WO2018041327A1

WO2018041327A1 - An industrial robot comprising a leakage detecting device configured to detect lubricant leaking from a gear unit

Info

Publication number: WO2018041327A1
Application number: PCT/EP2016/070311
Authority: WO
Inventors: Christian SÖRENSSON; Görgen Johansson; Roger Mellander
Original assignee: Abb Schweiz Ag
Priority date: 2016-08-29
Filing date: 2016-08-29
Publication date: 2018-03-08

Abstract

The present invention relates to an industrial robot comprising a rotatable shaft, a gear unit (8) connected to the rotatable shaft and comprising a fluid lubricant, a space (14) arranged to receive lubricant leaking from the gear unit, and a leakage detecting device (1) configured to detect lubricant leaking into the sealed space. The robot is provided with a recess (16) having an opening (18) facing the space and arranged for receiving lubricant from the space, and the leakage detecting device comprises a sensor (22) configured to detect the presence of lubricant in the recess.

Description

An industrial robot comprising a leakage detecting device configured to detect lubricant leaking from a gear unit

Field of the invention

The present invention relates to an industrial robot comprising a sealed space for receiving lubricant leaking from a gear unit, and a leakage detecting device configured to detect lubricant leaking into the sealed space.

Background of the invention

An industrial robot comprises a plurality of movable joints. Each of the joints is driven by a motor having a rotatable shaft connected to a gear unit provided with a lubricating oil. A problem is that it may occur a leakage of oil from the gear unit into the motor, which may cause damages to the robot. Thus, there is a need to detect leakage of lubricating oil from the gear unites as quickly as possible.

On the market advanced leakage detectors are available at a comparatively high price level.

JP2013111694 discloses an industrial robot including a main seal member forming a sealed first space in which a lubricant is charged, and a leakage detecting device configured to detect the lubricant leaking into a sealed second space between two adjacent seal members from the sealed first space. The leakage detecting device includes a sucking mechanism which sucks the lubricant out of the second space, and the sucking mechanism includes suction piping communicating with the interior of the second space, a negative pressure applying device which applies negative pressure into the second space through the suction piping, and a lubricant reservoir for storing the lubricant sucked out by the sucking mechanism. An operator can be based on the amount of the lubricant reserved in the lubricant reservoir determine whether lubricant is leaking from the first space. The robot further comprises a detection control unit provided with an alarm device. A disadvantage with this leakage detecting device is that it cannot detect small amounts of lubricating oil of droplet size. Another problem is that it is complicated to suck the lubricant out of the second space.

Object and summary of the invention

It is an object of the present invention to at least partly overcome the above problems, and to provide an improved detection of leaking lubricant in the robot.

This object is achieved by an industrial robot as defined in claim 1.

The robot comprises a rotatable shaft, a gear unit connected to the rotatable shaft and comprising a fluid lubricant, a space arranged to receive lubricant leaking from the gear unit, and a leakage detecting device configured to detect lubricant leaking into the space. The robot is provided with a recess having an opening facing the space and arranged for receiving lubricant from the space, and the leakage detecting device comprises a sensor configured to detect the presence of lubricant in the recess. The recess is arranged for receiving lubricant from the sealed space. Due to the gravity, lubricant present in the sealed space will drain into the recess. As long as there is no leakage of lubricant from the gear unit, there is no lubricant in the recess. However, if there is lubricant in the recess, there is a leakage of lubricant from the gear unit. Thus, by detecting the presence of lubricant in the recess, it is possible to determine whether or not there is a leakage of lubricant from the gear unit. By providing a recess for collecting the lubricant in the space, and detecting the presence of lubricant in the recess, there is no need of any sucking mechanism that sucks the lubricant out of the sealed space, as in the prior art.

The lubricant can be lubricating toil or a lubricating grease. I n both cases, the lubricant comprises oil and the sensor is configured to detect the presence of oil in the recess. Lubricating oil, such as mineral oil or polyalkylene glycol based oil, is commonly used in gear units of industrial robots.

According to an embodiment of the invention, the sensor is at least partly disposed in the recess. By arranging the sensor in the recess it is possible to detect small amounts of lubricating oil, and thus to improve the leakage detection. Suitably, the sensor protrudes into the recess to allow contact with lubricate in the recess.

According to an embodiment of the invention, the sensor comprises a piezoelectric crystal disposed in the recess so that it will receive lubricant leaking from the space. The piezoelectric crystal is arranged to allow contact with lubricate entering into the recess. Preferably, the crystal is disposed with a n upper surface facing an opening of the recess so that leaking lubricant will fall on the surface when lubricant enters the recess. The piezoelectric crystal will change its characteristic properties when a small droplet of lubricant has landed on the surface of the crystal. Thus, it is possible to detect small amounts of lubricant, and accordingly an early detection of a leakage is achieved. The piezoelectric crystal sensor is small and ca n easily fit in the recess. Further, a piezoelectric crystal sensor is cheap. Thus, it is possible to provide a reliable and cost-efficient detector, which is able to detect leakage of small amounts of lubricant from a gear unit of an industrial robot.

