英文资料圆钢送料机械手-上下料机械手大学设计.doc

英文资料The contral techniques drives and contrals handbookChapter A4Torque, speed and position controlA4.1 General principlesA4.1.1The ideal control systemMany applications exist where something has to be controlled to follow a reference quantity. For example, the speed of a large motor may be set from a low-power control signal. This can be achieved using a variable-speed drive as described in the following.矚慫润厲钐瘗睞枥庑赖。Ideally, the relationship between the reference and the motor speed should be linear, and the speed should change instantly with changes in the reference. Any control system can be represented as in Figure A4.1b, with an input reference signal, a transfer function F and an output. For the system to be ideal, the transfer function F would be a simple constant, so that the output would be proportional to the reference with no delay.聞創沟燴鐺險爱氇谴净。Figure A4.1 Variable-speed drive and motorA4.1.2Open-loop control Unfortunately, the transfer function of many practical systems is not a constant, and so without any form of feedback from the output to correct for the non-ideal nature of the transfer function, the output does not follow the demand as required. Using an induction motor supplied bya simple open-loop variable-speed drive as an example, the following illustrates some unwanted effects that can occur in practical systems:残骛楼諍锩瀨濟溆塹籟。Speed regulation. The output of a simple open-loop drive is a fixed frequency that is proportional to the speed reference, and so the frequency applied to the motor remains constant for a constant speed reference. The speed of the motor drops as load is applied because of the slip characteristic of the motor, and so the speed does not remain at the required level. 酽锕极額閉镇桧猪訣锥。Instability. It is possible under certain load conditions and at certain frequencies for the motor speed to oscillate around the required speed, even though the applied frequency is constant. Another major source of instability in rotating mechanical systems is low-loss elastic couplings and shafts. 彈贸摄尔霁毙攬砖卤庑。Non-linearity. There are many possible sources of non-linearity. If, for example, the motor is connected to a gearbox, the speed at the output of the gearbox could be affected by backlash between the gears. 謀荞抟箧飆鐸怼类蒋薔。Variations with temperature. Some aspects of the system transfer function may vary with temperature. For example, the slip of an induction motor increases as the motor heats up, and so for a given load the motor speed may reduce from the starting speed when the motor was cold. 厦礴恳蹒骈時盡继價骚。Delay. With a simple open-loop inverter and induction motor there can be a delay before the motor speed reaches the demanded level after a change in the speed reference. In very simple applications such as controlling the speed of a conveyor belt, this type of delay may not be a problem. In more complex systems, such as on a machine tool axis, delays have a significant effect on the quality of the system. 茕桢广鳓鯡选块网羈泪。These are just some of the unwanted effects that can be produced if an open-loop control system is used. One method that improves the quality of the controller is to use a measure of the output quantity to apply some feedback to give closed-loop control.鹅娅尽損鹌惨歷茏鴛賴。A4.1.3Closed-loop control The simple open-loop drive of Section A4.1.2 can be replaced with a control system as in Figure A4.2. This control system not only provides a means to correct for any error in the output variable, but also enable a stable response characteristic 籟丛妈羥为贍偾蛏练淨。A4.2 Controllers in a driveA4.2.1GeneralAlthough a modern variable-speed drive includes many features, the basic function of the drive is to control torque (or force), speed or position. Before proceeding to the specific details of how different types of variable-speed drive function, the theory of control for each of these quantities is discussed. A position control system is shown in Figure A4.5. This includes an inner speed controller, and within the speed controller there is an inner torque controller. It is possible to create a system where the position controller determines the mechanical torque that is applied to the load directly without the inner speed and torque loops. However, the position controller would need to be able to control the complex combined transfer function of the motor windings, the mechanical load and the conversion from speed to position.預頌圣鉉儐歲龈讶骅籴。Therefore it is more usual to use the format shown in Figure A4.5. The other advantage of this approach is that limits can be applied to the range or rate of change of speed and torque between each of the controllers. When a system is required to control speed only, the position controller is omitted, and when a system is required to control torque only, the position and speed controllers are omitted.渗釤呛俨匀谔鱉调硯錦。A position sensor is shown providing feedback for the system, but this may be replaced by a speed sensor or it may be omitted altogether as follows.