Method, system and apparatus for determining the resonant frequency of a mechanical system

Abstract

A method for determining a resonant frequency of a mechanical system (4), the mechanical system comprising a motor (6) coupled to a mechanical load (8), the method comprising: starting the motor by providing a supply voltage using a motor controller; applying a first excitation signal once the motor is running at a predefined target speed, the first excitation signal comprising a voltage pulse superimposed on the supply voltage; measuring a mechanical response of the mechanical system using a measurement system (18) coupled to the motor; and analyzing the measured response to determine at least one resonant frequency of the mechanical system.

Description

Technical Field

[0001] The present invention relates to methods, systems and apparatus for determining the resonant frequency of a mechanical system. Background Art

[0002] Aspects of the present invention are more particularly applicable to mechanical systems in which an electric machine is connected to a mechanical load.

[0003] In some applications, flexible connection devices are used to mechanically connect a load to a motor. As a result, due to the mechanical properties of the flexible connection device, the mechanical system as a whole can exhibit one or more resonant frequencies. If the system enters a resonant state during operation, there is a risk of damage or even destruction of certain mechanical components. Therefore, it is desirable to avoid operating the system near or at resonant frequencies, which means that the resonant frequencies must be determined in some way before operation.

[0004] Calculating theoretical values for resonant frequencies is not always possible in practice. However, experimental methods have been developed to determine at least some resonant frequencies.

[0005] For example, European patent application EP 1959324A2 discloses a method for determining resonant frequencies in a mechanical system including a motor driving a mechanical load. The method includes driving the motor at different speeds by varying the supply voltage during a test sequence while the load is connected to the motor. The voltage across the motor's input terminals is measured, and post-processing operations are used to determine the frequency response and identify one or more resonant frequencies.

[0006] A disadvantage of these known methods is that they require the system to operate within a wide frequency range including one or more resonant frequencies, and thus may lead to resonant conditions with all the associated disadvantages, even though the purpose of these methods is precisely to avoid operating the motor at such resonant frequencies. Summary of the Invention

[0007] It is therefore an object of the present invention to provide improved methods, systems and apparatus for determining the resonant frequency of a mechanical system.

[0008] According to one aspect, a method for determining a resonant frequency of a mechanical system, the mechanical system comprising a motor coupled to a mechanical load, comprises the steps of:

[0009] The motor is started by providing a supply voltage using a motor controller;

[0010] Once the motor is running at a predefined target speed, applying a first excitation signal, the first excitation signal comprising a voltage pulse superimposed on a power supply voltage;

[0011] measuring a mechanical response of a mechanical system using a measurement system coupled to the motor;

[0012] The measured response is analyzed to determine at least one resonant frequency of the mechanical system.

[0013] According to advantageous aspects, embodiments may include one or more of the following technical features, which may be used alone or in any possible technical combination:

[0014] - Measuring a mechanical response of the mechanical system comprises measuring a stator current of the electrical machine, and wherein a resonant frequency of the mechanical system is determined based on the measured stator current.

[0015] -Determining the resonant frequency of a mechanical system involves:

[0016] Calculating a corresponding stator current value represented in a synchronous frame of the motor based on the measured stator current;

[0017] identifying oscillations in corresponding stator current values;

[0018] A resonant frequency is determined based on the identified oscillations.

[0019] The method further comprises calculating a phase difference value between the calculated corresponding stator current values and determining a resonant frequency based on oscillations identified in the phase difference value.

[0020] - Measuring the mechanical response of the mechanical system comprises measuring a rotational speed of the electric machine, and wherein the resonant frequency of the mechanical system is determined based on the measured rotational speed.

[0021] - The method further comprises applying at least one additional excitation voltage pulse and measuring a corresponding response of the mechanical system, the resonant frequency being determined based on an analysis of the measured response.

[0022] - at least one additional excitation pulse differs from the first excitation pulse by at least one characteristic, such as pulse length, pulse amplitude or a time interval between two consecutive pulses.

