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Reference point return and fault diagnosis of vertical machining center

Reference point return and fault diagnosis of vertical machining centerReference point return and fault diagnosis of vertical machining center

The so-called vertical machining center reference point, also known as the origin or zero point, is the point where the mechanical origin and electrical origin of the machine tool coincide, and is the mechanically fixed point after the origin is returned. Each machine tool can have a reference origin, or multiple reference origins can be set as needed for automatic tool exchange (ATC) or automatic tray exchange (APC), etc. The reference point serves as the original reference system of the workpiece coordinate system. After the machine tool reference point is determined, each workpiece coordinate system is established accordingly. The so-called mechanical origin is the reference point of the basic mechanical coordinate system. Once the mechanical parts are assembled, the mechanical origin is established immediately. The so-called electrical origin is the reference point established by the grid signal or zero mark signal sent by the detection feedback element used by the machine tool. In order to make the electrical origin coincide with the mechanical origin, the distance from the electrical origin to the mechanical origin must be set with a parameter that sets the origin offset. This coincident point is the machine origin. In the process of using the machining center, the manual or automatic reference point operation of the machine tool is a frequently performed action. Regardless of whether the machine tool detection feedback element is equipped with an incremental pulse encoder or an absolute pulse encoder, in some cases, such as during ATC or APC, a certain axis or all axes of the machine tool must first return to the reference origin.

There are two ways to return to the machine tool reference point according to the way the machine tool detection element detects the origin signal. One is the grid point method, and the other is the magnetic switch method. In the grid point method, the detector generates a grid point or a zero pulse at the same time as the motor rotates a signal. After installing a deceleration bump and a deceleration switch on the mechanical body, the first grid point detected by the CNC system Or the zero signal is the origin. In the magnetic switch method, a magnet and a magnetic induction origin switch are installed on the machine body. When the magnetic induction origin switch detects the origin signal, the servo motor stops immediately, and the stop point is regarded as the origin. The characteristic of the grid method is that if the speed near the origin is less than a fixed value, the servo motor will always stop at the same point, that is, after the homing operation, the machine’s origin is maintained well. The characteristic of the magnetic switch method is that the software and hardware are simple, but the origin position drifts proportionally with the change of the servo motor speed, that is, the origin is uncertain. At present, almost all machine tools use the grid point method.
Several cases of using the grid point method to return to the origin of the machine are as follows:

  • 1. The machine tool that uses the incremental detection feedback element returns to the machine origin for the first time after it is turned on;
  • 2. The machine tool that uses the absolute detection feedback component is installed and commissioned for the first time after the machine is installed and commissioned;
  • 3. After the grid point offset parameter setting is adjusted, the machine tool returns to the origin manually for the first time.

According to the different measurement methods of the detection element, it is divided into zero reset by absolute pulse encoder and zero reset by incremental pulse encoder. In a system that uses an absolute pulse encoder as a measurement feedback element, after the machine is turned on for the first time before commissioning, after adjusting to a suitable reference point through parameter setting and machine zero return operation, as long as the backup battery of the absolute pulse encoder is valid, after Each time you turn on the machine, you do not need to go back to the reference point. In systems that use incremental pulse encoders, there are two modes of reference point return. One is that the axes return to the reference point manually in the reference point return mode after power-on. Manual return-to-origin operations are performed after each start-up; the other This is the G code command in memory mode to return to the origin during use.
There are generally the following three types of action processes for a machine tool that uses an incremental pulse encoder as a measurement feedback element when it is turned on and started manually:

