State monitoring device, rotation positioning device, and state monitoring method

Abstract

To provide a state monitoring device, a rotation positioning device, and a state monitoring method capable of diagnosing a degree and a position of damage to a gear that meshes in the rotation positioning device.SOLUTION: A state monitoring device comprises a drive actuator for generating drive force, a first gear connected to the output shaft of the drive actuator, a second gear that meshes with the first gear, and a rotary stage that is connected to the output shaft of the second gear and is rotated and driven, and monitors a state of a rotation positioning device. The state monitoring device also comprises: transmission error calculation means for calculating a transmission error from an angle position of an input shaft of the drive actuator and an angle position of the output shaft of the second gear; and damage evaluation means for evaluating damage to a gear on the basis of information obtained from the transmission error calculation means.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a condition monitoring device, a rotational positioning device, and a condition monitoring method. [Background technology]

[0002] There are many types of so-called rotational positioning devices that rotate relatively large stages, such as rotary table drives for machine tools, cab rotation devices for hydraulic excavators and other construction machinery, and nacelle rotation devices for wind power generation systems. These devices have a large gear that is integral with the stage, a small gear that meshes with the large gear, and an actuator that rotates the small gear. Many of these devices are automatically operated without an operator. In order to accurately determine the rotational position of the stage, such devices sometimes use a method in which an angle sensor is attached to the output shaft connecting the large gear and the stage, and the rotation of the actuator is controlled based on the angular position information of the stage output from the sensor.

[0003] In such equipment, even if the tooth surface of either the large or small gear wears out due to continued operation, or if dents or chipping occur due to excessive load, as long as the gear function is not lost, operation can continue based on the angle sensor signal of the output shaft.However, since the extent of damage is not clear, it can be difficult to evaluate how long operation can continue.

[0004] In response to these issues, a movement accuracy monitoring system has been proposed that includes a drive shaft side sensor that measures the position of the drive shaft, a driven part side sensor that measures the position of the driven part, a difference detection unit that forms differential data between the amount of movement of the drive shaft and the amount of movement of the driven part taking into account the gear ratio, and a monitoring control unit that forms information related to the movement accuracy of the driven part based on changes in the differential data.This system is said to be able to monitor and diagnose any decline in movement accuracy during actual operation. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2018-205895 Summary of the Invention [Problem to be solved by the invention]

[0006] On the other hand, in such a rotation positioning device, there are frequently used portions of the tooth pairs of the large and small gears that correspond to the angular positions of the stage, such as the standby position after work is completed or the work position where the work object is placed, and the teeth in such portions are prone to selective damage due to wear, chipping, etc. From the viewpoint of equipment maintenance, it is necessary to detect the location and extent of the damage and take measures to address it, but no measures to meet this requirement can be found in the prior art documents.

[0007] In a rotation stage with a fixed operating pattern, a particular angular position is frequently used, and even if teeth at that position are selectively damaged, it is desirable to be able to detect the position and the extent of the damage.

[0008] An object of the present invention is to provide a condition monitoring device, a rotational positioning device, and a condition monitoring method that are capable of diagnosing the extent and location of damage to gears that mesh with a rotational positioning device. [Means for solving the problem]

[0009] The condition monitoring device of the present invention monitors the condition of a rotational positioning device that includes a drive actuator that generates a drive force, a first gear connected to the output shaft of the drive actuator, a second gear that meshes with the first gear, and a rotation stage that is connected to the output shaft of the second gear and is driven to rotate, and is characterized by comprising: a transmission error calculation means that calculates a transmission error from the angular position of the input shaft of the drive actuator and the angular position of the output shaft of the second gear; and a damage evaluation means that evaluates damage to the gears based on information obtained by the transmission error calculation means.

[0010] Alternatively, the present invention provides a rotational positioning device comprising: a drive actuator that generates a drive force; a first gear connected to an output shaft of the drive actuator; a second gear that meshes with the first gear; a rotation stage that is connected to the output shaft of the second gear and is driven to rotate; an input shaft angle sensor that measures the angular position of the input shaft of the drive actuator; an output shaft angle sensor that measures the angular position of the output shaft of the second gear; transmission error calculation means that calculates a transmission error from the angular position measured by the input shaft angle sensor and the angular position measured by the output shaft angle sensor; transmission error storage means that stores information obtained by the transmission error calculation means; and damage evaluation means that evaluates damage to the gears based on the information from the transmission error storage means.

