Patent Application
20200173441
The present invention relates to a method for surveillance of an operating state of a pumping device connected to a process chamber in order to determine a failing operating state of the pumping device. The invention also concerns a pumping device and an installation employing said method.
Vacuum pumps include two rotors driven by a motor to turn inside the pump body (stator). During rotation, gas aspirated from the process chamber is trapped in the free space between the rotor and the stator to be discharged to the outlet.
Vacuum pumps are employed in particular in processes for fabricating semiconductors, flat screens or photovoltaic substrates, necessitating a pressure lower than atmospheric pressure. However, the gases employed in these processes may be converted into solid by-products that may be deposited in the form of a layer on the mobile and static parts of the pump and lead to clogging and then to jamming of the pump by seizing of the mechanism caused by excessive friction between the rotor and the stator.
Jamming of the pump can lead to irreversible damage to the product being fabricated in the associated process chamber (for example batches of semiconductor wafers). Considerable costs are induced by these unprogrammed interruptions of production.
At present the maintenance of vacuum pumps is based on both corrective and preventive action, the best situation being to be able to predict preventive maintenance before the vacuum pump fails and stops.
To this end, preventive maintenance operations are carried out at a period defined as a function of the application for which the vacuum pump is used. However, the period is not matched to the real conditions of use of the pump, which may vary as a function of the production load and can impact directly on the rate of wear or clogging of the pump, causing unnecessary or excessively late operations.
Methods of predicting vacuum pump failure have been developed to attempt to predict the onset of jamming of the pump and anticipate its replacement.
The document EP0828332 for example proposes surveillance of the performance of a vacuum pump in use to predict its future operating characteristics by measuring the rotation torque or the current of the motor. This method is complicated to implement, however. In fact, it is not possible to measure the rotation torque of the motor directly because torque sensors are much too bulky. Moreover, the measured motor current varies as a function of a multitude of situations independent of possible failure of the vacuum pump. This results in the necessity to cross reference this information with other measurements carried out on the vacuum line or on the vacuum pump in order to be able to interpret it better and not to cause untimely preventive maintenance.
Also known is a failure prediction method enabling determination of the time in which failure of a dry vacuum pump occurs. The duration of use of the vacuum pump before a failure occurs is estimated by statistical processing of the characteristic data of the pump (current, temperature, vibrations, etc.) combined with the characteristics of the fabrication process (gas flow, pressure, substrate temperature, etc.). This method is not self-contained, however. It is not able to predict the service life of the pump without taking into account operating conditions of the process. The analysis system depends on information provided by the production equipment necessitating the installation of a communication line between the equipment and the vacuum pump. Moreover, modification of the process conditions then necessitates modification of the model used by the analysis system, which is not a simple process during use of vacuum pumps.
Also known from the document EP 1 754 888 is a vacuum line failure prediction method. In that method, the evolution with time of a first functional parameter relating to the motor of the pump and a second functional parameter relating to the system for evacuation of gases from the pump are measured. The measured functional parameters are then correlated by statistical processing in order to predict the time of use of the vacuum pump before clogging occurs. The vacuum line is therefore able to carry out a self-diagnosis without correlation with external signals. This method is particularly well adapted to tracking the progress of the phenomenon of pollution by solid products in the vacuum line leading to clogging of the pump.
Although some of the prediction methods that have been developed enable these problems to be solved, there is a search for failure prediction methods that are simpler to implement, of relatively low cost, in particular by avoiding multiplication of the input parameters to be processed or the implementation of new sensors.
The problem is therefore to determine a failing operating state of the vacuum pump in order to protect the process chamber before the pump fails, with no indication of the treatment process taking place in the process chamber and with no knowledge of the conditions of use of the vacuum pump, by means of a method that is reliable and simple to implement.
