A laser lens monitoring system and method based on optical pressure sensing and memory alloy

Abstract

The application relates to the technical field of laser welding equipment, in particular to a laser lens monitoring system based on light pressure sensing and memory alloy, which can detect the power state of a laser through a laser state monitoring module, timely detects whether the power of the laser attenuates, timely grasps the real-time state of the laser, and avoids affecting the welding quality; the lens state of multiple lenses such as collimating mirrors, focusing mirrors and protective lenses is detected through a lens state monitoring module, that is, the lens state monitoring module is arranged at the position of each lens, timely monitoring and alarming are realized, monitoring is more comprehensive, the corresponding fault position can be quickly located, and the damage of the overall laser caused by the damage of one lens without timely discovery is avoided. Meanwhile, a monitoring method using the laser lens monitoring system is provided, the method is simple to operate, feedback is timely, and safety risks and losses are avoided.

Description

Technical Field

[0001] This invention relates to the field of laser welding equipment technology, specifically to a laser lens monitoring system and method based on photopressure sensing and shape memory alloys. Background Technology

[0002] In the field of laser welding technology, the laser output power of a laser gradually decreases due to dirt or damage during long-term use. If this decrease exceeds a critical value and is not detected in time, insufficient laser output power will result, leading to defects such as incomplete fusion and incomplete penetration, thus affecting the accuracy of the results or causing economic losses in production. Typically, a laser gun head mainly includes a collimating lens, a focusing lens, and a protective lens. Failure or damage to any of these lenses will cause insufficient laser output power. When damage to a lens in the laser gun head is not detected in time, the laser or the high temperature generated by the laser may cause the entire high-value laser gun head to be damaged, resulting in even greater losses.

[0003] In existing technologies, current lasers can only detect the overall temperature of the laser head. When a high-temperature alarm is triggered, engineers or technicians cannot quickly locate the fault. Furthermore, due to technological limitations or space constraints in the laser head, existing laser heads can only monitor for contamination of the protective lens, while lacking effective monitoring methods for the alignment lens and focusing lens, thus failing to provide comprehensive monitoring of multiple lenses within the laser. To address these issues, existing technologies require further improvement. Summary of the Invention

[0004] The purpose of this invention is to provide a laser lens monitoring system and method based on photopressure sensing and shape memory alloys, so as to solve the existing technical problems in the background art.

[0005] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: On one hand, this application provides a laser lens monitoring system based on photopressure sensing and shape memory alloy, including a collimating lens, a focusing lens, a protective lens, a laser status monitoring module, a lens status monitoring module, and a data processing module. The collimating lens, focusing lens, and protective lens are arranged sequentially downwards along the laser source. The laser status monitoring module is located between the collimating lens and the focusing lens and is used to detect the laser power to determine the laser status. The lens status monitoring modules are respectively located at the collimating lens, focusing lens, and protective lens and are used to detect the lens status of the collimating lens, focusing lens, and protective lens. The data processing module is electrically connected to both the laser status monitoring module and the lens status monitoring module.

[0006] Based on the above technical solution, the laser status monitoring module includes a fixed frame, a first reflector, a second reflector, a light pressure sensor, and a third reflector. The fixed frame has a hollow channel that allows the laser to pass through. The first and third reflectors are obliquely and symmetrically arranged in the hollow channel. The second reflector is arranged in the hollow channel on the side opposite to the first reflector. The light pressure sensor is fixedly arranged on the side of the second reflector away from the first reflector. The light pressure sensor is electrically connected to the data processing module.

[0007] Based on the above technical solution, a first status indicator light is provided on the outer side wall of the fixed frame.

[0008] Based on the above technical solution, the lens status monitoring module includes a lens base, a shape memory alloy metal ring, and a pressure sensor. The lens base has an annular through hole, and the inner wall of the annular through hole has a lens groove for mounting the lens. The shape memory alloy metal ring and the pressure sensor are arranged in the lens groove and are arranged sequentially below the lens. The pressure sensor is electrically connected to the data processing module.

[0009] Based on the above technical solution, a second status indicator light and a push-pull handle are provided on the outer side wall of the lens base.

