of the tilt circuit using a standard proportional-integral controller coupled with a dynamic valve transform. The dynamic valve transform is a function of engine speed (hydraulic flow) and rod end cylinder pressure. Inclusion of the dynamic valve transform in real-time control algorithm provides the liberalizing effect similar to a pressure compensated load sensing hydraulic system and results in lower component cost. Robust performance was verified through extensive system modeling and tests on aWTLmodel990 vehicle.系统辨识与实时控制斗轮式装载机的液压系统土方设备摘要土方设备行业的迅速整装待发，准备在近期实现数字化控制技术在其产品的快速部署的效率，性能，安全性和操作舒适巨大收益。世界上主要有两种类型的移动设备操作的最多:挖掘机和轮式装载机。现在挖掘机已受到业界的关注。轮式装载机产品在本文研究的是另一种高容量多功能机在配置频谱的另一端的例子。一个先进的电液开放中心的非压力补偿阀控制系统的状态进行了研究，以评估通过实施数字化速度 控制的 在收益。控制 是 要 ， 度要 ， 一个子系统， 以 受 高 的 控制。数字化速度的控制是功实施的一个轮式装载机currency1 准例“PI）和阀的动态fifl 的。动态fi阀是液压量功能，是一种发动机速和 压力端功能。性的性能进行了”，通过的系统 ，”，在大 轮式装载机型的。 行业已 在效率，性能，安全性巨大收益和最近在其产品的数字化控制技术的和快速部署的舒适度。 发 的行业 在迅速整装待发，准备在 期 实现类 的收益。主要有两种类型的 土设备 挖掘机和轮式装载机。 是 一个 的产品， 操作技 和力 最大度 高性能。 量其性能的 / 时，的 最 化的 作在本/ 的 式 本的 式。 的 是开发控制子系统， /机的性能， 本，作 控制子在 控制 的 制度。轮式装载机WTL中）有 多大 。 作 量范围从15000-350000和马力马力范围从100-1200。 到中型机 的 currency1程序例如，施 和 currency1）最而大的机 往往是currency1于矿山的 currency1 主。 WTL的一个常见的功能是利currency1 装载。 装载周期是一个 复的四个步骤，其中一些类型的是从股票桩 到 上。 个过程开始时， 的股票桩公羊和 的联系，以 升负载，而在同一时间向后摇桶步骤1 旅行，股票桩和挖）。当水桶已 载，逆向操作的化和旅行向时向后一个位置，使他有 够的空间，然后移到前进，前往 。 者继续 旅行期间 出的部，以便它清除床上时，他减慢，达到 步骤2 前往 ）负载。 员 着 桶倾倒从而释放负载到 床步骤3 储）。最后，操作 把水桶回其水平位置。同时，旅游 者进入反向化和 的联系，以 挖周期步骤4 行程开始位置）返回面。本研究的 点是数字化速度的控制的执行情况在执行子一个先进的轮式装载机移动设备的 电状态系统。 项技术的 currency1 仅于WTL以及实际上已 到了 的各种移动设备的 currency1。挖掘机液压子系统的控制问题已引起关注最近有液压子在一般系统。描述轮式装载机的子系统土方设备 细 4个子系统。1）动力装置，2）制动系统，三） 作装置，4）液压传动 。上电列一个电源，通常是一个柴油发动机。电力传输到一种通过液力矩 然后连到差距机械传动，驱动 和最后轮胎。 通常是对WTL。挖掘机有水文静态驱动列即，液压泵和马达）连 到一个轨道。几个发动机功率起飞通过泵 供动力向液压系统 行，制动系统是典型的液压和液压执行系统。液压驱动系统包含面从事 具， 供了力量和 动，从事土壤或其他需要 。2.1 度出租 者被 有六个 度 三线性前，后部，横向， 向）和三个 ， ，roll0。 斗有两个 度。 是一种 的 程度。 ， 有着9 的 程度。 了化即 行的 ，两制在系统 。 先，前 动 对于后。 ，后制在一个平面上。 一个 除了 度向） 位从 中。 个 除了三 度 横向 线 动， 和 动 动的。 ， 有个 度在 的 型 电和倾实桶和操作前，部和 线 动和 的。2.2. 连currency1 有几种类型的WTL的实现使currency1 前的联系。一个非常“的例如，的Z型连。它是 的两个 位四联动 升fi，fl， ，桶）和两个的非对液压升机及倾），个 关 连在一起。2.3. 主要液压系统一个通液压系统通常currency1实现控制量的电WTL的倾和一currency1开放中心非压力补偿阀型阀。 个系统包含一个水泵/释压阀的传到主fi向阀从而 到液压和”。和它的 作是最的 容液压通过各种 行 的电 。最 的 是在有 从放大 电。电线和倾， 中在E/ 时阀电 被。在种情况，通过发泵 向阀的负载量最大操作设置压力）的倾阀。于 阀 中心，动收益上的电阀， 是 中心，返回水。 开放中心的事实，即当阀在中 的 ，从通过到的阀泵。如 倾阀中心及电阀 移，一开 泵 期三）而受到制。泵的压力 起，” 了负载 回阀发向 端HE）的对在另一 的升油 PC）的泵。在同时，从端 ）电在另一 量的 CT）和 ，进行”。 ，电 高一个 能在桶中的负载。如 电阀 移 反的情况。从他 对”的CT，从泵动以及整个阀如 有 够的移 全关的PT 平 横 能源的电。 ，电 回桶 到面。如 电阀 中和倾阀被时，倾油的行 类 于电。在WTL ，电 道与阀系列倾。 配置被 倾 先事项， 电 的倾量需 以通过 和关电电 。 ， 安全阀 能被 到个RE和他如 个部