According to an embodiment of the invention, the piezoelectric crystal is a quartz crystal a nd the sensor is a quartz crystal resonator. A quartz crystal resonator has been proven to be particularly suitable for detecting lubricating oil, such as mineral oil and polyalkylene glycol based oil, which is typically used in gear units in industrial robots. By using a quartz crystal resonator it is possible to detect very small amounts of lubricating oil, in the order of a few microliters, in a moving and spinning system with temperatures in between 5-80 °C.

According to an embodiment of the invention, the piezoelectric crystal is a quartz crystal microbalance (Q.CM). A Q.CM has been proven to be able to detect lubricating oil with very high measuring accuracy, i.e. very small amounts of lubricating oil ca n be detected using a Q.CM . Further, it has been proven that Q.CM provides very fast detection of a small amount of lubricating oil. Thus, this embodiment makes it possible to quickly and reliably detect leakage of lubricating oil in a robot.

According to an embodiment of the invention, sensor comprises an oscillation circuit electrically connected to the piezoelectric crystal and configured to produce an oscillating output signal, and the leakage detecting device is configured to detect changes in the oscillating output signal, and to detect the presence of lubricant in the recess based on the detected changes.

According to an embodiment of the invention, the leakage detecting device comprises a measuring device configured to measure the oscillating output signal, and the leakage detecting device is configured to detect changes in the measured output signal.

According to an embodiment of the invention, the measuring device is configured to measure the voltage of the oscillating output signal and to detect whether the measured voltage is above or below a threshold value, a nd the leakage detecting device is configured to indicate leakage of lubricate from the gear unit if the measured voltage is below the threshold value. For example, a simple voltage meter can be used for measuring the voltage of the oscillating output signal. This em bodiment provides a simple and cost-effective detection of changes in the oscillating output signal.

According to an embodiment of the invention, the robot comprises a housing at least partly surrounding the rotatable shaft, and the space is formed between the rotatable shaft and the housing, and the recess is formed in the housing.

According to an embodiment of the invention, the robot comprises a motor including the rotatable shaft, and the housing is a motor housing surrounding the motor, and the space is formed inside the motor housing. It is important to detect when lubricant leaks into the motor. This embodiment provides detection of lubricant entering the motor housing.

According to an embodiment of the invention, the space is a sealed space formed between two spaced apart sealing elements.

According to an embodiment of the invention, the recess is a through-hole. The through-hole is, for example, a drain hole for draining the space from lubricant. Many robots are today provided with such drain hole for draining a sealed space from lubricant. It is convenient to use the existing drain holes for providing leakage detection. For example, the sensor is fitted in the through-hole so that a bottom part of the through-hole is sea led. Thus, no lubricant is leaking from the space to the surrounding of the robot.

According to an embodiment of the invention, the lea kage detecting device is configured to provide an alarm upon detecting leakage of lubricate from the gear unit. Brief description of the drawings

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures.

Fig. 1 shows a part of an industrial robot including a leakage detecting device according to an embodiment of the invention.

Fig. 2a shows a cross-section A-A through the robot part shown in figure 1.

Fig. 2b shows a cross-section B-B through the robot part shown in figure 2a.

Fig. 3 shows an example of a leakage detecting device including a quartz crystal resonator. Fig. 4 shows an example of how the output from the quartz crystal resonator will change upon detecting lubricating oil.

Detailed description of preferred embodiments of the invention

Figure 1 shows a part of an industrial robot including a leakage detecting device according to an embodiment of the invention. Figure 2a shows a cross-section A-A through the robot part shown in figure 1 and figure 2b shows a cross-section B-B through the robot part shown in figure 2a.