[0010] Based on the above technical solution, the lens status monitoring module also includes a lens clamping plate, which is fixedly connected to the lens base and is used to press the lens onto the lens base.

[0011] Based on the above technical solution, the shape memory alloy metal ring is made of nickel-titanium alloy.

[0012] On the other hand, this application also provides a laser lens monitoring method based on photopressure sensing and shape memory alloy, which employs the aforementioned laser lens monitoring system and includes the following steps: The laser is turned on and introduced into the collimating lens and its lens status monitoring module, the laser status monitoring module, the focusing lens and its lens status monitoring module, and the protective lens and its lens status monitoring module in sequence. The pressure value corresponding to the actual laser power detected by the laser status monitoring module; The data processing module determines the power status of the laser based on the pressure value corresponding to the actual laser power. The lens temperature change is detected by the lens status monitoring module. The data processing module determines the dirt status of the lens based on temperature changes.

[0013] Based on the above technical solution, determining the power state of the laser includes the following steps: Establish the relationship between laser power and pressure, and store the data in the data processing module corresponding to the laser status monitoring module; The data processing module reads the preset theoretical power of the laser and retrieves its corresponding theoretical pressure value F1; After the laser passes through the collimating lens, it enters the laser status monitoring module. The data processing module extracts the actual pressure value F2 through the optical pressure sensor and compares it with the theoretical pressure value F1. If the error is within the preset range, the laser is in normal condition; if the error exceeds the preset range, the laser is in abnormal condition, and the laser status monitoring module will trigger an alarm.

[0014] Based on the above technical solution, the determination of the lens's soiling condition includes the following steps: When the lens temperature changes, the temperature of the shape memory alloy metal ring increases; When the temperature exceeds the critical temperature of the shape memory alloy ring, the shape memory alloy ring undergoes a phase transition and deforms. The pressure sensor detects the pressure generated by the deformation of the shape memory alloy metal ring; When the pressure exceeds the preset value, the data processing module outputs a signal and issues an alarm.

[0015] The beneficial effects of the technical solution provided by this invention are as follows: This invention provides a laser lens monitoring system based on photopressure sensing and shape memory alloys. By setting up a laser status monitoring module, the system can detect the laser power status and promptly detect whether the laser power has attenuated, thus keeping track of the laser's real-time status and preventing any impact on welding quality. By setting up a lens status monitoring module, the system can detect the status of multiple lenses, such as collimating lenses, focusing lenses, and protective lenses. That is, a lens status monitoring module is set up at the location of each lens, which can promptly monitor and alarm, making the monitoring more comprehensive and quickly locating the corresponding fault location, thus preventing damage to the entire laser from going undetected by a single lens.

[0016] This application also provides a laser lens monitoring method based on photopressure sensing and shape memory alloy. Using the above-mentioned laser lens monitoring system, the operation is simpler, the system feedback is timely, and it is convenient for operators to identify changes in the state of the laser or lens in a timely manner. At the same time, it can also quickly locate the fault location and avoid potential safety risks and usage risks. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure and principle of the laser in this invention; Figure 2 This is a schematic diagram of the laser status monitoring module in this invention; Figure 3 This is a cross-sectional schematic diagram of the laser status monitoring module in this invention; Figure 4 This is a schematic diagram of the lens status monitoring module in this invention; Figure 5 This is a cross-sectional schematic diagram of the lens status monitoring module in this invention; Figure 6 This is a schematic diagram of the shape memory alloy metal ring in this invention at low temperature; Figure 7 This is a schematic diagram of the shape memory alloy metal ring in this invention at high temperature; Detailed Implementation The present invention will be further described below with reference to the accompanying drawings and embodiments: In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0018] In the description of this invention, it should be understood that the terms "left", "right", "front", "rear", "top", "bottom", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0019] like Figures 1 to 7 As shown, a laser lens monitoring system based on optical pressure sensing and shape memory alloy includes a collimating lens 1, a focusing lens 2, a protective lens 3, a laser status monitoring module 4, a lens status monitoring module, and a data processing module. The collimating lens 1, focusing lens 2, and protective lens 3 are arranged sequentially downwards from the laser source. The laser status monitoring module 4 is located between the collimating lens 1 and the focusing lens 2 and is used to detect the laser power to determine the laser status. The lens status monitoring modules are respectively located at the collimating lens 1, focusing lens 2, and protective lens 3 and are used to detect the lens status of the collimating lens 1, focusing lens 2, and protective lens 3. The data processing module is electrically connected to the laser status monitoring module 4 and the lens status monitoring module.