[0011] Alternatively, a condition monitoring method of the present invention is a condition monitoring method for monitoring the condition of a rotational positioning device comprising a drive actuator that generates a drive force, a first gear connected to the output shaft of the drive actuator, a second gear that meshes with the first gear, and a rotation stage that is connected to the output shaft of the second gear and is driven to rotate, the method comprising calculating a transmission error from the angular position of the input shaft of the drive actuator and the angular position of the output shaft of the second gear, and evaluating damage to the gears based on the calculated transmission error. [Effects of the Invention]

[0012] According to the present invention, it is possible to provide a condition monitoring device, a rotational positioning device, and a condition monitoring method that are capable of diagnosing the extent and location of damage to gears that mesh with a rotational positioning device. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a configuration diagram of a rotation positioning device and a state monitoring device. [Figure 2] FIG. [Figure 3] FIG. 2 is a diagram illustrating the functions of the condition monitoring device. [Figure 4] An explanatory diagram of an example of application to a large-scale system. DETAILED DESCRIPTION OF THE INVENTION

[0014] Specific embodiments of the present invention will now be described with reference to the accompanying drawings, in which like reference numerals denote like or corresponding parts. [Example]

[0015] The rotational positioning device and condition monitoring device of this embodiment will be described using Figures 1 and 2. Figure 1 shows a configuration diagram of the rotational positioning device and condition monitoring device of this embodiment, and Figure 2 is an explanatory diagram of the transmission error waveform. This embodiment shows a relatively small-scale configuration and can be applied to, for example, the rotational stage of a machine tool or power shovel, or the nacelle base of a wind turbine.

[0016] Rotation positioning device 42 includes rotation stage 30, the rotational position of which needs to be determined, output shaft 26, which serves as the rotation axis of rotation stage 30, and output shaft bearing 28, which is a bearing for output shaft 26. Output shaft 26 is rotatably supported by output shaft bearing 28. Coaxially provided are large gear 25, which is a second gear that rotationally drives output shaft 26, and output shaft brake 29, which suppresses rotation of output shaft 26. Output shaft brake 29 can change its holding force in stages, and functions as a braking force that can be used while the brake is sliding.

[0017] A pinion gear 24, which is a first gear, is installed so as to mesh with the large gear 25, and a drive actuator 41, which is made up of a reducer 23 and a drive motor 22, is connected coaxially to the pinion gear 24. The large gear 25 and the pinion gear 24 are housed in a gearbox 27. In other words, the rotation positioning device includes the drive actuator 41 that generates a driving force, the pinion gear 24, which is a first gear, connected to the output shaft of the drive actuator 41, the large gear 25, which is a second gear that meshes with the first gear, and the rotation stage 30 that is connected to the output shaft of the second gear and is driven to rotate.

[0018] Here, there are frequently used portions of the tooth pairs of the large and small gears that correspond to the angular positions of the rotary stage 30. In such a situation, this embodiment is suitable for application.

[0019] An output shaft angle sensor 11 that measures the angular position of the rotation stage 30 is provided on the side of the rotation stage 30, and an input shaft angle sensor 12 that measures the angular position of the drive motor is provided coaxially with the drive motor. The operation of the drive motor 22 is controlled by an operation control device 21 connected to the drive motor 22 based on the angle information from the output shaft angle sensor 11.

[0020] The output shaft angle sensor 11 and the input shaft angle sensor 12 are connected to a transmission error calculation means 13, which calculates the transmission error, which is the difference between the theoretical angular position of the rotating stage 30, obtained by correcting the angular position of the drive motor 22 measured using the input shaft angle sensor 12 with the reduction ratio of the reducer 23 and the large and small gears, and the actual angular position of the stage based on the signal from the output shaft angle sensor 11.

[0021] This transmission error information is stored in the transmission error storage means 14 as differential transmission error information and a transmission error waveform associated with the rotation angle of the rotation stage 30, and the damage evaluation means 15 connected to this reads out the transmission error waveform and performs damage evaluation.