[0023] -The method further comprises:

[0024] modifying at least one motor operating parameter to drive the motor at a different speed;

[0025] Once the motor is running at the new target speed, another excitation signal comprising voltage pulses superimposed on the supply voltage is applied using a pulse generator;

[0026] measuring a new mechanical response of a mechanical system using a measurement system;

[0027] The measured response is analyzed to determine at least one resonant frequency of the mechanical system.

[0028] Starting the motor includes boosting the supply voltage to a threshold voltage value, and wherein the motor controller is placed in an open-loop control mode once the motor is running at a predefined target speed.

[0029] - The method comprises determining a type of mechanical coupling between the load and the motor based on the measured mechanical response.

[0030] According to another aspect, a method for driving a mechanical system including a motor coupled to a mechanical load includes:

[0031] Determine the resonant frequency of the mechanical system using the method described above;

[0032] At least one operating parameter of the electric machine is modified to avoid excitation near or at the determined resonant frequency.

[0033] According to another aspect, an apparatus for determining a resonant frequency of a mechanical system, the mechanical system comprising a motor coupled to a mechanical load, is configured to:

[0034] The motor is started by providing a supply voltage using a motor controller;

[0035] Once the motor is running at a predefined target speed, applying a first excitation signal, the first excitation signal comprising a voltage pulse superimposed on a power supply voltage;

[0036] measuring a mechanical response of a mechanical system using a measurement system coupled to the motor;

[0037] The measured response is analyzed to determine at least one resonant frequency of the mechanical system. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Many aspects of the present invention will be better understood by reading the following description of some embodiments provided by way of example only and with reference to the accompanying drawings, in which:

[0039] Figure 1 is a schematic diagram of a mechanical system and apparatus for determining at least one resonant frequency of a mechanical system according to an embodiment of the present invention;

[0040] Figure 2 is a simplified diagram of an exemplary mechanical system including a flexible coupling device between a motor and a mechanical load;

[0041] Figure 3 is a flow chart depicting an exemplary method for determining a resonant frequency of a mechanical system according to an embodiment of the present invention;

[0042] Figure 4 It is depicted in the execution Figure 3 The method is applied during Figure 1a graph of the magnitude of an exemplary excitation voltage signal for a motor of a mechanical system;

[0043] Figure 5 is depicted in the applied Figure 4 After the excitation voltage signal Figure 1 A graph of the response of the motor of the mechanical system;

[0044] Figure 6 It is used to determine similar Figure 1 a flowchart of an exemplary method for determining mechanical characteristics of a mechanical system;

[0045] Figure 7 It is used for operations similar to Figure 1 A flow chart of an exemplary method of a mechanical system. DETAILED DESCRIPTION

[0046] Figure 1 An exemplary apparatus 2 for determining mechanical properties of a mechanical system 4 comprising an electric machine 6 connected to a mechanical load 8 is shown.

[0047] For example, the motor 6 is an alternating current (AC) motor, such as a synchronous motor or an asynchronous motor. In some other embodiments, the motor 6 may be a direct current (DC) motor.

[0048] In many embodiments, the motor 6 can be used in industrial facilities, such as industrial production lines, to drive machinery and / or industrial equipment, such as pumps, fans, conveyors, valves, robots, elevators, movable parts (such as doors or shutters), etc. In other examples, the motor 6 can be used as a prime mover, for example in a vehicle such as a passenger car or an industrial vehicle.

[0049] The motor 6 is mechanically connected to the load 8 via a flexible connection device so as to drive the load 8 when the motor 6 rotates.

[0050] In many embodiments, motor 6 is driven by providing a suitable voltage to the input terminals of motor 6 using a controller 10 powered by a power source 12 , such as a variable speed drive.

[0051] In many embodiments, device 2 is more generally configured to perform the following steps:

[0052] - starting the motor 6 by supplying a supply voltage to the controller 10;

[0053] - once the motor 6 is running at a predefined target rotation speed, applying a first excitation signal, the first excitation signal comprising a voltage pulse superimposed on the supply voltage;

[0054] -Measure the mechanical response of mechanical systems;

[0055] - Analyzing the measured response to determine at least one resonant frequency of the mechanical system.