[0020] This invention provides a laser lens monitoring system based on photopressure sensing and shape memory alloy. By setting up a laser status monitoring module 4, the system can detect the laser power status and promptly detect whether the laser power has attenuated, thus keeping track of the laser's real-time status and preventing any impact on welding quality. By setting up a lens status monitoring module, the system can detect the status of multiple lenses, such as collimating lens 1, focusing lens 2, and protective lens 3. That is, a lens status monitoring module is set at the location of each lens, which can promptly monitor and alarm, making the monitoring more comprehensive and quickly locating the corresponding fault location, thus preventing damage to the entire laser from going undetected by a single damaged lens.

[0021] The collimating lens converts the randomly diverging laser beam into a parallel laser beam; the focusing lens focuses the parallel laser beam into a converged laser beam; and the protective lens prevents or avoids damage to the focusing lens from smoke, dust, and high-temperature splashes.

[0022] Meanwhile, placing the laser status monitoring module 4 between the collimating lens 1 and the focusing lens 2 can ensure detection accuracy. This is also related to the function of the collimating lens mentioned above. That is, the laser is not parallel light before entering the collimating lens and there is scattered light, which will affect the detection accuracy.

[0023] Based on the above technical solution, the laser status monitoring module 4 includes a fixed frame 41, a first reflector 42, a second reflector 43, a light pressure sensor 44, and a third reflector 45. The fixed frame 41 has a hollow channel that allows the laser to pass through. The first reflector 42 and the third reflector 45 are obliquely and symmetrically arranged in the hollow channel. The second reflector 43 is arranged in the hollow channel on the side opposite to the first reflector 42. The light pressure sensor 44 is fixedly arranged on the side of the second reflector 43 away from the first reflector 42. The light pressure sensor 44 is electrically connected to the data processing module.

[0024] Traditional laser power measurement mainly employs calorimetry and sampling methods. Calorimetry involves larger and heavier equipment, and the higher the power being measured, the larger the equipment becomes. The surface of the absorbing material in the measuring device is easily damaged by the laser. While sampling methods have the advantage of smaller equipment size, they require extremely high accuracy and stability in laser sampling ratios, resulting in larger measurement errors and poorer stability compared to calorimetry.

[0025] The laser condition monitoring module of this application employs an optical pressure sensor 44, successfully solving the aforementioned problems of complex or inaccurate laser power measurement and poor stability. Although photons have no rest mass, they possess momentum. When a laser beam irradiates an object's surface, it generates pressure. Utilizing this pressure effect, laser power can be directly traced back to mass, providing a novel approach for high-power laser metrology. Compared to traditional calorimetric methods, this method offers advantages such as fast response speed and high measurement accuracy. Furthermore, the higher the measured laser power, the better the signal-to-noise ratio of the result. Theoretically, the greater the measured laser power, the smaller the measurement uncertainty. Therefore, the optical pressure method has significant advantages in measuring high-power lasers.

[0026] Based on the above technical solution, a first status indicator light 46 is provided on the outer wall of the fixed frame 41. By providing the first status indicator light, the power status of the laser can be fed back in a timely manner, making it easier for operators to detect changes in the laser's power status and simplifying operation.

