1. Introduction
The reciprocating stroke precision control of the honing machine has always been the bottleneck restricting the high-speed development of the honing machine. This paper introduces a method of using the pulses emitted by the rotary encoder as CPU counting pulses to achieve precise control of the reciprocating stroke of the honing machine, thereby achieving a qualitative leap in the improvement of the quality of the honing machine tool.
In the past, the honing machine stroke control relied on the mechanical sprocket to convert the linear motion into the collision of the rotating collision block or the collision of the slider to change the reciprocating direction of the honing machine. The reciprocating stroke was achieved by adjusting the position of the slider and the collision block. It has many inconveniences:
(1) Due to manual operation, it is difficult to adjust the reciprocating stroke to the ideal position, and it is not convenient to adjust. (2) In addition, the collision of the slider may cause wear and looseness, which may cause excessive deviation in repeated positioning accuracy when reciprocating in the opposite direction. (3) The position of the collision block needs to be corrected frequently. (4) It cannot be set when a small reciprocating stroke is required. (5) Since contact collision can easily damage the device, the maintenance cost is too high. For this reason, we use rotary encoders as control elements in CNC honing machines and successfully overcome the above shortcomings.
2. Encoder selection
As we all know, the pulses sent by the rotary encoder are divided into A-phase pulses and B-phase pulses. With A and B-phase pulses, the CPU high-speed counting input of the PLC can determine whether the rotary encoder is rotating forward or reverse according to the order of the arrival of A and B-phase pulses. If the forward rotation of the rotary encoder is set as plus counting, then the reverse rotation is minus counting. Since this machine tool uses the Omron CJ1M programmable controller, it has a 100kHz high-number counting unit, which limits its receiving pulse frequency, and the encoder is selected based on this. The reciprocating speed of a general honing machine is 3 to 30 m/min, that is, the maximum reciprocating speed is 500 mm/s. Assuming that the encoder is driven by a pulley directly, the encoder pulley diameter is 60 mm, and the encoder pulley circumference L = πD = 3.14 × 60 = 188.4 mm, then the maximum speed of the encoder is 500 mm/188.4 mm/s = 2.65 r/s. If the encoder outputs 10000 P/R pulses per revolution, the maximum frequency of the encoder is 2.65ı10000 P/R = 26.5 kHz, which is much less than 100 kHz. The encoder selected for this machine tool is OMRON E6B2-CWZ6C-2000P/R can output 2000 A and B pulses per revolution, and the CPU of CJ1M takes the rising and falling edge jump signals of the pulses at the high-speed input end as counting signals, which is equivalent to quadrupling the frequency of the pulse signal emitted by the rotary encoder, that is, when the rotary encoder rotates one revolution, the high-speed counting unit of the CPU counts at 2000P/R×4=8000P/R. Even so, it will not exceed the maximum counting frequency of the CPU, so there is no need to add other high-speed counting unit hardware.
3. Setting of high-speed counting unit
The high-speed counting input terminal of the CJ1M programmable controller has linear and cyclic counting modes. The counting input terminal of this machine tool is set according to the differential linear counting mode.
4. Principle
Place the coordinate values of the upper and lower reversing points and the water ring position of the honing machine in decimal (hexadecimal needs to be converted) in different DM addresses in the CJ1M data storage area, and use these values as target values. The accumulated count or accumulated subtracted count value transmitted from the high-speed counting input terminal is the current value. The current value is compared with several target values. The comparison result sends an interrupt to control whether the spindle reciprocates downward or upward. (See the attached diagram)
As mentioned above, the encoder pulley diameter D = 60mm, the encoder pulley circumference L = 188.4mm, the number of pulses sent by the encoder per revolution: 2000 × 4 times = 8000, each pulse of the encoder represents the distance of reciprocating movement, that is, the pulse equivalent, pulse equivalent = 188.4/8000 = 0.02356mm/P, based on this pulse equivalent, the distance (number of pulses) from the zero point of the water circle to the reciprocating upper reversing point, lower reversing point and upper limit point can be calculated, and the values of these distances can be used as the coordinates of their target values. The difference between the coordinates of the upper and lower reversing points is the distance of the reciprocating stroke. When the reciprocating stroke of the spindle is determined, changing the coordinate values of the upper and lower reversing points can change the reciprocating stroke range of the spindle. These values can be set directly through the touch screen. According to the communication protocol between the touch screen and CJ1M, they are transmitted to the DM area register of CJ1M (the communication between the touch screen and CJ1M is no longer explained).