[0022] The condition monitoring device 43 mainly comprises a transmission error calculation means 13 that calculates the transmission error from the angular position of the input shaft and the angular position of the output shaft of the second gear, and a damage evaluation means 15 that evaluates damage to the gears based on the information obtained by the transmission error calculation means 13. With this configuration, it is possible to diagnose the extent and location of damage to the gears that mesh with the rotation positioning device.

[0023] In addition, an output shaft angle sensor 11 that measures the angular position of the output shaft of the second gear, an input shaft angle sensor 12 that measures the angular position of the input shaft, and a transmission error storage means 14 that stores information on the transmission error are provided, making it possible to easily measure and calculate the transmission error.

[0024] The angular positions of the drive motor 22 and the rotary stage 30 may be measured constantly while the rotary positioning device is in operation, but it is preferable to periodically perform measurements in a preset transmission error measurement mode. In this case, the holding force of the output shaft brake 29 is set to a small value and the drive motor 22 is rotated while the brake is slipping, thereby preventing the gear tooth flanks from separating as occurs when there is no load, and allowing the transmission error to be measured with high accuracy. In other words, by providing the output shaft brake 29 coaxially with the rotary stage 30 and calculating the transmission error while applying a constant load using the output shaft brake 29, the transmission error can be measured with high accuracy.

[0025] When the rotation stage is rotated in the normal direction (for example, clockwise when viewed from the side opposite the large gear) with the large and small gears undamaged, a healthy transmission error waveform 51 during normal rotation, as shown in Figure 2, is obtained. Spur gears are sometimes used as the large and small gears, and in these cases, positions 56 where the transmission error is small and positions 57 where the transmission error is large alternately. This is because, as the meshing progresses, the number of teeth that mesh simultaneously changes, causing a change in the meshing stiffness of the teeth. The transmission error is small at positions where the meshing stiffness is high, and large at positions where the meshing stiffness is low, with the period of this fluctuation coinciding with the meshing pitch 54 of the teeth.

[0026] Similarly, when the rotary stage 30 is rotated in the reverse direction, a normal transmission error waveform 52 during reverse rotation is obtained. Normally, there is a gap between the tooth surface used in the forward direction and the tooth surface used in the reverse direction, which is called backlash 53. A normal transmission error waveform during reverse rotation is roughly symmetrical to a normal transmission error waveform during forward rotation, with respect to the horizontal axis representing the angular position of the stage, but due to the influence of backlash, the positions of the peaks where the transmission error becomes large often do not coincide on the normal and reverse rotation sides.

[0027] Gears can become eccentric if excessive load is applied during operation, bending the gear shaft. For example, if the pinion is eccentric, a transmission error waveform similar to transmission error waveform 58, which indicates eccentric gear, is obtained. The transmission error waveform can be distinguished from the pinion because it increases and decreases with the period of the pinion's rotational pitch 55 in addition to the fluctuating component of the meshing pitch period. The same is true for the gear.

[0028] Damage to an individual tooth will result in, for example, a transmission error waveform 60, which indicates localized deterioration. If a specific tooth on the reverse side is partially damaged, the amplitude of the transmission error will increase sharply locally at position 61 where the tooth is partially damaged. Similarly, if a specific tooth surface is worn, the amplitude of the transmission error will increase locally at position 62 where the tooth is locally worn. If such localized damage is on the pinion tooth, the above characteristics will appear at the rotational pitch of the pinion gear, and if it is on the gear tooth, the characteristics will appear at the rotational pitch of the gear.

[0029] The transmission error waveform measured in this way is stored in the transmission error storage means together with the date and time of measurement, and can be called up at will.

[0030] In the transmission error measurement mode, the transmission error of not only the tooth flank in the forward rotation direction of the stage but also that in the reverse rotation direction is measured. Furthermore, by repeatedly rotating the stage forward and backward at a predetermined angular position, the backlash at that position is measured. In other words, by measuring the backlash by rotating the drive actuator 41 forward and backward at a predetermined meshing position between the first gear and the second gear, it is possible to grasp the deterioration of the gears. Furthermore, by automatically executing the transmission error measurement mode at a preset frequency, the progress of deterioration of the large and small gears can be grasped.

[0031] In rotary positioning devices, certain angular positions of the stage are often used frequently, and damage to these parts is not uncommon. In such cases, by changing the frequently used position and controlling operation so that the damaged part is not used, as well as by formulating and executing a repair plan, it is possible to maintain the indexing performance of the stage and minimize unplanned equipment downtime.