To this end, the invention consists in a method for surveillance of an operating state of a pumping device connected to a process chamber in order to determine a failing operating state of the pumping device, the pumping device comprising at least one rough-vacuum pump comprising:
characterized in that a failing operating state is determined when in steady-state operation the difference between the first input parameter and the output parameter is equal to or exceeds 10% of the first input parameter for a predetermined time greater than 3 seconds.
In the case of progressive clogging of the rough-vacuum pump that the aim is to diagnose, the rough-vacuum pump load fluctuates. In fact, the frequency decreases and increases alternately without going below alert or alarm thresholds of the pumping device. The variable speed drive can then make good any frequency reductions caused by current surges for times of the order of a few seconds or a few minutes. The surveillance method enables these situations to be detected in order to predict sudden stopping of the pumping device within a few seconds, or even a few minutes, which leaves time to isolate the pumping device from the process chamber situated on the upstream side and to shut off the arrival of the gases in the process chamber.
According to one or more features of said surveillance method, separately or in combination:
The invention also consists in a pumping device including at least one rough-vacuum pump including:
characterized in that the pumping device includes a surveillance unit configured for surveillance of an operating state of a pumping device in accordance with a surveillance method as described above.
The pumping device may include a Roots Blower mounted in series with and on the upstream side of the rough-vacuum pump.
The invention also consists in an installation including a process chamber connected by a pipe to the intake of a pumping device as described above.
Other advantages and features will become apparent on reading the following description of one particular but nonlimiting embodiment of the invention and from the appended drawings, in which:
In these figures, identical elements bear the same reference numbers.
The following embodiments are examples. Although the description refers to one or more embodiments, this does not necessarily mean that each reference concerns the same embodiment or that the features apply only to only one embodiment. Single features of different embodiments may equally be combined or interchanged to provide other embodiments.
A rough-vacuum pump is defined as a positive displacement vacuum pump which with the aid of two rotors aspirates, transfers and then discharges the gas to be pumped at atmospheric pressure. The rotors are carried by two shafts driven in rotation by a motor of the rough-vacuum pump.
A Roots Blower is defined as a positive displacement vacuum pump which with the aid of Roots type rotors aspirates, transfers and then discharges the gas to be pumped. This vacuum pump is mounted on the upstream side of and in series with a rough-vacuum pump. The rotors are carried by two shafts driven in rotation by a motor of the Roots Blower.
By “upstream” is meant an element that is placed before another relative to the direction of flow of the gas. Acontrario, by “downstream” is meant an element placed after another relative to the direction or flow of the gas to be pumped, the upstream element being at a lower pressure than the downstream element.
The installation 1 represented in
The pumping device 5 includes at least one rough-vacuum pump 7 including a pump body inside which two rotors 10 can be driven in rotation by a drive motor M1 of the rough-vacuum pump 7 (
The rough-vacuum pump 7 is a multistage pump, that is to say includes a plurality of, at least three, for example five, pumping stages T1, T2, T3, T4, T5, mounted in series between an intake and a discharge of the rough-vacuum pump 7 and in which a gas to be pumped can flow.
Each pumping stage T1-T5 comprises a respective inlet and a respective outlet. The successive pumping stages T1-T5 are connected in series one after the other by respective interstage pipes connecting the outlet of the preceding pumping stage to the inlet of the following pumping stage. The inlet of the first pumping stage T1 communicates with the intake of the vacuum pump 7 and the outlet of the last pumping stage T5 communicates with the discharge 6 of the vacuum pump 2.
As can be seen better in
The rotors 10 feature lobes with identical profiles. The rotors represented are of “Roots” type (“8” or “bean” shape section). Of course, the invention applies equally to other dry type multistage rough-vacuum pump types, such as the “Claw” type or spiral type or screw type or in accordance with another principle similar to a positive displacement vacuum pump.
