Position control device

Abstract

To provide a position control device capable of preventing follow-up delay during extremely low and constant speed operation.SOLUTION: A position control device that controls the position of a controlled object sequentially calculates a speed command value V and a torque command value τc based on a position command value Pos_ref and a position detection value Pos, determines whether the controlled object is operating at a constant speed in an extremely low speed region that is a speed range equal to or less than a predetermined threshold value Vth based on the speed command value V calculated from the position command value Pos_ref, and if it determines that the controlled object is operating at an extremely low and constant speed based on an acceleration command value A, adds a friction compensation amount τs that compensates for friction occurring during the extremely low and constant speed operation to at least one of the torque command value τc, the speed command value V, and the position command value Pos_ref.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an improvement in a position control device for a machine tool or the like. [Background technology]

[0002] As an example of a conventional position control device applied to a machine tool, a block diagram corresponding to the position control device disclosed in Patent Document 1 is shown in Fig. 3. The configuration and operation of the position control device 112 shown in Fig. 3 will be described below. A position command value Pos_ref is input to the position control device 112 from a higher-level device (not shown).

[0003] The position command value Pos_ref is time-differentiated by differentiators 101 and 104, and the respective outputs become a speed command value V and an acceleration command value A. The acceleration command value A is amplified by an acceleration torque conversion constant K by amplifier 105 to become an acceleration torque command value τaa for generating the acceleration in target system 110. The speed command value V and the acceleration torque command value τaa are added to the output of position controller 102 and the output of speed controller 108, respectively. This series of processes constitutes a well-known feedforward intended to improve the responsiveness of the position detection value Pos with respect to the position command value Pos_ref.

[0004] The subtractor 100 subtracts the position detection value Pos from the position command value Pos_ref to calculate the position deviation Pos_ref-Pos. The position detection value Pos is a position detection signal obtained by detecting the position of the control target of the target system 110 using a linear scale or the like (not shown).

[0005] Position controller 102 proportionally amplifies the position error Pos_ref-Pos using a position loop gain. An adder 103 adds the output of this position controller 102 to a speed command value V and a startup friction compensation value Vsfc to obtain a final speed command value Vc. A subtractor 107 subtracts a speed detection value Vel, which is a speed of the target system 110 (i.e., the controlled object) detected using a linear scale or the like (not shown), from the final speed command value Vc to calculate a speed error Vc-Vel. The speed detection value Vel is, for example, a time differential value of the rotational angle position of a position detector coupled to a servo motor inside the target system 110, or the output of a speed detector coupled to the servo motor. A speed controller 108 is a typical PI controller that amplifies the speed error Vc-Vel using the proportional gain of the speed loop and the integral gain of the speed error.

[0006] The output of the speed controller 108 is added to an acceleration torque command value τaa in an adder 109 to become a torque command value τc. The torque command value τc is input to a target system 110 to operate the target system 110.

[0007] An overview of a conventional position control device 112 shown in Figure 3 will be described. Friction force may be discontinuous before and after a controlled object starts moving from a stopped state, i.e., at startup. The conventional position control device 112 is provided with a startup friction compensation calculation unit 111 to perform torque compensation at startup. When the startup friction compensation calculation unit 111 detects from the speed command value V that the target system 110 stops and the direction of movement before and after startup are different, the startup friction compensation calculation unit 111 generates a friction compensation value Vsfc in a predetermined form at startup that corresponds to the motor torque generated at startup detected from the torque command value τc and the amount of change in friction torque immediately after startup, thereby improving the tracking characteristics at startup. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 3840429 Summary of the Invention [Problem to be solved by the invention]

[0009] In conventional position control, when the controlled object is decelerated to an extremely low speed and then moves at a constant extremely low speed in the same direction without coming to a complete stop, there is a problem in that the position tracking accuracy deteriorates immediately after the start of the extremely low constant speed movement.