铙誅卧泻噦圣骋贶頂廡。Position information is required by the torque controller to function in an a.c. motor drive (see the dotted line). If position feedback is provided the speed feed-back is derived as the change of speed over a fixed sample period. Sensorless schemes are possible for speed and torque control of a.c. motors, in which case the sensor is not required. 擁締凤袜备訊顎轮烂蔷。Position feedback is not necessary for the torque controller in a d.c. motor drive, so a speed feedback device such as a tacho-generator can be used to provide the feed-back for the speed controller. Again, sensorless schemes are possible where a speed feedback device is not required. 贓熱俣阃歲匱阊邺镓騷。A4.2.2Torque controlA torque controller for a rotary motor, or a force controller for a linear motor, is the basic inner loop of most variable-speed drives. Only torque control is discussed here, but the principles also apply to force control for a linear application. In order to explain the principles of torque control, the simple d.c. motor system in Figure A4.6 is used as an example. The analysis of torque control in an a.c. motor can be done in exactly the same way, provided suitable transformations are carried out in the drive. These transformations will be discussed later.坛摶乡囂忏蒌鍥铃氈淚。The torque demand or reference (Te*) is converted by the torque controller into a current in the motor armature, and the motor itself converts the current into torque蜡變黲癟報伥铉锚鈰赘。Figure A4.6 Torque and current controllers in a d.c. motor drive: (a) torque control;買鲷鴯譖昙膚遙闫撷凄。(b) current control to drive the mechanical load. Figure A4.6b shows the system required to convert the torque reference into motor current. The torque reference (Te*) is first transformed into a current reference (ia*) by including the scaling effect of the motor flux. The motor flux, controlled by the motor field current (if ), is normally reduced from its rated level at higher speeds when the terminal voltage would exceed the maximum possible output voltage of the power circuit without this adjustment. Current limits are then applied to the current reference so that the required current does not exceed the capa-bilities of the drive. The current reference (limited to a maximum level) becomes the input for the PI controller. The electrical equivalent circuit of the motor consists of a resistance (Ra), an inductance (La) and a back emf that is proportional to flux and speed綾镝鯛駕櫬鹕踪韦辚糴。(Kevc/crated).The PI controller alone could successfully control the current in this circuit because as the speed increases, the voltage required to overcome the back emf would be pro-vided by the integral term. The integral control is likely to be relatively slow, so to improve the performance during transient speed changes a voltage feed-forward term equivalent to Kevc/crated is included. The combined output of the PI controller and the voltage feed-forward term form the voltage reference (va*), and in response to this the power circuit applies a voltage (va) to the motors electrical circuit to give a current (ia). The current is measured by a sensor and used as feedback for the current controller.驅踬髏彦浃绥譎饴憂锦。As well as the linear components shown in Figure A4.6, the current control loop in a digital drive includes sample delays as well as delays caused by the power circuit. In practice, the response of the controller is dominated by the proportional gain. In particular, if a voltage feed-forward term is used, the integral term has very little effect on the transient response.猫虿驢绘燈鮒诛髅貺庑。 Setting of the control loop gains is clearly very important in optimising the per-formance of the control loop. One of the simplest methods to determine a suitable proportional gain is to use the following equation:锹籁饗迳琐筆襖鸥娅薔。where La is the motor inductance and Ts the current controller sample time. K is a con-stant that is related to the current and voltage scaling, and the delays present in the control system and power circuit. Most modern variable-speed drives include auto-tuning algorithms based on measurement of the electrical parameters of the motor taken by the drive itself, and so the user does not normally need to adjust the current controller gains.構氽頑黉碩饨荠龈话骛。It is useful to know the closed-loop transfer function of the torque controller (i.e. Te/Te*) so that the response of a stand-alone torque controller, or the effect of an inner torque controller on outer loops such as a speed controller, can be predicted. As the response is dominated by the system delays it is appropriate to represent the closed-loop response as simple gains and a unity gain transport delay as shown in Figure A4.7.輒峄陽檉簖疖網儂號泶。