[0032] As described above, by associating the transmission error with the angle information of the output shaft and generating a transmission error waveform, or by associating the transmission error with the angle information of the input shaft and generating a transmission error waveform, the transmission error is associated with the angular position of the stage, allowing it to be evaluated as a transmission error waveform and the location and extent of various damages that occur in the large and small gears to be measured. In addition, by storing this information in the transmission error storage means 14, various pieces of information can be used.

[0033] In addition, a transmission error measurement mode is provided, which automatically performs measurements at a predetermined frequency and stores the transmission error waveform data each time, making it possible to accurately grasp the changes in gear deterioration over time, change control to minimize the impact on regular operations, and formulate repair plans.In other words, by measuring the transmission error at predetermined time intervals and evaluating its changes over time, the progress of damage can be evaluated.

[0034] In this embodiment, the input shaft angle sensor is provided on the drive motor shaft, but it may also be provided on the pinion shaft. Furthermore, while the transmission error is correlated with the angular position of the stage, it may also be correlated with the angular position of the drive motor or pinion. Furthermore, while the drive actuator is comprised of a drive motor and a reducer, the drive actuator may also be comprised of only a drive motor.

[0035] According to this embodiment, the location and extent of damage that has selectively occurred on the teeth of the gear or pinion can be accurately evaluated, and unplanned shutdowns of the equipment can be prevented by taking appropriate measures. [Example]

[0036] A second embodiment of the state monitoring device according to the present invention will be described with reference to FIG. 3, which is a diagram illustrating the functions of the state monitoring device.

[0037] The output shaft angle sensor 11 and the input shaft angle sensor 12 are angle-to-voltage signal converters such as rotary encoders or pulse gears, and as both sensors rotate, they generate square waves or sine waves corresponding to the rotation angle, such as the output shaft angle pulse 71 and the input shaft angle pulse 72.

[0038] In the transmission error calculation means 13, both are converted into a rotation angle using the number of pulses per rotation, and the angular position information of the input shaft is corrected using the reduction ratio of the reducer and large and small gears to obtain theoretical angular position information of the rotation stage. The transmission error is obtained by subtracting the actually measured angular position information of the stage from this value, and this is organized by the angular position of the stage to obtain the transmission error waveform 73, which is stored in the transmission error storage means 14.

[0039] The damage assessment means 15 reads and analyzes the transmission error waveform stored in the transmission error storage means to assess the damage state of the large and small gears. A method that often uses a Fourier transform of the transmission error waveform is to perform a Fourier transform. Because the transmission error waveform corresponds to the angular position of the stage, the result of the Fourier transform becomes the fluctuation amplitude relative to the rotational order of the stage, i.e., the rotational order of the large gear, and analytical results such as the large gear rotational component 74 of the transmission error, the pinion rotational component 75 of the transmission error, and the gear meshing component 76 of the transmission error are obtained.

[0040] The results of this analysis are merely an extraction of waveform characteristics, and are suitable for evaluating the amount of gear eccentricity and the overall degree of tooth wear, but are not suitable for evaluating the selective deterioration of specific teeth.

[0041] When evaluating damage to a specific tooth on a gear set, the transmission error waveform 73 is divided into transmission error evaluation blocks A, B, C, and D in meshing pitch units, and the wave height of the transmission error waveform in each block is evaluated. An upper limit value 77 of the transmission error waveform is set in each block, and if this limit is exceeded, it is considered to be tooth damage.

[0042] As described above, by dividing the meshing pitch of the first gear and the second gear into blocks, evaluating the wave height of the transmission error waveform for each block, and acquiring and analyzing the transmission error waveform, it is possible to determine the average deterioration of the teeth of the large and small gears, as well as the degree and location of local damage.

[0043] In this embodiment, the angular position information of the input shaft is corrected using the reduction ratio of the reducer and large and small gears to obtain the theoretical angular position information of the rotation stage, and the actually measured angular position information of the stage is subtracted from this value to calculate the transmission error. However, the rotational position information of the stage may also be corrected using the reduction ratio to obtain the theoretical angular position information of the input shaft, and the actually measured angular position information of the input shaft may be subtracted from this value to obtain the transmission error.