[0010] It is widely known that the guide mechanism, particularly the sliding guide, included in the target system 110 can be in a state of boundary lubrication, mixed lubrication, or hydrodynamic lubrication depending on the operating speed. The friction coefficient resulting from these states is known to exhibit a large friction force in the low-speed region, then decrease once, before increasing again, as represented by the Stribeck curve.

[0011] Specifically, consider the operation of the target system 110, which starts deceleration from a speed range in a hydrodynamic lubrication state, decelerates to an extremely low speed corresponding to the boundary lubrication region, and then moves. The torque command value τc output by the position control device 112 is smaller than the drive torque required in the mixed lubrication region during deceleration because frictional force acts in the deceleration direction during deceleration. In this state, if an attempt is made to transition to extremely low constant speed operation in the boundary lubrication region, the torque required is greater than in the hydrodynamic lubrication region, resulting in insufficient torque. As a result, position tracking is delayed.

[0012] The time waveform of the above-mentioned tracking delay is shown in Fig. 4. In the example of Fig. 4, the speed command value V drops to the low speed region at time T1. Then, after time T1, a speed command value V is input to move at a constant speed in the same direction while maintaining the low speed without stopping. In the example of Fig. 4, due to the phenomenon explained in

[0011] , a tracking delay occurs in the position deviation Diff immediately after time T1.

[0013] Patent Document 1 discloses a technique for performing compensation to reduce the tracking delay when restarting after stopping. However, in the case of the transition to the extremely low speed, the target system 110 does not assume a zero speed, so compensation cannot be performed and the tracking delay cannot be resolved. [Means for solving the problem]

[0014] The position control device disclosed in this specification is a position control device that controls the position of a control object, and is characterized in that it is configured to sequentially calculate a speed command value and a torque command value based on a position command value and a position detection value, determine whether the control object is operating at a constant speed in an extremely low speed region that is a speed range equal to or less than a predetermined threshold value (hereinafter referred to as extremely low speed constant speed operation) based on the speed and acceleration calculated from the position command value, and if it is determined that the control object is operating at an extremely low speed constant speed, add a friction compensation amount that compensates for friction that occurs in the extremely low speed constant speed operation to at least one of the torque command value, the speed command value, and the position command value. [Effects of the Invention]

[0015] According to the position control device of the present invention, it is possible to effectively reduce the follow-up delay in a drive system in which a follow-up delay occurs due to friction in an extremely low speed region. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a block diagram illustrating an embodiment of the present invention. [Figure 2] FIG. 10 is a diagram showing a time response waveform when the present invention is applied. [Figure 3] FIG. 1 is a block diagram showing a conventional technology having a friction compensation function at the start of a feed axis. [Figure 4] FIG. 10 is a diagram showing a time response waveform when the present invention is not implemented. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a block diagram showing an example of the configuration of a position control device according to an embodiment of the present invention. The same parts as those in Fig. 3 above are given the same names and numbers, and the description of those parts will be omitted. A target system 110 is an object to be controlled by the position control device.

[0018] The friction compensation calculation unit 3 of the present invention will be described below. A friction compensation amount τs for compensating for a tracking delay due to friction is calculated by a friction compensation timing detection unit 1 and a friction compensation amount generation unit 2. The friction compensation amount τs is added to the output of an adder 109 by an adder 6 to become a torque command value τc in this embodiment.

[0019] The friction compensation timing detection unit 1 determines whether the target system 110 is operating at an extremely low constant speed. Specifically, the friction compensation timing detection unit 1 determines that the target system 110 is in an extremely low speed region when the absolute value of the speed command value V is equal to or less than a preset speed threshold Vth and greater than 0, and determines that the target system 110 is not in an extremely low speed region when the absolute value of the speed command value V is greater than the preset speed threshold Vth. The friction compensation timing detection unit 1 then retains the determination result. The speed threshold Vth that defines the extremely low speed region may be set according to the characteristics of the target system 110 (i.e., the controlled object). For example, the speed threshold Vth may be the speed at which the guiding mechanism of the controlled object transitions from a mixed lubrication state to a boundary lubrication state. Alternatively, the speed threshold Vth may be the speed at which the guiding mechanism of the controlled object has the smallest friction coefficient.

