On the elimination of oscillation in CNC system

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Talking about the elimination methods of oscillation in CNC system


the oscillation phenomenon of CNC system has become a common problem of CNC full closed-loop system. Especially in the horizontal Z-axis with column and the b-axis of rotating CNC workbench, the system has a high frequency of oscillation. This problem has become one of the important factors affecting the normal use and height of CNC equipment

1. Reasons for oscillation the international influence of plastic extruder continues to rise analysis

there are many reasons for oscillation. Chen pointed out that there are many factors that cannot be eliminated in the mechanical aspect, such as transmission clearance, elastic deformation, friction resistance and so on, and the influence of relevant parameters of the servo system is also an important aspect

servo system can be divided into AC and DC. This paper mainly discusses the oscillation of DC servo system caused by the influence of parameters. Most CNC machine tools adopt the full closed-loop mode, and its control schematic diagram is shown in Figure 1

Figure 1 servo full closed loop system

it can be seen from Figure 1 that there are roughly four reasons for the vibration of the servo system:

a. the output voltage is unstable due to poor position loop

b. vibration caused by poor speed ring

c. the adjustable positioner of servo system is too human, causing voltage output distortion

d. the clearance of transmission mechanism (such as screw rod) is too large

the main factors causing vibration are the distortion of output parameters of these control rings or too much clearance of mechanical transmission devices. Both of them can be optimized by servo control system

2. Basic method of eliminating vibration

some CNC servo systems use semi closed-loop devices, while the full closed-loop servo system must adjust the parameters on the premise that the local semi closed-loop system does not vibrate, so the two are similar. In order to avoid repetition, this paper only discusses the parameter optimization method in the case of full closed-loop

2.1 reduce the position loop gain

there are reference standard values in the servo system, such as 3000 for fanuco-c series and 1666 for Siemens 3 system. In case of oscillation, the gain can be reduced appropriately, but not too much, because the steady-state error of the system should be guaranteed

2.2 reduce the load inertia ratio

the load inertia ratio is generally set at about 70% of the parameters shown when vibration occurs. If the fault cannot be eliminated, it is not appropriate to continue to reduce the parameter value

2.3 add proportional calculus (PID)

proportional calculus is a multi-functional controller, which can only effectively proportional gain the current and voltage signals. At the same time, it can adjust the output signal so as to expand the total production capacity of the product to 9000 tons per year. The problem of lag or excess is sometimes caused by the lag or advance of the output current and voltage. At this time, the phase of the output current and voltage can be adjusted by PID

2.4 using high-frequency suppression function

the above discussion is about the parameter optimization method in low-frequency oscillation, and sometimes the NC system will produce feedback signals containing high-frequency harmonics due to some mechanical oscillation reasons, which makes the output torque not constant, resulting in vibration

for this high-frequency oscillation, a first-order low-pass filter can be added to the speed loop, that is, the torque filter. Its control principle is shown in Figure 2

Figure 2 torque filtering function diagram

Figure 2 shows that the speed command and speed feedback signals are converted into torque signals by the speed controller, and the torque signals cut off the high-frequency components through the first-order filtering link, so as to obtain effective torque control signals. By adjusting the parameters, the frequency above 100Hz generated by the machine can be cut off, so as to achieve the effect of eliminating high-frequency oscillation

2.5 adopt dual position feedback function

dual feedback is a method to change the control mode. You can choose semi closed loop or full closed loop mode in the same system. Its schematic diagram is shown in Figure 3

Figure 3 Schematic diagram of dual position feedback control

it can be seen from Figure 3 that error controllers ER1 and ER2 are used in full closed loop and semi closed loop systems respectively. The transfer function of the first-order delay link is (1 +) τ S) -1, it can be seen that the actual error Er mainly depends on the time constant in the first-order delay link τ, Value of:

if τ= O. Then (1+ τ s) -1 = 1, er = ER1 + (er2-er1) = ER2, it can be seen that in this case, the system is in the full closed-loop control error, so the full closed-loop error control method can be used to eliminate the vibration fault

if τ= ∞, then (1+ τ s) -1 = O, er = ER1. It can be seen that in this case, the system is in the semi closed loop control error, so the semi closed loop error control method can be used to eliminate the vibration fault

to sum up, the system can be operated in full closed loop and semi closed loop by using double position feedback, which greatly improves the adjustment range of the system and increases the adjustment parameters of the system. It can be seen from the time constant that the system can adjust the full closed-loop error in the stop state and the semi closed-loop error in the transition state. Taking fanuco-c as an example, the specific parameter adjustment process is briefly introduced

first, set the parameter p84ll (dpfb) to 1, that is, it is mainly used to select the double position feedback function for the tensile performance experiment of steel bar welded joints welded by various welding methods; P8449 is the maximum amplitude of position feedback, which is generally set to 0; P8478 (numerator) and p8479 (denominator) are the constant settings of the position conversion link in the above figure, which can be set according to requirements; P8480 is the parameter setting code of the first-order delay link, and its setting range is 10 ms ~ 300 ms, which is generally set to about 100ms; P8481 is the zero point amplitude, which is generally 0, but it can be adjusted higher due to oscillation

the dual position feedback function is a relatively flexible error correction method, which has a good function of parameter optimization and ensuring system stability in the process of system debugging

2.6 adopt the mechanical feedback function

after ensuring the stability of the semi closed loop, use the full closed loop system to adjust the relevant parameters of the system. If the elastic oscillation fault is caused by the mechanical link, the mechanical feedback link should be used to adjust the parameters, which can achieve good results. Its principle control diagram is shown in Figure 4

where: k1v is the velocity integral gain; K2v is the speed proportional gain; α Is the mechanical speed feedback gain

Figure 4 Schematic diagram of mechanical speed feedback control

it can be seen from Figure 4 that the elastic mechanical device between the motor and the workbench may be deformed, and the position detection mainly comes from the position encoder, and the speed feedback directly comes from the motor encoder. From the whole closed-loop process, the position feedback in the full closed-loop lags behind other speed links due to the deformation of the mechanical elastic device, resulting in system oscillation

if the mechanical speed feedback link is added according to figure 4, the mechanical speed lag can be compensated, so as to achieve the effect of eliminating oscillation. For example, for the fanuco-c system, people in the industry said that the mechanical speed feedback device can be enabled after setting the parameter p8421 l to 1. Then set other relevant parameters as required, which can be adjusted by referring to the system parameter manual

the above mainly discusses several basic methods to eliminate the oscillation fault of CNC system. Different methods can be selected for parameter optimization according to different systems. It is mainly to understand the causes of oscillation before taking corresponding elimination methods for adjustment. It is not allowed to modify parameters blindly, so as not to affect the stability and reliability of the whole system

3. Conclusion

this paper discusses several common methods to eliminate numerical control oscillation faults. Various control links have detailed control theory as the basis, which can effectively eliminate oscillation problems and improve production efficiency while ensuring the stability and reliability of the control system. Of course, there are still some problems in some control links, which need to be further understood and solved in the future

the oscillation problem of CNC system is a common fault in the commissioning or operation of CNC machine tools. For the production and processing process, deal with the fault problem in time to ensure the normal operation of production. At the same time, import processing centers need to be maintained. For common oscillation problems, it takes a long time to accumulate and make rational judgments on the problems in order to achieve effective maintenance. Therefore, different diagnosis methods should be adopted for different CNC systems, and effective troubleshooting measures should be formulated according to the characteristics of CNC systems to improve production and operation capacity and ensure processing efficiency. (end)

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