[0044] As described above, the angle signal of input shaft angle sensor 12, which measures the angular position of the input shaft, is corrected by the reduction ratio to obtain theoretical angle position information, and the angle signal of output shaft angle sensor 11, which measures the angular position of the output shaft, is obtained as actual angle position information; alternatively, the angle signal of output shaft angle sensor 11 is corrected by the reduction ratio to obtain theoretical angle position information, the angle signal of input shaft angle sensor 12 is obtained as actual angle position information, and the difference between the theoretical angle position information and the actual angle position information is obtained as the transmission error, thereby making it possible to determine the extent and location of gear damage. [Example]

[0045] A third embodiment of the state monitoring device of the present invention will be described with reference to FIG. 4, which is an explanatory diagram of an example of application to a large-scale system.

[0046] A slewing brake disc 84 and a slewing bearing 82 are installed on the top of a cylindrical wind turbine tower 83, a large gear 25 is formed on the outer periphery of the stationary side of the slewing bearing, and a nacelle base 81 is installed to rotate freely via the slewing bearing. On the nacelle base, which corresponds to the rotation stage, are installed a slewing brake caliper 85 that clamps the slewing brake disc, a drive actuator 41 consisting of a drive motor 22 and a reducer 23, and a pinion 24 that meshes with the large gear, coaxially with the drive actuator.

[0047] An output shaft angle sensor 11 is installed on the nacelle base, and an input shaft angle sensor 12 is installed coaxially with the drive motor, and both are connected to transmission error calculation means 13. The above constitutes a slewing drive system 87. Multiple sets of drive actuator, pinion and input shaft angle sensor are provided for each wind turbine, and they rotate the wind turbine nacelle 86.

[0048] A wind turbine farm 91 is equipped with multiple wind turbines E, F, and G, and communication lines are drawn from the transmission error calculation means placed in the wind turbine nacelles and connected to an external network 92, which is also connected to the external network and is connected to a monitoring computer 94 placed at a status monitoring site 93.

[0049] The equipment configuration and the procedure for measuring transmission error are generally the same as in Examples 1 and 2, except that multiple drive actuators, pinion gears, and input shaft angle sensors are provided for each wind turbine. While one set of drive actuators is measuring the transmission error, the operation of the other drive actuators is temporarily suspended, and the pinion gear is left free to rotate, assisting in the measurement of transmission error.

[0050] If there are two or more sets of drive actuators, first gears, and input shaft angle sensors, and in the transmission error measurement mode, drive actuators other than the set being measured are stopped and allowed to rotate freely, it becomes possible to calculate the transmission error even in large-scale systems with multiple pinion gears.

[0051] Wind turbine farms are often located in remote areas, such as mountainous regions, and inspectors may be unable to access the wind turbine nacelle due to adverse weather conditions, such as snowfall. Therefore, the transmission error communication line drawn from the wind turbine is connected to an external network, and the transmission error waveform is stored and analyzed by a monitoring computer installed at the monitoring site. Therefore, the transmission error storage means and damage assessment means are integrated into the monitoring computer.

[0052] The above configuration and method make it possible to measure transmission errors even in large-scale systems with multiple pinion gears, and to store and analyze the transmission error measurement results remotely. [Explanation of symbols]

[0053] 11... output shaft angle sensor, 12... input shaft angle sensor, 13... transmission error calculation means, 14...transmission error storage means, 15...damage evaluation means, 21...operation control device, 22... drive motor, 23... reducer, 24... pinion, 25... gear, 26... output shaft, 27... gear box, 28... output shaft bearing, 29... output shaft brake, 30... rotation stage, 41... drive actuator, 42... rotation positioning device, 43... condition monitoring device, 51...Normal transmission error waveform during forward rotation, 52...Normal transmission error waveform during reverse rotation, 53...Backlash, 54...Meshing pitch, 55...Rotational pitch of pinion, 56... Position where transmission error is small, 57... Position where transmission error is large, 58...Transmission error waveform with gear eccentricity, 60...Transmission error waveform with localized deterioration, 61...Location where there is a scratch on a part of the tooth, 62...Location where there is localized wear on the tooth, 71...output shaft angle pulse, 72...input shaft angle pulse, 73...transmission error waveform, 74...Large gear rotation component of transmission error, 75...Pinion gear rotation component of transmission error, 76...Gear meshing component of transmission error, 77...Upper limit of transmission error waveform, 81... nacelle base, 82... slewing bearing, 83... wind turbine tower, 84...swing brake disc, 85...swing brake caliper, 86...wind turbine nacelle, 87...slewing drive system, 91...windmill farm, 92...external network, 93...Status monitoring site, 94...Monitoring computer