[0056] For this purpose, the device 2 comprises a pulse generator 14 which is able to generate voltage pulses and to inject these voltage pulses into the motor 6 , for example by superimposing the generated voltage pulses on the supply voltage fed to the motor 6 .

[0057] In various embodiments, the pulse generator 14 may be implemented by the controller 10 or may be a separate device from the controller 10. This separate device may be connected to an input terminal of the controller 10 or an input terminal of the motor 6.

[0058] The system 2 further comprises a control system 16 programmed to implement the above-described method steps for determining mechanical properties of the system 4 , such as determining one or several resonant frequencies of the system 4 .

[0059] For example, the control system 16 includes electronic circuitry and may include a general purpose processor, such as a microcontroller, and computer memory storing software code and / or executable instructions that, when executed by the processor, cause the processor to implement the described method.

[0060] In alternative embodiments, control system 16 may include a digital signal processor (DSP), a programmable logic controller (PLC), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or any suitable combination thereof. Additionally, analog circuits may be used to implement many of the same functions.

[0061] Control system 16 is coupled to controller 10 and pulse generator 14, such as by cables or wires or a wireless communication link.

[0062] The system 2 also includes a measurement system 18 configured to measure a mechanical response of the mechanical system 4 in response to the excitation signal.

[0063] In a preferred embodiment, the measurement system 18 comprises one or several current sensors coupled to the input terminals of the stator of the electric machine 6 in order to measure the currents of at least some of the electrical phases of the stator, and preferably of all the electrical phases of the stator.

[0064] In other words, the measurement system 18 is configured to measure the electrical response of the system 4. The electrical response includes information about the mechanical response of the system 4 (such as rotational speed and / or mechanical vibrations).

[0065] For measuring the mechanical response of the motor 6 this is more preferable and more efficient than measuring different mechanical values such as torque or speed or voltage.

[0066] Preferably, the current sensor is integrated with the motor 6. Thus, the current sensor already installed in the motor 6 can be reused, and there is no need to add a dedicated current sensor. In alternative embodiments, the current sensor can be part of the controller 10, or can be a dedicated current sensor.

[0067] In other embodiments, the measurement system 18 may be configured to measure the rotational speed of the motor 6 .

[0068] In other further embodiments, the measurement system 18 may be configured to measure the rotational speed and the current of the motor 6 .

[0069] The device 2 further comprises a signal processing unit 20 configured to analyse the response measured by the measurement system 18 and to automatically determine the resonant frequency based on the measured response.

[0070] Processing device 20 may be implemented by any type of circuitry in the same manner as described with reference to control system 16 .

[0071] In some embodiments, processing unit 20 is implemented by control system 16, for example, as software code executed by a processor and / or as dedicated circuitry. In other embodiments, processing unit 20 may be a remote computer server.

[0072] Figure 2 An exemplary mechanical system 30 is shown, wherein the moment of inertia is J m The motor 6 is coupled to the rotating inertia J through a flexible mechanical coupling device 10 with a stiffness coefficient K. l Mechanical load 8.

[0073] Reference mark "T m ", "ω m ” and “θ m ” respectively correspond to the torque, speed and angular position of the rotor of the motor 6. l ", "ω l ” and “θ l ” correspond to the torque, speed and angular position of the load 8 respectively.

[0074] In practice, even though the theoretical value of the resonant frequency can be calculated from analytical formulas based on classical mechanical models, doing so is often difficult because it is not always possible to know the stiffness coefficient K, the moment of inertia J, and the l and moment of inertia J m The exact value of .

[0075] An embodiment of a method for determining a resonant frequency will be described with reference to Figure 3 Flowchart and Figure 4 and Figure 5.