[0027] The working principle of the laser status monitoring module 4 is as follows: 1. Establish the relationship between laser power and pressure, and write the data into the laser status monitoring module; 2. During operation, the laser status monitoring module reads the laser power set by the laser software or equipment system and retrieves the corresponding pressure F1. 3. After the laser passes through the collimating lens, it enters the laser status monitoring module, which extracts the pressure value F2 through the 34 optical pressure sensor; 4. The laser status monitoring module compares F2 with F1. If the error is within the set range, the laser is considered to be in normal condition, and the first status indicator light is green. If the error exceeds a certain set range, the laser is considered to be in abnormal condition, the laser status monitoring module system alarms, reminding process or production personnel to pay attention to the laser status, and the first status indicator light is yellow. If the error exceeds a certain larger set range, the laser is considered to be in seriously abnormal condition, the first status indicator light is red, the laser status monitoring module outputs a signal to interrupt the welding process and prevent greater losses.

[0028] Based on the above technical solution, the lens status monitoring module includes a lens base 51, a shape memory alloy metal ring 52, and a pressure sensor 53. The lens base 51 is provided with an annular through hole, and a lens groove 54 for mounting the lens is provided on the inner side wall of the annular through hole. The shape memory alloy metal ring 52 and the pressure sensor 53 are arranged in the lens groove 54 and are arranged sequentially below the lens. The pressure sensor 53 is electrically connected to the data processing module.

[0029] It is understood that the lens mentioned here can be a collimating lens, a focusing lens, or a protective lens, depending on the actual installation location.

[0030] Based on the above technical solution, a second status indicator light 55 and a push-pull handle 56 are provided on the outer side wall of the lens base 51. The second status indicator light 55 provides timely feedback on the lens status, allowing operators to promptly detect changes in the lens status and making operation more convenient. Simultaneously, the push-pull handle 56 on the outer side of the lens base 51 facilitates lens inspection or replacement.

[0031] Based on the above technical solution, the lens status monitoring module further includes a lens clamping plate 57, which is fixedly connected to the lens base 51 and used to press the lens firmly onto the lens base 51. By providing the lens clamping plate, the lens, the shape memory alloy metal ring, and the pressure sensor can be pressed together. The pressure sensor can promptly and effectively detect the pressure signal generated by the change in the shape memory alloy metal ring, resulting in more accurate detection.

[0032] The working principle of the lens status monitoring module is as follows: When the lens is not contaminated, the second status indicator 55 is green; when the lens is contaminated, the laser transmittance decreases, causing the lens temperature to rise; at the junction of the lens and the shape memory alloy metal ring 52, the temperature of the shape memory alloy metal ring 52 rises; when the temperature exceeds the critical temperature of the shape memory alloy, the shape memory alloy metal ring 52 deforms, the pressure sensor 53 outputs pressure, and when the pressure value exceeds the set value, the data processing system outputs a signal, and the status indicator changes to yellow or red; when the second status indicator 55 is yellow, it reminds the process or production personnel to pay attention to the lens status; when the second status indicator 55 is red, the data processing system outputs a signal to interrupt the welding process to prevent greater losses.

[0033] Based on the above technical solution, the shape memory alloy metal ring 52 is made of nickel-titanium alloy.

[0034] The deformation temperature range of shape memory metals mainly depends on changes in their composition and crystal structure. The phase transformation temperature (Af) of shape memory metals can be precisely controlled between -100℃ and +110℃ with an error of no more than 1°. This embodiment uses a nickel-titanium alloy shape memory alloy as an example. Above 40℃, its crystal structure exhibits high symmetry, while below 40℃, the crystal structure transforms into a martensitic variant. When the metal wire is stretched and deformed, the martensitic variant gradually unifies into a single orientation. If the temperature is suddenly raised above 40℃, the martensite undergoes a reverse transformation, reverting to the original high-temperature phase with a single orientation.

[0035] In this embodiment, a metal ring made of nickel-titanium shape memory alloy is used, such as Figure 6 and Figure 7 The ring structure in the material, under normal conditions, the nickel-titanium shape memory alloy has a wavy structure, which is used in this application to process it into the following form. Figure 7 The flat ring structure shown will deform at high temperatures (exceeding its phase transition critical temperature) during use, and will return to its initial state. Figure 6 The wave-like structure shown is illustrated.