An industrial robot comprises a plurality of movable joints. Each of the joints is driven by a motor 3 including a rotatable shaft 5 for driving the joint. Each of the motors 3 is connected to a gear unit 8. Each of the gear units 8 is connected to one of the joints. The motor 3 has a housing 10 at least partly surrounding the rotatable shaft 5. The rotatable shaft 5 extends from the motor 3 to the gear unit 8. The gear units 8 are provided with a lubricant in the form of lubricating oil, for example mineral oil or any other suitable lubricating oil. The gear units 8 can also be provided with lubricating grease. To avoid that the lubricant enters the motor, the motor is provided with two spaced apart sealing elements 12a and 12b arranged between the rotatable shaft 5 and the housing 10 so that a sealed space 14 is formed between the rotatable shaft 5 and the housing 10. It happens that lubricant leaks from the gear unit 8 and flows via the rotatable shaft 5 and passes through the sealing elements 12b into the motor 3. The sealed space 14 receives the lubricant leaking from the gear unit. The housing 10 is provided with a recess 16 arranged to receive lubricant from the sealed space 14. The recess 16 is arranged for receiving lubricant from the sealed space. The recess 16 is disposed in a lower part of the housing 10 in order to allow lubricant present in the sealed space 14 to drain into the recess 16 due to the gravity. The recess 16 can, for example, be a drilled hole. In this embodiment, the recess 16 is a through-hole extending from the sealed space 14 to the surrounding of the housing 10. An upper end of the through-hole 16 has an opening 18, as shown in figure 2a, facing the sealed space 14 and a lower end of the through- hole 16 has an opening 20 facing the surroundings of the motor 3 and thereby allowing the lubricant to leave the sealed space 14. The through-hole can be a drain hole for draining the sealed space from oil. In an alternative embodiment of the invention, the recess can be a cavity in the housing adapted for collecting lubricant entering into the sealed space. The robot further comprises a leakage detecting device 1 configured to detect lubricant leaking into the sealed space 14. The leakage detecting device comprises a sensor 22 configured to detect the presence of lubricant in the recess 16. The sensor 22 is at least partly disposed in the recess 16 to be able to come into contact with the lubricant entering the recess. The sensor can be of different types.

Figure 3 shows an example of a leakage detecting device 1 including a quartz crystal resonator. The leakage detecting device 1 is configured to detect changes in the output from the quartz crystal resonator, and to detect the presence of lubricant in the recess based on the detected changes. In this embodiment, the sensor 22 is a quartz crystal resonator including a piezoelectric crystal 24 and an oscillation circuit 26 electrically connected to the piezoelectric crystal. Preferably, the piezoelectric crystal 24 is a quartz crystal and more specifically a quartz crystal microbalance (Q.CM). The oscillation circuit 26 is configured to produce an oscillating output signal in dependence on the resonance frequency of the crystal resonator. Such oscillation circuits are well-known in the prior art, and are not described here. The piezoelectric crystal 24 can be located in the through-hole so that it forms a bottom of the recess and seals the bottom part of the recess. Alternatively, the piezoelectric crystal 24 can be located in the through-hole so that a space is formed between the housing and the piezoelectric crystal, and by that providing the ability to let the lubricant pass through the piezoelectric crystal. Thus, the through-hole will still function as a drain hole.

The leakage detecting device 1 further comprises a measuring device 28 configured to measure the oscillating output signal from the oscillation circuit 26, and the leakage detecting device is configured to detect changes in the oscillating output signal based on the measurements from the measuring device 28. The leakage detecting device 1 may also comprise a monitoring unit 30 arranged to provide an alarm upon detecting the presence of lubricant in the recess 16. The monitoring unit 30 can, for example, be a part of the control system of the robot, or an external computer unit.

The physical basis of operation of the Q.CM originates in the converse piezoelectric effect, in which the application of an electric field across a piezoelectric material induces a deformation of the material. Q.CM comprises a thin disk sliced from a single crystal of AT-cut alpha-quartz. The disk is fitted between metal electrodes, often gold, that are vapour deposited on either side of the crystal. The crystal starts to oscillate when an alternating electric field is applied over its electrodes. The resonance frequency is related to the thickness of the crystal. Typica l frequencies are within the range of 5 to 10 MHz. The fundamental frequency of the Q.CM decreases with increasing mass, increasing viscosity of liquid, and with increasing roughness of its electrodes. There is a change of resonance frequency and/or impedance of a crystal oscillation circuit when the lubricant gets in contact with the quartz crystal.

The sensor 22 comprises the piezoelectric crystal 24 protruding into the recess 16 so that it will receive lubricant from the sealed space 14 when there is lubricant present in the sealed space. When a small drop of lubricant is received on the piezoelectric crystal 24, a thin film of lubricant will cover the surface of the piezoelectric crystal and accordingly the properties of the piezoelectric crystal will change. When lubricant has been received on a surface of the piezoelectric crystal 24, the characteristics, such as the resonance frequency and the impedance, of the quartz crystal resonator will change and the monitoring unit 30 will detect a change in the output signal from the quartz crystal resonator. The piezoelectric crystal 24 is provided with a casing. The casing can be fitted in the recess 16 by a thread, be glued, bolted or mechanically attached to the wall in any other appropriate way.