[0076] At step S100 , the motor 6 is started using the controller 10 by feeding a suitable supply voltage to the stator windings of the motor 6 , for example by providing a three-phase AC voltage between input terminals connected to the stator windings.

[0077] Preferably, the maximum amplitude of the supply voltage is gradually increased by following a specific start-up sequence (eg linear ramp-up).The controller 10 increases the supply voltage, for example by applying a V / f control strategy, until a predefined voltage setpoint value corresponding to the target speed is reached.

[0078] Once the startup sequence is complete, the maximum amplitude remains equal to the voltage set point value.

[0079] For example, at this stage, the controller 10 operates in an open-loop control mode.

[0080] At the end of step S100 , the electric machine 6 is running in a so-called permanent regime, also called steady state operation, having a constant or nearly constant rotational speed equal to the target rotational speed.

[0081] In step S102 , the device 2 applies at least one excitation signal comprising voltage pulses superimposed on a supply voltage using the pulse generator 14 .

[0082] Preferably, the amplitude of the voltage pulse is significantly higher than the voltage set point value, for example 50% higher than said set point value.

[0083] More preferably, the duration of the voltage pulse is shorter than 500 milliseconds (ms), or shorter than 100 ms or shorter than 50 ms.

[0084] In response to the voltage pulse, the torque applied by the motor 6 changes briefly, resulting in a sudden change in speed. The mechanical system 4 enters an excited state before quickly settling back into the previous so-called permanent state or steady state operation.

[0085] This sudden change feature can be observed in the stator current due to the magnetic coupling between the magnetic circuit on the stator side and the rotating magnetic flux generated by the rotor of the electric machine 6.

[0086] At step S104 , the measurement system 18 measures the mechanical response of the mechanical system 4 , for example by measuring the stator current of each electrical phase at the input terminals of the stator coils.

[0087] In alternative embodiments, measurement system 18 may measure any other relevant operating parameter of motor 6, such as rotational speed, instead of or in addition to the current measurement. The rotational speed may be measured directly using one or more rotation sensors, such as encoder wheels or Hall effect sensors, or may be estimated from electrical values measured by controller 10.

[0088] In some embodiments, several voltage pulses may be applied continuously, for example, by repeating step S102 once or multiple times.

[0089] The repeated voltage pulses may be identical or may differ from the first voltage pulse by at least one pulse parameter, such as pulse length, pulse amplitude, time interval between two consecutive pulses or the like.

[0090] For example, at step S105, the control system 16 determines whether one or more additional voltage pulses need to be applied. If so, then at step S107, the control system 16 changes at least one pulse parameter before generating the pulse.

[0091] Step S104 may be repeated after each voltage pulse for detecting a response to a single excitation pulse, although in alternative embodiments step S104 may be performed only after several pulses have been applied, in order to detect responses to several excitation pulses.

[0092] In step S106 , the measured response is analyzed to determine one or more resonant frequencies of the system 4 , for example by automatically identifying oscillations in the stator current.

[0093] For example, in a first sub-step, a stator current represented in a synchronous reference frame of the motor 6 is calculated from the measured stator currents, such as the quadrature current I q and DC current I d .

[0094] This calculation may be performed using a Park transform or any other suitable method.

[0095] In the second sub-step, the calculated stator current I d , I q The resonant frequency is then determined by identifying the frequency of the oscillation. For example, the current oscillation frequency is equal to the resonant frequency of the mechanical system 4.

[0096] In some embodiments, the frequency analysis may include a Fast Fourier Transform or any similar operation to identify the frequency spectrum of the current signal and then determine the resonant frequency.

[0097] In some optional but advantageous embodiments, in order to more easily detect the resonant frequency and use less computing resources, the resonant frequency can be calculated based on the stator current Id and I q The phase difference between the oscillations is identified instead of the calculated stator current I d and I q Identify oscillations in itself.

[0098] For example, during the second sub-step, the stator current I d and I q , use the following formula to calculate the phase difference Ψ: Ψ=atan2(I d / I q ), where “atan2(…)” is the inverse tangent trigonometric function defined on the interval [-π;π].