[0020] Furthermore, the friction compensation timing detection unit 1 determines that the target system 110 is operating at an extremely low constant speed when a preset time has elapsed in the extremely low speed region and the acceleration command value A is zero. The friction compensation timing detection unit 1 also outputs a friction compensation trigger signal Tr. When the target system 110 is operating at an extremely low constant speed, the friction compensation trigger signal Tr has the same sign as the speed command value V and its absolute value is 1. On the other hand, when the target system 110 is not operating at an extremely low constant speed, the friction compensation trigger signal Tr is 0. The determination of whether the acceleration command value A is zero may be based on whether it truly matches zero, or it may be based on whether the acceleration command value A is approximately zero, for example, when the absolute value of the acceleration command value A is equal to or less than a predetermined value. In the extremely low speed region and when the acceleration command value A is zero, the direction of movement does not reverse. Therefore, this function is independent of the compensation function for compensating for the tracking delay during reversal.

[0021] The friction compensation amount generator 2 outputs the friction compensation amount τs according to equation (1), taking the time t=0 as the point in time when it detects that the value of the friction compensation trigger signal Tr has changed from 0 to 1 or −1. τs(t)=Tr×τca×exp(-t÷Td) (1)

[0022] The compensation amount amplitude τca and the compensation time constant Td in Equation (1) are both coefficients that are specified in advance. The compensation amount amplitude τca may be determined in advance, for example, by experiment or simulation. For example, with the friction compensation amount τs set to 0, an extremely low constant speed operation may be started, and the torque command value τc at the timing when the position error Diff changes from an increasing trend to a decreasing trend (i.e., time T3 in FIG. 4) may then be set as the compensation amount amplitude τca. Alternatively, the compensation amount amplitude τca may be determined by changing and searching for a torque command value that balances with the friction generated at a speed equal to the speed threshold Vth (i.e., a reference speed for determining the extremely low speed region) based on the Stribeck characteristic of the controlled object.

[0023] The compensation time constant Td may also be determined in advance through experiments or simulations. For example, the compensation time constant Td may be determined by executing an extremely low-speed constant-speed operation in advance and determining the time elapsed from the start of the operation until the position error Diff changes from an increasing trend to a decreasing trend, i.e., T3-T1 in FIG. 4 . Alternatively, the compensation time constant Td may be determined by changing and searching for a time that reduces the cumulative value of the position error Diff that occurs until the position error Diff changes from an increasing trend to a decreasing trend. In either case, according to Equation (1), the friction compensation amount τs takes a large value immediately after the start of the extremely low-speed constant-speed operation and then rapidly decreases over time. This allows appropriate compensation for the torque shortage immediately after the start of the extremely low-speed constant-speed operation, effectively preventing follow-up delay.

[0024] In this embodiment, the friction compensation amount τs is output according to equation (1), but the output format may be changed as appropriate as long as the friction compensation amount τs according to the characteristics of the target system 110 can be obtained. For example, the friction compensation amount τs may be an impulse signal. Furthermore, instead of calculating the friction compensation amount τs from an equation, the friction compensation amount τs corresponding to each elapsed time may be stored in advance in the form of a table or map, and the friction compensation amount τs may be determined from the table or map.

[0025] The time response according to this embodiment is shown in Figure 2. In Figure 2, the target system 110 initially moves at a constant speed, begins deceleration at time T0, and operates at an extremely low constant speed from time T1. In this embodiment, an extremely low speed region is detected at time T1, and the point in time when the acceleration command value A is zero is used as the compensation start timing, at which point the friction compensation trigger signal Tr (not shown) becomes 1 and compensation begins. Therefore, at time T2, the friction compensation amount τs is added to the torque command value τc, and as a result, the position error Diff decreases. Here, the dashed line in Figure 2 for Diff is the response waveform according to the prior art shown in Figure 4.