The rotors 10 are angularly offset and driven to turn synchronously in opposite directions in the central housing 9 of each stage T1-T5. During rotation, the gas aspirated from the inlet is trapped in the volume generated by the rotors 10 and the stator 8 and is then driven by the rotors 10 to the next stage (the direction of flow of the gases is illustrated by the arrows G in
The rough-vacuum pump 7 is termed “dry” because in operation the rotors 10 turn inside the stator 8 with no mechanical contact with the stator 8, which allows the total absence of oil in the pumping stages T1-T5.
The discharge pressure of the rough-vacuum pump 7 is atmospheric pressure.
The pumping device 5 may include a Roots Blower 11 mounted in series with and on the upstream side of the rough-vacuum pump 7, as can be seen in
The Roots Blower 11 is, like the rough-vacuum pump 7, a positive displacement vacuum pump which, with the aid of Roots type rotors, aspirates, transfers and then discharges the gas to be pumped.
The Roots Blower 11 includes at least one pumping stage B1 in which two rotors are driven in rotation by a motor M2 of the Roots Blower 11. The rotation frequency of the motor M2 is for example set to between 30 Hz and 100 Hz, for example 55 Hz.
The Roots Blower 11 differs from the rough-vacuum pump 7 mainly in greater dimensions, greater clearance tolerances and the fact that the Roots Blower 11 does not discharge at atmospheric pressure but has to be used mounted in series on the upstream side of a rough-vacuum pump 7.
The rough-vacuum pump 7 further includes a variable speed drive 12 configured to control the rotation speed of the motor M1 of the rough-vacuum pump 7. The motor M1 is for example an asynchronous motor, for example having a motor power of the order of 5 kW.
The variable speed drive 12 is configured on the one hand to receive a first input parameter corresponding to a set point frequency C and a second input parameter corresponding to a motor current i and on the other hand to deliver to the motor M1 an output parameter corresponding to a control frequency F (
The set point frequency C may be a parameter set by the user, for example by modification of an analogue input signal of the variable speed drive 12 or by means of a manual adjuster knob of the variable speed drive 12. The set point frequency C is for example between 30 Hz and 60 Hz inclusive, for example 60 Hz.
The motor current i is the current consumed by the motor M1. The motor current i increases if the load on the motor M1 increases and decreases if the load on the motor M1 decreases.
According to one embodiment, the variable speed drive 12 controls the control frequency F in an open loop, for example by pulse width modulation (PWM).
The inverter of the variable speed drive 12 controls the motor M1 using a stream of pulses that determines both the voltage and the frequency.
Open loop control does not use sensors to measure the rotation speed or the angular position of the shaft. This configuration without sensors is economical and relatively simple to implement.
On starting the rough-vacuum pump 7 (starting phase D,
Then, once the set point frequency C has been reached, the variable speed drive 12 controls the control frequency F so that it is constant and equal to the set point frequency C (steady state operation H).
An acceleration current threshold S1 may be used to protect the variable speed drive 12 against current surges when the control frequency F increases. A current surge may occur for example if the rise time is too short in the starting phase D.
If the motor current i exceeds the acceleration current threshold S1 the variable speed drive 12 blocks the increase in the control frequency F until the motor current i decreases and passes again below the acceleration current threshold S1 (see for example the starting overload phases E1 in
The acceleration current threshold S1 may be adjustable.
An operating current threshold S2 is provided to protect the variable speed drive 12 against current surges in steady state operation H where the variable speed drive 12 sets the control frequency F to the set point frequency C.
If the motor current i exceeds the operating current threshold S2 the variable speed drive 12 reduces the control frequency F until the motor current i decreases and passes again below the operating current threshold S2 (see for example the in-operation overload phases E2 in
If the value of the motor current i is below the operating current threshold S2 less a few percent the variable speed drive 12 again increases the control frequency F until the set point frequency C is reached.
The decrease and the increase of the control frequency F under the control of the variable speed drive 12 in steady state operation H may be linear. The operating current threshold 52 may be adjustable. It is for example less than the acceleration current threshold S1.