[0036] This embodiment also provides a laser lens monitoring method based on photopressure sensing and shape memory alloys. Using the aforementioned laser lens monitoring system, the method includes the following steps: The laser is turned on and introduced into the collimating lens 1 and its lens status monitoring module, the laser status monitoring module 4, the focusing lens and its lens status monitoring module, and the protective lens and its lens status monitoring module in sequence. Receive the pressure value corresponding to the actual laser power detected by laser status monitoring module 4; The data processing module determines the power status of the laser based on the pressure value corresponding to the actual laser power. The lens temperature change is detected by the lens status monitoring module. The data processing module determines the dirt status of the lens based on temperature changes.

[0037] This application also provides a laser lens monitoring method based on photopressure sensing and shape memory alloy. Using the above-mentioned laser lens monitoring system, the operation is simpler, the system feedback is timely, and it is convenient for operators to identify changes in the state of the laser or lens in a timely manner. At the same time, it can also quickly locate the fault location and avoid potential safety risks and usage risks.

[0038] Based on the above technical solution, determining the power state of the laser includes the following steps: Establish the relationship between laser power and pressure, and store the data in the data processing module corresponding to laser status monitoring module 4; The data processing module reads the preset theoretical power of the laser and retrieves its corresponding theoretical pressure value F1; After the laser passes through the collimating lens 1, it enters the laser status monitoring module 4. The data processing module extracts the actual pressure value F2 through the optical pressure sensor 44 and compares it with the theoretical pressure value F1. If the error is within the preset range, the laser is in normal condition; if the error exceeds the preset range, the laser is in abnormal condition, and the laser status monitoring module will trigger an alarm.

[0039] In a preferred embodiment, if the error between the actual pressure value F2 and the theoretical pressure value F1 exceeds a preset range A, the laser power status is considered abnormal, the laser status monitoring module 4 alarms, and reminds process or production personnel to pay attention to the laser status. In a more preferred embodiment, if the error between the actual pressure value F2 and the theoretical pressure value F1 exceeds a preset range B, wherein the preset range B is larger than the preset range A, then the laser power state is determined to be seriously abnormal. In this case, the laser state monitoring module 4 outputs a corresponding signal to interrupt the welding process and prevent greater losses.

[0040] Based on the above technical solution, the determination of the lens's soiling condition includes the following steps: When the lens temperature changes, the temperature of the shape memory alloy metal ring 52 increases; When the temperature exceeds the critical temperature of the shape memory alloy metal ring 52, the shape memory alloy metal ring 52 undergoes a phase transformation and deforms. Pressure sensor 53 detects the pressure generated by the deformation of shape memory alloy metal ring 52; When the pressure exceeds the preset value, the data processing module outputs a signal and issues an alarm.

[0041] Specifically, when the lens is undamaged, i.e., in its normal state, the status indicator light is green. When the lens is damaged, the laser transmittance decreases, causing the lens temperature to rise. At the junction of the lens and the shape memory alloy metal ring 52, the temperature of the shape memory alloy metal ring 52 also rises. When the temperature exceeds the critical temperature of the shape memory alloy, the metal sheet deforms, and the pressure sensor 53 outputs pressure. If the pressure exceeds the preset value, the data processing module outputs a signal, and the second status indicator light 55 turns yellow or red. When the second status indicator light 55 is yellow, it reminds the process or production personnel to pay attention to the lens status. When the second status indicator light 55 is red, the data processing system outputs a signal to interrupt the welding process to prevent greater losses.

[0042] It is understood that the data processing module in this application uses a PLC module, a common control method in this technical field.

[0043] The foregoing has shown and described the basic principles and main features of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments. Therefore, the embodiments should be considered as exemplary and not restrictive. The scope of the present invention is defined by the appended claims rather than the foregoing description. Therefore, it is intended that all variations falling within the meaning and scope of the equivalents of the claims be included within the present invention.