[0026] In this embodiment, the configuration shown in FIG. 1 does not specify the order of calculations, and it is of course possible to rearrange the configuration as appropriate, for example, by switching the order of calculations of adder 6 and adder 109 or combining them into one calculation unit.

[0027] Furthermore, the position control device to which the present invention is applied is not limited to the feedback control system of FIG. 1, and may, for example, be a state feedback type feedback control system in which velocity and position are fed back to a single controller.

[0028] The configuration of the position control device according to this embodiment can be applied to control devices for not only machine tools but also robots and various other industrial machines having a drive shaft driven by an electric motor and subject to friction.

[0029] The position control device shown in FIG. 1 may physically be a computer equipped with a processor and memory and performing various calculations. For example, the configuration of the position control device may be realized by a CPU included in the position control device executing various software stored in a storage device. The position control device may also be realized by a configuration mainly composed of hardware such as an FPGA. This target system 110 (i.e., the control target) includes, for example, a motor, a driven body driven by the motor, an inverter that applies power to the motor according to a torque command value τc, and a position sensor that detects the position of the motor, the driven body, or both.

[0030] In this embodiment, the friction compensation amount τs output by the friction compensation amount generation unit 2 is configured to be added to the torque command value τc, but for example, instead of adding it to the torque command value τc, an equivalent function can be provided by integrating the friction compensation amount τs and adding the result to the speed command value V. Similarly, an equivalent function can be provided by integrating the friction compensation amount τs twice and adding the result to the position command value Pos_ref. [Explanation of symbols]

[0031] 1 Friction compensation timing detection unit, 2 Friction compensation amount generation unit, 3 Friction compensation calculation unit, 4 Position control device, 100, 107 Subtractor, 101, 104 Differentiator, 102 Position controller, 105 Amplifier, 108 Speed ​​controller, 6, 103, 106, 109 Adder, 110 Target system, 111 Start-up friction compensation calculation unit, 112 Position control device.

Claims

1. A position control device that controls the position of a control object, A speed command value and a torque command value are calculated in sequence based on the position command value and the position detection value. determining whether the controlled object is operating at a constant speed in an extremely low speed region, which is a speed range equal to or less than a predetermined threshold, based on the speed and acceleration of the controlled object calculated from the position command value; when it is determined that an extremely low speed constant speed operation is being performed, a friction compensation amount for compensating for friction occurring during the extremely low speed constant speed operation is added to at least one of the torque command value, the speed command value, and the position command value; A position control device characterized by being configured as follows.

2. 2. The position control device according to claim 1, 10. A position control device according to claim 9, wherein the friction compensation amount is a value whose absolute value decreases over time from a predetermined compensation amount amplitude and whose sign matches the speed command value.

3. 3. The position control device according to claim 2, wherein Tr is a constant that is 1 when the speed command value is positive and is −1 when the speed command value is negative, τca is the compensation amount amplitude, Td is a compensation time constant, and t is an elapsed time from the start of the extremely low constant speed operation, the position control device is configured to calculate the friction compensation amount according to equation (1). τs(t)=Tr×τca×exp(-t÷Td) (1)

4. 3. The position control device according to claim 2, a compensation amount amplitude that is determined to be a torque command value that balances with friction that occurs at a speed threshold that is determined to be in an extremely low speed region based on the Stribeck characteristic of the controlled object.

5. 3. The position control device according to claim 2, wherein the compensation amount amplitude and the compensation time constant are the torque command value and the elapsed time at a timing when the position error changes from an increasing trend to a decreasing trend after the start of the extremely low constant speed operation under the condition that the friction compensation amount is set to zero.

6. 3. The position control device according to claim 2, the compensation amount amplitude and the compensation time constant are values ​​found by changing the compensation amount amplitude and the compensation time constant so as to reduce an accumulated value of the position error that occurs from the start of the extremely low constant speed operation until the position error changes from an increasing trend to a decreasing trend.

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