The position detection element is composed of a detection element (sensor) and a signal processing device, and is an important part of the horizontal cnc lathe machine closed-loop servo system. Its function is to detect the actual value of the position and speed of the worktable, and send feedback signals to the numerical control device or servo device, thereby forming a closed-loop control. The detection element generally uses the principle of light or magnetism to complete the detection of position or speed.
The position detection element is divided into direct measurement element and indirect measurement element according to the detection method. Linear detection elements are generally used when measuring the linear movement of the machine tool, which is called direct measurement, and the position closed-loop control formed is called full-closed-loop control. The measurement accuracy mainly depends on the accuracy of the measuring element and is not affected by the accuracy of the machine tool transmission. Since the linear displacement of the machine tool table has an accurate proportional relationship with the rotation angle of the drive motor, the method of driving and detecting the rotation angle of the motor or screw can be used to indirectly measure the moving distance of the table. This method is called indirect measurement. The position closed-loop control is called semi-closed-loop control. The measurement accuracy depends on the accuracy of the detection element and the machine tool feed drive chain. The machining accuracy of closed-loop CNC machine tools is largely determined by the accuracy of the position detection device. CNC machine tools have very strict requirements for position detection elements, and their resolution is usually between 0.001 and 0.01 mm or less.
1. The requirements of the feed servo system for the position measuring device
The feed servo system has high requirements for the position measuring device:
1) Little influence of temperature and humidity, reliable operation, good accuracy retention, and strong anti-interference ability.
2) It can meet the requirements of accuracy, speed and measurement range.
3) Easy to use and maintain, adapt to the working environment of machine tools.
4) Low cost.
5) It is easy to realize high-speed dynamic measurement and processing, and easy to realize automation.
Position detection devices can be divided into different categories according to different classification methods. According to the form of the output signal, it can be classified into digital and analog; according to the type of measurement base point, it can be classified as incremental; according to the movement form of the position measuring element, it can be classified into rotary and linear.
2. Diagnosis and elimination of faults in the detection device
Compared with the numerical control device, the probability of failure of the detection element is relatively high, and the phenomenon of cable damage, element contamination, and collision deformation often occurs. If it is suspected to be the fault of the detection element, first check whether there is cable breakage, fouling, deformation, etc., and you can also determine the quality of the detection element by measuring its output, which requires proficiency in the working principle and output signal of the detection element . The following takes the SIEMENS system as an example for description.
(1) Input the signal. The connection relationship between the position control module of the SIEMENS CNC system and the position detection device.
The output signal of incremental rotary measuring device or linear device has two forms: di is a voltage or current sinusoidal signal, and EXE is a pulse shaping interpolator; di is a TTL level signal. Take HEIDENHA1N's sinusoidal current output grating ruler as an example. The grating is composed of grating ruler, pulse shaping interpolator (EXE), cables and connectors.
During the movement of the machine tool, three sets of signals are output from the scanning unit: two sets of incremental signals are generated by four photocells, and two photocells with a 180° phase difference are connected together, and their push-pull forms a phase difference of 90° and amplitude. The two sets of Ie1 and Ie2 with a value of about 11μA are similar to sine waves. A set of reference signals are also connected in push-pull form by two photocells with a difference of 180°. The output is a spike signal Ie0 with an effective component of about 5.5μA. The signal is only generated when it passes the reference mark. The so-called reference mark is that a magnet is installed on the housing of the grating ruler, and a reed switch is installed on the scanning unit. When the reed switch is close to the magnet, the reference signal can be output.
The two sets of incremental signals Ie1 and Ie2 enter the EXE through the transmission cable and connectors, and after amplification and shaping, two square wave signals Ua1 and Ua2 with a phase difference of 90° and the reference signal Ua0 are output. These signals are properly combined and processed. That is, five pulses can be generated in one signal cycle, that is, 5 times the frequency is processed, and sent to the CNC position control module through the connector.
(2) EXE signal processing. The function of the pulse shaping interpolator (EXE) is to amplify, reshape, frequency multiply and alarm the incremental signal output by the grating ruler or encoder, and output it to the CNC for position control. EXE is composed of basic circuit and subdivision circuit.
The basic circuit printed circuit board contains channel amplifier, shaping circuit, drive and alarm circuit, etc. The subdivision circuit is made into a circuit board as an optional function, and the two boards are connected through the J3 connector.