[0099] This example is not limiting, and the phase difference may be calculated differently.

[0100] Optionally, during a final step S108 , the identified resonance frequency is stored in a memory and / or sent to a remote computer server and / or displayed to an operator using a human-machine interface.

[0101] In other embodiments, the above method steps may be performed in a different order. Some steps may be omitted. The above exemplary embodiments do not prevent other embodiments from including one or more other method steps, which may be performed together with or in place of some of the above steps.

[0102] In some embodiments, step S100 may be repeated one or more times, preferably by changing one or more operating parameters, such as the slope of the voltage boost, the voltage set point value, or any parameter related to the power supply voltage, so as to set the motor 6 to a different speed.

[0103] Once the motor 6 is running at the new target speed in a so-called permanent state, step S102 is repeated to apply another excitation voltage pulse superimposed on the supply voltage. Steps S104 and S106 are then repeated to measure the new response of the mechanical system 4 and analyze the measured response to determine at least one resonant frequency of the mechanical system.

[0104] Figure 4 There is shown a supply voltage signal 40 suitable for driving the motor 6 during a start-up sequence. The voltage amplitude (expressed in arbitrary units) is plotted as a function of time (expressed in seconds).

[0105] During a first phase P1 corresponding to step S100, the controller 10 increases the maximum amplitude supply voltage, for example from zero to a set point voltage using a linear ramp 42. Then, during a second phase P2 corresponding to step S102, several voltage pulses 44 are applied on top of the constant supply voltage.

[0106] Figure 5 The mechanical system 4 is shown in response to Figure 4 Example of the behavior of the stimulus signal.

[0107] exist Figure 5 , item (a) shows a graph 50 using the same Figure 4 The same time scale is used to show the magnitude of the stator currents 52 , 54 (expressed in normalized units) as a function of time.

[0108] Item (b) is a magnified view of graph 50 that highlights some characteristics of stator currents 52 and 54 for a particular time window.

[0109] In this example, stator currents 52 and 54 are both shown in the synchronous frame of the motor 6. In this example, current 52 is the quadrature current I q , current 54 is the DC current I d .

[0110] Currents 52 and 54 both exhibit spikes 56, which occur simultaneously for both currents 52 and 54. Each current spike highlights the response of mechanical system 4 to excitation voltage pulse 44. For explanatory purposes, two consecutive current spikes 56 are labeled 56a and 56b. Peaks 56a and 56b can be seen in inset (b). It can be seen that between spikes 56, currents 52 and 54 oscillate at a specific frequency (oscillations 58) that is equal to the resonant frequency of the mechanical system.

[0111] refer to Figure 6 , describes an additional embodiment in which the type of mechanical coupling between the load 8 and the motor 6 (eg, resonant coupling or damped coupling) is determined during step S106 before attempting to determine the resonant frequency.

[0112] For example, during step S110 , the type of coupling is determined based on the mechanical response measured during step S104 .

[0113] If, during step S112 , the coupling type is found to correspond to a resonant coupling, then during the following step S114 , the resonant frequency of the system 4 is automatically determined as described above.

[0114] In some embodiments, if the coupling type is found to correspond to non-resonant coupling (such as damped coupling), step S114 is omitted.

[0115] At step S116 , the results of the analysis (including the identified coupling type and resonant frequency) are stored in a memory and / or transmitted to a remote computer server and / or displayed to an operator using a human-machine interface.

[0116] refer to Figure 7 , a method for improving the operation of a mechanical system 4 is described.

[0117] In step S120 , the resonant frequency of the mechanical system 4 is determined using a method according to one of the above-described embodiments.

[0118] At step S122, at least one operating parameter of the controller 10 is modified to avoid excitation near or at the resonant frequency identified during step S120. For example, one or more parameters of the controller 10 may be modified automatically by the control system 16. In an alternative embodiment, a filtering device, such as a band-stop filter tuned to the resonant frequency, may be added between the motor 6 and the controller 10.