Claims

1. A status monitoring device for monitoring the status of a rotation positioning device including a drive actuator that generates a drive force, a first gear connected to an output shaft of the drive actuator, a second gear that meshes with the first gear, and a rotation stage that is connected to the output shaft of the second gear and is rotationally driven, a transmission error calculation means for calculating a transmission error from the angular position of the input shaft of the drive actuator and the angular position of the output shaft of the second gear; and damage evaluation means for evaluating gear damage based on the information obtained by the transmission error calculation means.

2. The condition monitoring device according to claim 1, A condition monitoring device characterized in that the diameter of the second gear is larger than that of the first gear.

3. The condition monitoring device according to claim 1, The transmission error calculation means an angle signal from an input shaft angle sensor that measures the angular position of the input shaft is corrected by a reduction ratio to obtain theoretical angular position information, and an angle signal from an output shaft angle sensor that measures the angular position of the output shaft of the second gear is obtained to obtain actual angular position information; or Alternatively, the angle signal of the output shaft angle sensor is corrected by the reduction ratio to obtain theoretical angle position information, The angle signal of the input shaft angle sensor is used as actual measured angle position information, A condition monitoring device characterized in that a difference between the theoretical angular position information and the actually measured angular position information is determined as a transmission error.

4. The condition monitoring device according to claim 1, The transmission error is stored as a transmission error waveform in association with angle information of the output shaft of the second gear. Alternatively, the condition monitoring device is characterized by comprising a transmission error storage means for storing the transmission error as a transmission error waveform in association with angle information of the input shaft.

5. 5. The condition monitoring device according to claim 4, a condition monitoring device that divides the transmission error waveform into blocks using the meshing pitch of the first gear and the second gear as a unit, and evaluates the wave height of the transmission error waveform for each block.

6. The condition monitoring device according to claim 1, A condition monitoring device characterized in that the transmission error is measured at preset time intervals and the progress of damage is evaluated by evaluating the change over time.

7. a drive actuator that generates a drive force; a first gear connected to an output shaft of the drive actuator; a second gear meshing with the first gear; a rotation stage connected to an output shaft of the second gear and driven to rotate; an input shaft angle sensor that measures the angular position of the input shaft of the drive actuator; an output shaft angle sensor that measures the angular position of the output shaft of the second gear; a transmission error calculation means for calculating a transmission error from the angular position measured by the input shaft angle sensor and the angular position measured by the output shaft angle sensor; transmission error storage means for storing information obtained by the transmission error calculation means; and damage evaluation means for evaluating damage to the gear based on information from said transmission error storage means.

8. 8. The rotary positioning device according to claim 7, A rotational positioning device having a transmission error measurement mode for calculating the transmission error.

9. 8. The rotary positioning device according to claim 7, a rotational positioning device including an output shaft brake coaxial with the rotation stage, and calculating the transmission error while applying a constant load by the output shaft brake;

10. 8. The rotary positioning device according to claim 7, A rotational positioning device characterized in that backlash is measured by rotating the drive actuator forward and backward at a predetermined meshing position between the first gear and the second gear.

11. 9. The rotary positioning device according to claim 8, two or more sets of the drive actuator, the first gear, and the input shaft angle sensor are provided; In the transmission error measurement mode, the drive actuators other than the set to be measured are stopped and allowed to rotate freely.

12. A status monitoring method for monitoring the status of a rotation positioning device including a drive actuator that generates a drive force, a first gear connected to an output shaft of the drive actuator, a second gear that meshes with the first gear, and a rotation stage that is connected to the output shaft of the second gear and is rotationally driven, comprising: calculating a transmission error from the angular position of the input shaft of the drive actuator and the angular position of the output shaft of the second gear; A condition monitoring method characterized by evaluating gear damage based on the calculated transmission error.

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