1) Channel amplifier. When the grating detects and generates sine wave current signals Ie1, Ie2 and Ie0, through the channel amplifier, a certain amplitude of sine current voltage is output.
2) Shaping the circuit. Based on the amplification of Ie1, Ie2 and Ie0, the shaping circuit converts them into three corresponding square wave signals Ua1, Ua2 and Ua0. The TTL high level is greater than or equal to 2.5V, and the low level is less than or equal to 0.5V. .
3) Alarm circuit. When the grating causes the output signal of the channel amplifier to be zero due to the break of the input cable, the pollution of the grating or the damage of the bulb, the alarm signal is driven by the drive circuit and then output to the CNC system by the connector J2.
4) Subdivision circuit. In the position control of some high-precision CNC machine tools (such as CNC grinders), high resolution is required for position measurement. For example, the accuracy of the grating ruler alone cannot be satisfied. For this reason, a subdivision circuit must be used to improve the resolution. Rate to meet the needs of high-speed machine tools. The output signal of the basic circuit channel amplifier is connected to the subdivision circuit through the connector J3. After being processed by the subdivision circuit, the output signal of the two channels with a phase difference of 90° and a duty ratio of 1:1 in one cycle is output through the connector J3. Subdivide the square wave signal. After the two square wave position numbers are driven by the drive circuit in the basic circuit, they are the corresponding Ua1 and Ua2 channel signals, which are output to the CMC system by the connector J2.
In addition, the purpose of the synchronization circuit is to obtain square wave reference pulses corresponding to the leading and trailing edges of the square wave signals Ua1 and Ua2.
3. Common forms of faults in detection devices
(1) Mechanical oscillation (during acceleration/deceleration)
1) The pulse encoder is malfunctioning. At this time, check whether the voltage of the feedback line terminal on the speed unit drops at a certain point. If there is a drop, it indicates that the pulse encoder is defective, and the encoder should be replaced.
2) The pulse encoder cross coupling may be damaged, causing the shaft speed to be out of sync with the detected speed. The coupling should be replaced.
3) If the tachometer generator fails, the tachometer should be repaired or replaced.
(2) Mechanical runaway (speeding). In the case of checking the position control unit and speed control unit, the following points should be checked:
1) Check whether the wiring of the pulse encoder is wrong, check whether the wiring of the encoder is positive feedback, and whether the phase A and phase B are connected reversely.
2) Check whether the pulse encoder coupling is damaged. If it is damaged, replace the coupling.
3) Check whether the terminal of the tachogenerator is connected reversely and whether the excitation signal wire is connected incorrectly.
(3) The spindle cannot be oriented or the orientation is not in place. Check the setting and adjustment of the orientation control circuit, check the orientation board, and the adjustment of the spindle control printed circuit board. At the same time, check whether the position detector (encoder) is defective.
(4) Coordinate axis vibration feed. After checking whether the motor coil is short-circuited, whether the mechanical feed screw is well connected to the motor, and whether the entire servo system is stable, check whether the pulse code is good, whether the coupling connection is stable and reliable, and whether the tachometer is reliable.
(5) The alarm caused by program error and operation error in NC alarm. For example, NC reports 090# and 091# of FAUNUC-6ME system. NC alarm occurs, which may be caused by the main circuit failure and the feed speed is too low. At the same time, it is also possible that the pulse encoder is bad; the pulse encoder power supply voltage is too low. At this time, adjust the 15V of the power supply voltage so that the voltage value on the +5V terminal of the main circuit board is within 4.95～5.10V; there is no input pulse The one-turn signal of the encoder cannot perform the reference point return normally.
(6) Alarm of the servo system. Such as FAUNUC-6ME system's servo alarm 416#, 426#, 436#, 446#, 456#, SINUMERIK880 system's servo alarm I364#, SINUMERIK8 system's servo alarm 114#, 104#, etc. When the above alarm number appears, it may be: axis pulse encoder feedback signal is broken, short circuit and signal loss, use an oscilloscope to measure the A-phase and B-phase one-revolution signal; the encoder is contaminated, too dirty, and the signal cannot be received correctly .
In short, in the failure of CNC equipment, the failure rate of the detection components is relatively high. As long as the correct use and strengthening of maintenance, and in-depth analysis of the problems that occur, the failure rate will be reduced, and the failure can be resolved quickly to ensure The normal operation of the equipment.