[0119] The above-described embodiments and alternatives can be combined with one another in order to construct new embodiments of the invention.

Claims

1. A method for determining a resonant frequency of a mechanical system (4), the mechanical system comprising an electric motor (6) coupled to a mechanical load (8), the method comprising: Starting (S100) the motor (6) by providing a power supply voltage using a motor controller (10); Once the motor is running at a predetermined target speed, applying (S102) a first excitation signal, the first excitation signal comprising a voltage pulse superimposed on the power supply voltage; measuring a response of the mechanical system by measuring (S104) a stator current of the motor using a measurement system (18) coupled to the motor, the measurement system comprising one or more current sensors coupled to input terminals of the stator of the motor; analyzing ( S106 , S108 ) the measured response to determine at least one resonant frequency of the mechanical system based on the measured stator current; Wherein, determining the resonant frequency of the mechanical system includes: Calculating a corresponding stator current value represented in a synchronous frame of the motor based on the measured stator current; calculating a phase difference between the calculated corresponding stator current values; identifying oscillations in the calculated phase difference values; The resonant frequency is determined based on the identified oscillation.

2. The method according to claim 1, wherein Measuring the response of the mechanical system includes measuring ( S104 ) a rotational speed of the motor, and wherein a resonant frequency of the mechanical system is determined based on the measured rotational speed.

3. The method according to claim 1, wherein The method further comprises applying at least one additional excitation voltage pulse (S102, S107), and measuring a corresponding response of the mechanical system, the resonant frequency being determined based on an analysis of the measured response.

4. The method according to claim 3, wherein: The at least one additional excitation pulse (S107) differs from the first excitation pulse by at least one parameter.

5. The method according to claim 4, wherein The at least one parameter comprises a pulse length, a pulse amplitude, or a time interval between two consecutive pulses.

6. The method according to any one of claims 1 to 5, wherein The method further comprises: Using the motor controller (10) to change the power supply voltage to set the motor (6) to different speeds; Once the motor is running at the new target speed, another excitation signal comprising voltage pulses superimposed on the supply voltage is applied (S102) using a pulse generator (14); measuring (S104) a new response of the mechanical system using the measurement system (18); The measured new response is analyzed (S106, S108) to determine at least one resonant frequency of the mechanical system.

7. The method according to any one of claims 1 to 5, wherein Starting the motor includes increasing the supply voltage to a threshold voltage value, and wherein the motor controller (10) is placed in an open-loop control mode once the motor is running at a predefined target speed.

8. The method according to any one of claims 1 to 5, wherein The method includes determining (S110) a type of mechanical coupling between the load (8) and the motor (6) based on the measured response.

9. A method for driving a mechanical system (4), the mechanical system comprising an electric motor (6) coupled to a mechanical load (8), the method comprising: determining (S120) a resonant frequency of the mechanical system using a method according to any one of claims 1 to 8; At least one operating parameter of the motor (6) is modified (S122) to avoid excitation near or at the determined resonant frequency.

10. A device (2) for determining a resonant frequency of a mechanical system (4), the mechanical system comprising an electric motor (6) coupled to a mechanical load (8), the device (2) being configured to: starting the motor (6) by providing a power supply voltage using a motor controller (10); Once the motor is running at a predefined target speed, applying a first excitation signal, the first excitation signal comprising a voltage pulse superimposed on the power supply voltage; measuring a response of the mechanical system by measuring a stator current of the motor using a measurement system (18) coupled to the motor, the measurement system comprising one or more current sensors coupled to input terminals of the stator of the motor; analyzing the measured response to determine at least one resonant frequency of the mechanical system based on the measured stator current; Wherein, determining the resonant frequency of the mechanical system includes: Calculating a corresponding stator current value represented in a synchronous frame of the motor based on the measured stator current; calculating a phase difference between the calculated corresponding stator current values; Oscillations in the calculated phase difference values are identified; and the resonant frequency is determined based on the identified oscillations.

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