Figure 4 shows an example of how the output from the quartz crystal resonator including a Q.CM will change when lubricating oil is disposed on the Q.CM. A Q.CM with a frequency of 10.7 MHz was used and connected to an oscillating circuit. The output was measured in DC-voltage. The sensor was installed on the robot and the voltage output from the oscillating circuit was measured during several hours. Measurements were performed each 2 seconds. The output was first measured without contact with the oil, and then 50 microliters of oil were dropped on the crystal and the output signal from the oscillating circuit was measured. As seen from the figure, the output signal from the quartz crystal resonator is oscillating with a stable frequency as long as the Q.CM is allowed to operate freely without any contact with the lubricating oil. As seen from the figure, the frequency as well as the output voltage from the quartz crystal resonator decreases to a value close to zero when the drop of oil was dropped on the Q.CM. When the drop of oil is added on the Q.CM, the output signal becomes too weak to be able to be measured, and accordingly a quick and reliable detection could be performed.

In this embodiment, the measuring device 28 is configured to measure the voltage of the oscillating output signal and to detect whether the measured voltage is above or below a threshold value, and the leakage detecting device 1 is configured to indicate leakage of lubricate from the gear unit 8 if the measured voltage is below the threshold value. The threshold value is, for example, selected between 2 - 3 V. In another embodiment of the invention, the measuring device 28 is configured to measure the frequency of the oscillating output signal and to detect when the measured frequency is below a threshold value of the frequency.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. For example, the leakage detecting device 1 may include a photo transmitter that emits visible or invisible light. The intensity of the light is measured by a receiver. In case of lubricate between the transmitter and receiver a shift in intensity will be used for detection.

In the embodiment described above, the recess and the sensor are arranged inside the motor and leakage of lubricant from the gear unit to the motor is detected. However, in another embodiment, the space can be formed outside the motor. In an alternative embodiment of the invention, the recess and the sensor can arranged in an area where a movable arm part of the robot is connected to the gear unit, in order to detect leakage of lubricant from the gear units to the joins of the robot. In this embodiment a rotatable shaft from the gear unit is connected the movable arm part and a leakage of lubricant may occur to the outside the robot. For example, it is important to immediately detect leakage of lubricant from robots working in the food industry.

Claims

1. An industrial robot comprising:

- a rotatable shaft (5),

- a gear unit (8) connected to the rotatable shaft and comprising a fluid lubricant,

- a space (14) arranged to receive lubricant leaking from the gear unit, and

- a leakage detecting device (1) configured to detect lubricant leaking into the space, characterized in that the robot is provided with a recess (16) having an opening (18) facing the space and arranged for receiving lubricant from the space, and the leakage detecting device comprises a sensor (22) configured to detect the presence of lubricant in the recess.

2. The industrial robot according to claim 1, wherein said sensor (22) is at least partly disposed in the recess (16).

3. The industrial robot according to claim 1 or 2, wherein said sensor (22) comprises a piezoelectric crystal (24) disposed in the recess (16) so that it will receive lubricant from the space (14).

4. The industrial robot according to claim 3, wherein said piezoelectric crystal (24) is a quartz crystal and the sensor (22) is a quartz crystal resonator.

5. The industrial robot according to claim 3 or 4, wherein said piezoelectric crystal (24) is a quartz crystal microbalance.

6. The industrial robot according to any of the claims 3 - 5, wherein said sensor (22) comprises an oscillation circuit (26) electrically connected to the piezoelectric crystal (24) and configured to produce an oscillating output signal, and the leakage detecting device (1) is configured to detect changes in the oscillating output signal, and to detect the presence of lubricant in the recess (16) based on the detected changes.

7. The industrial robot according to claim 6, wherein the leakage detecting device (1) comprises a measuring device (28) configured to measure the oscillating output signal, and the leakage detecting device is configured to detect changes in the measured output signal.

8. The industrial robot according to claim 7, wherein said measuring device (28) is configured to measure the voltage of the oscillating output signal and to detect whether the measured voltage is above or below a threshold value, and the leakage detecting device (1) is configured to indicate leakage of lubricate from the gear unit (8) if the measured voltage is below the threshold value.

9. The industrial robot according to any of the previous claims, wherein said lubricant comprises oil.

10. The industrial robot according to any of the previous claims, wherein said lubricant is any of lubricating oil or lubricating grease.

11. The industrial robot according to any of the previous claims, wherein the robot comprises a housing (10) at least partly surrounding the rotatable shaft, and said space (14) is formed between the rotatable shaft (5) and the housing (10), and said recess is formed in the housing.

12. The industrial robot according to claim 11, wherein the robot comprises a motor (3) including the rotatable shaft (5), and said housing (10) is a motor housing surrounding the motor, and said space (14) is formed inside the motor housing.

13. The industrial robot according to any of the previous claims, wherein said recess (16) is a through-hole.

14. The industrial robot according to any of the previous claims, wherein said space (14) is a sealed space formed between two spaced apart sealing elements (12a-b).

15. The industrial robot according to any of the previous claims, wherein the leakage detecting device (1) is configured to provide an alarm upon detecting leakage of lubricate from the gear unit.