  • 1. When manually returning to the origin, the origin axis first moves to the origin at the fast feed rate set by the parameter. When the origin deceleration bump presses the origin deceleration switch, the servo motor decelerates to the origin approach speed set by the parameter and continues to move forward. When the deceleration bumper releases the home deceleration switch, when the CNC system detects the first grid point or zero mark signal sent by the encoder, the homing axis stops, and this stop point is the machine tool reference point.
  • 2. The homing axis first moves to the origin direction at a rapid feed rate. When the home deceleration switch is pressed by the deceleration bump, the homing axis brakes to zero speed, and moves in the opposite direction at a speed close to the origin. When the deceleration hits the block After the origin proximity switch is released, when the CNC system detects the first grid point or zero mark signal sent by the detection feedback element, the homing axis stops, and this point is the machine origin.
  • 3. When homing, the homing axis first moves to the homing direction at the rapid feed rate. When the homing deceleration block presses the homing deceleration switch, the homing axis brakes until the speed is zero, and then jogs in the opposite direction. When the block releases the home position deceleration switch, the return-to-zero axis moves in the reverse rapid traverse direction. When the deceleration striker presses the home position deceleration switch again, the return-to-zero axis moves forward at a speed close to the origin. After the deceleration striker releases the deceleration switch When the CNC system detects the first grid point or zero mark signal, the zero return axis stops and the machine origin is established accordingly.

The machine tool using incremental detection feedback components is powered on for the first time. Each servo axis is manually returned to the origin. Most of the return-to-origin uses a bump-type return. Afterwards, each return to the origin can be returned to the first origin at high speed at a rapid feed rate with G code instructions. The reference point position memorized during the reset.

Further analyze the return of the machine tool origin from the control process of the CNC system. When the machine tool returns to the machine tool origin mode, the servo motor rotates toward the origin at a feed speed greater than a fixed speed. When the CNC system detects a signal of the motor, the CNC The reference counter in the system is cleared. If the grid offset is set by the parameter, the reference counter is also automatically set to a value equal to the grid offset. After that, the reference counter becomes a ring counter. When the counter counts the movement command pulse to the value set by the reference counter, it is reset, and a grid point is generated with the appearance of the one-turn signal. When the deceleration bumper depresses the home deceleration switch, the motor decelerates to a speed close to the home speed. After the bumper releases the home deceleration switch, the motor stops at the next grid point, generates a homing completion flag signal, and the reference position is reset. After the power is turned on, it returns to the origin for the second time. Since the reference counter has been set and the grid has been established, it can directly return to the origin. When the machine tool using the absolute detection feedback element returns to the origin for the first time, the CNC system communicates with the absolute detection feedback element to establish the current position, and calculates the distance from the current position to the machine origin and the distance from the current position to the nearest grid point , Assign the calculated value to the counter, and the grid point is established.

When the machining center returns to the reference point and fails, first check from simple to complex. First check whether the origin deceleration block is loose, whether the deceleration switch is fixed firmly, and whether the switch is damaged. If there is no problem, the drift of the mechanical relative position should be further checked with a dial indicator or laser measuring instrument, the length of the deceleration bumper should be checked, and the back The relationship between the home position of the origin, the position of the deceleration switch and the origin position, check the homing mode, whether it is the first homing after turning on, whether an absolute pulse encoder is used, the movement amount of the servo motor per revolution, the command multiple and detection Multiplier ratio, check whether the parameter setting of the rapid return speed to the origin, the parameter setting of the speed close to the origin and the parameter setting of the rapid feed time constant are appropriate, check whether the system is fully closed loop or semi closed loop, and check whether the reference counter setting is appropriate.

Back to the original fault phenomenon and diagnostic adjustment steps are as follows:
Check whether the absolute point encoder is used after the machine tool returns to the origin. If it is used, see the origin drift when the machine tool using the absolute pulse encoder is used for the diagnosis and adjustment steps; if the machine tool uses the incremental pulse encoder, the system should be determined Full closed loop or semi closed loop. If it is a full closed loop system, see the origin offset in the full closed loop system for diagnosis and adjustment steps; if it is a half closed loop system, use a dial indicator or laser measuring instrument to check whether the relative position of the machine drifts. If there is no drift, but the position display is deviated, check whether the work coordinate system offset is invalid. After the machine returns to the origin, the CRT position of the machine is displayed as a non-zero value, which depends on some parameter settings such as work coordinate system offset. If the relative position of the machine deviates, determine the offset. If the offset is a grid, the diagnosis method sees the processing step of the origin drifting by one grid point. If the amount of drift is a few pulses, see the diagnostic steps for a few pulses of origin drift. Otherwise, check whether the number of pulses and the value of the reference counter match. If it does not match, modify the value of the reference counter to match; if it matches, the pulse encoder is broken and needs to be replaced.

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