The torque reference could be in N m, but it is more conventional to use a value that is a percentage of the rated motor torque. Figure A4.7a gives the transfer function when the torque controller is used alone. Kt is the torque constant of the motor in N m A21. If the torque controller is used with an outer speed controller a slightly different representation must be used, as in Figure A4.7b. The speed controller pro-duces a torque reference where a value of unity corresponds to a current level that is specified for the size or rating of the drive. From a control perspective it is unimportant whether this is the maximum current capability of the drive, the rated current or some other level. The actual level尧侧閆繭絳闕绚勵蜆贅。 used is defined as Kc (in amperes), and should be included in the transfer function as shown. These simple models allow the drive user to predict the performance of a stand-alone torque controller or a torque controller with an outer speed loop.识饒鎂錕缢灩筧嚌俨淒。A4.2.3Flux controlThe motor flux and hence the motorterminal voltage for a given speed are defined by the flux producing current. In the example of a simple d.c. motor drive used pre-viously, the motor flux level is set by the field current, if. The flux controller (Figure A4.8) includes an inner current loop and an outer loop that maintains rated flux in the motor until the armature terminal voltage reaches its maximum limit. When the motor speed increases above rated speed it then controls the field current and hence the flux, so that the armature voltage remains at the maximum required level.凍鈹鋨劳臘锴痫婦胫籴。A4.2.4Speed controlA4.2.4.1 Basic speed controlClosed-loop speed control can be achieved by applying a simple PI controller around the torque controller described previously. For the purposes of this analysis it is assumed that the load is an inertia J, with a torque Td that is not related to speed (friction is neglected). The resulting system is shown in Figure A4.10恥諤銪灭萦欢煬鞏鹜錦。Figure A4.10 Speed controllerIf the PI controller is represented as Kp Ki/s, the torque controller is assumed to be ideal with no delays so that the unity transport delay can be neglected, and the inertia load is represented as 1/Js then the forward loop gain in the s domain is given by鯊腎鑰诎褳鉀沩懼統庫。The closed-loop transfer function in the s domain v(s)/v*(s) is given by G(s)/ 1 G(s). Substituting for G(s) and rearranging gives 硕癘鄴颃诌攆檸攜驤蔹。If the natural frequency of the system is defined as vn (KcKtKi=J ) and the damping factor is defined as j vnKp/(2Ki) then阌擻輳嬪諫迁择楨秘騖。As with the torque controller, it is useful to know the closed-loop response so that the response of a stand-alone speed controller, or the effect of an inner speed controller on an outer position loop, can be predicted. If a moderate response is required from the speed controller it is not significantly affected by system delays, and a linear transfer function such as equation (A4.4) can be used. All the constants in these equations and the delays associated with the current controllers are normally provided to users so that calculations and/or simulations can be carried out to predict the performance of the speed controller.氬嚕躑竄贸恳彈瀘颔澩。In addition to providing the required closed-loop step response, it is important for the system to be able to prevent unwanted movement as the result of an applied torque transient. This could be because a load is suddenly applied or because of an uneven load. The ability to prevent unwanted movement is referred to as stiffness. The com-pliance angle of the system is a measure of釷鹆資贏車贖孙滅獅赘。Figure A4.11 Responses of an ideal speed controller: (a) closed-loop step response;怂阐譜鯪迳導嘯畫長凉。Figure A4.12 Unwanted delays in a practical digital drive谚辞調担鈧谄动禪泻類。Dynamics 115UMC 3 000 rpm servo motor (Kt 1.6 N m A21, J 0.00078 kg m2) with the speed controller gains set to Kp 0.0693j and Ki 14.32.嘰觐詿缧铴嗫偽純铪锩。As the damping factor is increased, the closed-loop response overshoot is reduced and the speed of response improves. The closed-loop response includes 10 per cent overshoot with a damping factor of unity because of the s term in its numerator.熒绐譏钲鏌觶鷹緇機库。As the damping factor is increased, the overshoot of the response to a torque tran-sient is reduced and the response becomes slower. In this case there is no s term in the numerator and the response includes no overshoot with a damping factor of unity.鶼渍螻偉阅劍鲰腎邏蘞。It would appear from these results that the higher the proportional gain, and hence the higher the damping factor the better the responses; however, the results so far assume an ideal torque controller and no additional unwanted delays. In a real digital drive system the delays given in Figure A4.12 are likely to be present. A delay is included to represent the sample period for speed measurement, but this is only relevant if the speed feedback is derived from a position feedback device such as an encoder and is measured as a change of position over a fixed sample period.纣忧蔣氳頑莶驅藥悯骛。The effect of the unwanted delays can be seen in the closed-loop step response for a real system as shown in Figure A4.13. In each case the response of the real s