If the difference between the first input parameter corresponding to the set point frequency C and the output parameter corresponding to the control frequency F is equal to or exceeds 90% of the first input parameter over a long time period greater than one minute, an alert or an alarm is triggered. This may indicate for example binding of the rough-vacuum pump 7 in the starting phase D, the latter not succeeding in starting for example.
The pumping device 5 further includes a surveillance unit 13 configured for surveillance of an operating state of the pumping device 5 in accordance with a surveillance method described hereinafter.
The surveillance unit 13 includes one or more microcontrollers or computers with memories and programs adapted for surveillance of an operating state of the pumping device 5. The surveillance unit 13 may be that of the variable speed drive 12 or a remote unit connected to the variable speed drive 12.
The surveillance unit 13 monitors the control frequency F sent to the motor M1 by the variable speed drive 12 at least in steady state operation H, that is to say when the variable speed drive 12 is not in the starting phase D or in a stopped phase. In steady state operation, the set point frequency C is constant at least during a time period greater than or equal to a predetermined time T, for example three seconds.
An operating state necessitating maintenance intervention on the rough-vacuum pump 7 in steady state operation H is determined if the difference between the first input parameter corresponding to the set point frequency C and the output parameter corresponding to the control frequency F is equal to or exceeds 10% of the first input parameter for a predetermined time T greater than 3 seconds, for example greater than 10 seconds, the first input parameter being greater than the output parameter. In other words, the operating state necessitates intervention if C−F≥0.1*C, that is to say if F≤0.9*C (
For example, imminent failure of the rough-vacuum pump 7 is determined if the control frequency F sent by the variable speed drive 12 to the motor M1 is less than or equal to 54 Hz for the predetermined time T.
According to a more restrictive criterion, an operating state necessitating maintenance intervention on the rough-vacuum pump 7 is determined if in steady state operation H the difference between the first input parameter and the output parameter is equal to or exceeds 4% of the first input parameter for the predetermined time T. In other words, the operating state necessitates intervention if C−F≥0.04*C, that is to say if F≤0.96*C, i.e. earlier than for F≤0.9*C.
For example, imminent failure of the rough-vacuum pump 7 is determined if the control frequency F sent by the variable speed drive 12 to the motor M1 is less than 57.6 Hz for the predetermined time T.
The predetermined time T may be measured by a timer relay or directly by the surveillance unit 13. The predetermined time T may be less than 30 seconds.
Accordingly, and as can be seen in the example from
This detection of imminent failure of the pumping device 5 enables prediction of sudden stopping of the pumping device 5 within a time of the order of a few seconds, between 1 second and 2 minutes and here of the order of 10 to 20 seconds, which leaves time to isolate the pumping device 5 from the upstream process chamber 2 and possibly a turbomolecular vacuum pump and to shut off the feeding of the gases.
In the case of progressive clogging of the rough-vacuum pump 7 that the aim is to diagnose, the load of the rough-vacuum pump 7 fluctuates. In fact, the control frequency F decreases and increases alternately without falling below the alert or alarm threshold. The variable speed drive 12 can then compensate the frequency reductions caused by the current surges for a few seconds or a few minutes. The surveillance method therefore enables detection of this situation in order to enable the user to be able to take measures and to protect the process chamber 2 before the clogging becomes excessive.
The surveillance unit 13 may for example switch a logic output if imminent failure is determined or switch a relay connected in series with the alert and alarm relays of the rough-vacuum pump 7. These relays can be used to shut isolating valves arranged between the process chamber 2 and the pumping device 5 and/or to shut off the arrival of the gases in the process chamber 2.
The surveillance method therefore enables simple and robust identification of abnormal behaviour of the pumping device 5 on the one hand with no indication of the treatment process taking place in the process chamber 2 and on the other hand without being impacted by disturbances to or modifications of the treatment process.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1754979 | Jun 2017 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2018/062606 | 5/15/2018 | WO | 00 |