[0044] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A laser lens monitoring system based on photopressure sensing and shape memory alloy, characterized in that, The system includes a collimating lens (1), a focusing lens (2), a protective lens (3), a laser status monitoring module (4), a lens status monitoring module, and a data processing module. The collimating lens (1), the focusing lens (2), and the protective lens (3) are arranged sequentially downwards from the laser source. The laser status monitoring module (4) is located between the collimating lens (1) and the focusing lens (2) and is used to detect the laser power to determine the laser status. The lens status monitoring modules are respectively located at the collimating lens (1), the focusing lens (2), and the protective lens (3) and are used to detect the lens status of the collimating lens (1), the focusing lens (2), and the protective lens (3). The data processing module is electrically connected to the laser status monitoring module (4) and the lens status monitoring module. The laser status monitoring module (4) includes a fixed frame (41), a first reflector (42), a second reflector (43), a light pressure sensor (44), and a third reflector (45). The fixed frame (41) has a hollow channel that allows the laser to pass through. The first reflector (42) and the third reflector (45) are inclined and symmetrically arranged in the hollow channel. The second reflector (43) is arranged in the hollow channel on the side opposite to the first reflector (42). The light pressure sensor (44) is fixedly arranged on the side of the second reflector (43) away from the first reflector (42). The light pressure sensor (44) is electrically connected to the data processing module. A first status indicator light (46) is provided on the outer wall of the fixed frame (41). The lens status monitoring module includes a lens base (51), a shape memory alloy metal ring (52), and a pressure sensor (53). The lens base (51) has an annular through hole, and the inner side wall of the annular through hole has a lens groove (54) for mounting the lens. The shape memory alloy metal ring (52) and the pressure sensor (53) are located in the lens groove (54) and are arranged sequentially below the lens. The pressure sensor (53) is electrically connected to the data processing module. A second status indicator light (55) is provided on the outer side wall of the lens base (51).

2. The laser lens monitoring system based on photopressure sensing and shape memory alloy according to claim 1, characterized in that, A push-pull handle (56) is provided on the outer wall of the lens base (51).

3. The laser lens monitoring system based on photopressure sensing and shape memory alloy according to claim 1, characterized in that, The lens status monitoring module also includes a lens clamp (57), which is fixedly connected to the lens base (51) and is used to press the lens onto the lens base (51).

4. The laser lens monitoring system based on photopressure sensing and shape memory alloy according to claim 1, characterized in that, The shape memory alloy metal ring (52) is made of nickel-titanium alloy.

5. A laser lens monitoring method based on photopressure sensing and shape memory alloy, employing the laser lens monitoring system described in any one of claims 1 to 4, characterized in that, Includes the following steps: Turn on the laser and introduce the laser to pass through the collimating lens (1) and its lens status monitoring module, the laser status monitoring module (4), the focusing lens and its lens status monitoring module, and the protective lens and its lens status monitoring module in sequence. Receive the pressure value corresponding to the actual laser power detected by the laser status monitoring module (4); The data processing module determines the power status of the laser based on the pressure value corresponding to the actual laser power. The lens temperature change is detected by the lens status monitoring module. The data processing module determines the dirt status of the lens based on temperature changes.

6. The laser lens monitoring method based on photopressure sensing and shape memory alloy according to claim 5, characterized in that, Determining the power state of a laser includes the following steps: Establish the relationship between laser power and pressure, and store the data in the data processing module corresponding to the laser status monitoring module (4); The data processing module reads the preset theoretical power of the laser and retrieves its corresponding theoretical pressure value F1; When the laser passes through the collimating lens (1) and enters the laser status monitoring module (4), the data processing module extracts the actual pressure value F2 through the optical pressure sensor (44) and compares it with the theoretical pressure value F1. If the error is within the preset range, the laser is in normal condition; if the error exceeds the preset range, the laser is in abnormal condition, and the laser status monitoring module will trigger an alarm.

7. A laser lens monitoring method based on photopressure sensing and shape memory alloy according to claim 5, characterized in that, Determining the condition of a lens involves the following steps: When the lens temperature changes, the temperature of the shape memory alloy metal ring (52) increases; When the temperature exceeds the critical temperature of the shape memory alloy metal ring (52), the shape memory alloy metal ring (52) undergoes a phase transformation and deforms. The pressure sensor (53) detects the pressure generated by the deformation of the shape memory alloy metal ring (52); When the pressure exceeds the preset value, the data processing module outputs a signal and issues an alarm.

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