5. Debugging
Determination of the zero point of the water circle position: In order to achieve accurate reciprocating control, a unique starting reference point for honing to start reciprocating—the zero point of the water circle—must be found to ensure accuracy. This raises a difficulty: how to make the zero point of the water circle position unique and unchanged. Since the induction signal of the contactless proximity switch is within a certain area, if the induction block is at any position within the signal range of the water circle contactless switch at the moment of powering on the machine, the CPU reads the value of the zero point of the water circle at this time, and its position is not a fixed point in space. The maximum difference between the upper and lower ranges can be more than ten millimeters, which makes it impossible to uniquely determine the position of the water circle, let alone achieve accurate control.
If the machine tool is powered on, the sensor block is not within the signal range of the water circle non-contact switch, there are two situations: one is that the sensor block is above the water circle switch; the other is that the sensor block is below the water circle switch. In order for the CPU to read the value of the water circle zero point, the sensor block must first approach the signal range of the water circle switch. Then, they have an upper approach point and a lower approach point, and the difference between the two is more than ten millimeters, so the result is not unique.
If the zero point is placed at the upper limit position, although the spindle cannot move up further when it reciprocates upward to the upper limit position at the highest point, and the spindle can only move downward to the upper limit position, the spindle must be moved up to the upper limit position before the machine tool is powered on and reciprocated each time, and work starts from the upper limit position. This is troublesome and affects work efficiency. It is very ineffective and cannot be broken through. In the subsequent debugging, after careful observation and analysis, it was concluded that the spindle has three stop positions at the moment the machine tool is powered on (see attached figure):
(1) Stop in area A above the upper limit and the water circle switch where no signal is sent;
(2) Stop in area B where the water circle switch sends a signal;
(3) Stop in area C where the water circle switch does not send a signal.
In the first case, if the spindle is to reciprocate normally, the induction block must pass through the water ring switch signal range B area. In the second case, the induction block itself is in the water ring switch signal range B area. In the third case, if the spindle is to reciprocate normally, the spindle must be controlled in the reciprocating area. To make it controlled, a unique coordinate reference water ring position zero point must be established. Therefore, before reciprocating, the spindle must first reach the water ring switch signal range B area upward. In this way, the first and third cases are summarized as the second case - coordinate value zero point registration. From the zero point registration to the spindle entering the reciprocating area and reciprocating normally, the spindle must receive a downward reciprocating instruction. Combining the above three cases, the spindle must leave the water ring switch position and go downward from signaling to non-signaling when it enters the normal reciprocating area. We take the lower edge signal from signaling to loss of signal as the zero point registration assignment signal. This result is unique. Regardless of whether the main axis approaches the water circle switch from above or below, only the moment when it "downwards" leaves the water circle switch and loses the signal is taken as the benchmark, because we know that for the same contactless proximity switch, under the same other conditions, the signal detection distance from signal transmission to signal loss is certain, that is, as long as the position of the switch remains unchanged, the spatial position of the sensor block leaving the water circle switch downward and causing the switch to lose the signal is certain. This successfully solves the problem that the zero point of the water circle position is the only one, making it a breakthrough.
If the coordinate of the spindle at the zero point of the water ring position is set to 0, and the spindle rotates downward, the encoder count is positive, then the count values in the reciprocating area are all positive, and when the spindle rises to the upper limit area above the water ring switch position, the count value is negative, which brings inconvenience to data processing. In order to ensure that the data are all positive, the coordinate assignment is set to 5000 when registering the zero point, that is, 5000×pulse equivalent=5000×0.02356=118mm. The distance from the water ring switch to the upper limit switch of this machine tool is less than 100mm. Even if the spindle rises to the upper limit position and stops, the counting result will not be negative. For example, the zero point registration value of the water circle switch is 5000, the coordinates of the upper reversing point are set to 6300, and the coordinates of the lower reversing point are set to 18000. The distance between the upper reversing point and the water circle switch is: (6300-5000)×0.02356=30.6mm The distance between the lower reversing point and the water circle switch is: (18000-5000)×0.02356=306mm The reciprocating stroke of the spindle is: (18000-6300)×0.02356=275mm. By changing the coordinate settings of the upper and lower reversing points, the size of the reciprocating stroke and the reciprocating area of the spindle can be adjusted.
6. Conclusion
The use of rotary encoders to control the reciprocating stroke of the honing machine is flexible and efficient. It can overcome the many shortcomings of the reciprocating commutation of the honing machine controlled by collision of the collision block or the collision of the slider. It is extremely convenient to use. The reciprocating stroke area and the up and down commutation position can be set arbitrarily within the specified range. Due to the use of non-contact and collision-free control, maintenance-free can be achieved. For this reason, the use of rotary encoders as the control element of the honing machine can lay a certain foundation for the high-speed development of the honing machine.
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