Electrical control of CNC machine tool motion coordinates - Database & Sql Blog Articles

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The electrical control of a motion coordinate of a CNC machine consists of a series of current (torque) control loops, speed control loops and position control loops.

(1) The current loop is a circuit that supplies torque to the servo motor. In general, its matching adjustment with the motor has been made by the manufacturer or the corresponding matching parameters are specified, and the feedback signal is also connected in the servo system, so wiring and adjustment are not required.

(2) The speed loop is a circuit that controls the motor speed, that is, the speed of the coordinate axis. The speed regulator is a proportional integral (PI) regulator. The P and I adjustment values ​​are completely dependent on the load size of the driven coordinate axis and the mechanical characteristics of the mechanical transmission system (rail, transmission) transmission stiffness and transmission clearance. When the characteristics change significantly, the mechanical transmission system needs to be repaired first, and then the speed loop PI regulator is re-adjusted.

The optimal adjustment of the speed loop can be completed under the condition of the position loop open loop, which is easier for the horizontal motion coordinate axis and the rotation coordinate axis, and for the vertical motion coordinate axis, the position will automatically fall when the position is open loop. In the event of danger, it is possible to take off the motor no-load adjustment first, and then install the motor together with the position ring to adjust it or directly adjust it with the position ring. At this time, it requires some experience and care.

See the “Speed ​​Measurement” section above for the feedback loop of the speed loop.

(3) The position loop is a control link that controls the precise positioning of each coordinate axis according to the command position. The position loop will ultimately affect the positional accuracy and working accuracy of the axes. There are two aspects to this:

The first is the matching of the accuracy of the position measuring component with the pulse equivalent of the CNC system. The number of pulses emitted by the measuring unit per unit moving distance is matched with the resolution specified by the numerical control system after being multiplied by the external frequency multiplier circuit and/or the CNC internal multiplication factor. For example, the position measuring component 10 pulses/mm, and the numerical control system resolution, ie, the pulse equivalent is 0.001 mm, the pulse sent by the measuring component must be matched by 100 times frequency.

The second is the correct setting and adjustment of the position loop gain coefficient Kv value. Usually, the Kv value is set as the machine data. In the numerical control system, the setting address and numerical unit of the Kv value are specified for each coordinate axis. The setting of the Kv value after the speed loop optimization adjustment becomes an important factor reflecting the performance of the machine tool and affecting the final accuracy. The Kv value is a direct expression of the performance of the machine's motion coordinates and is not arbitrarily magnified. There are two problems to be paid about the setting of Kv value. First, the following formula must be satisfied: Kv=v/Δ where v——coordinate running speed, m/minΔ——tracking error, mm

Note that the units used by different CNC systems may be different. When setting, pay attention to the units specified by the CNC system. For example, if the unit of coordinate running speed is m/min, the unit of Kv value is m/(mm·min), and if the unit of v is mm/s, the unit of Kv should be mm/(mm·s). Secondly, the Kv values ​​of the linkage axes must be the same to ensure the accuracy of the synthetic motion. Usually the axis with the lowest Kv value is taken as the standard.

Position feedback (see the previous section "Position Measurement") has three cases: one is that there is no position measuring component, the position is open loop control, that is, there is no position feedback, the stepping motor drive is generally open loop; the other is semi-closed loop control. That is, the position measuring component is not on the final moving part of the coordinate axis, that is to say, there are still some transmission links outside the position closed-loop control. This situation requires that the outer ring transmission part should have considerable transmission rigidity and transmission precision, and add backlash compensation. After the pitch error compensation, high position control accuracy can be obtained; the third is full closed-loop control, that is, the position measuring component is mounted on the final moving part of the coordinate axis. In theory, the positional accuracy of this control is the best, but It requires higher or lower than the entire mechanical transmission system. If it does not, it will seriously affect the dynamic accuracy of the two coordinates, so that the machine can only work under the condition of reducing the speed loop and position accuracy. Another important issue affecting the accuracy of full-closed control is the precise mounting of the measuring components, so don't underestimate it.

(4) Feedforward control is opposite to feedback. It is an adjustment circuit that pre-adds the command value extraction part to the rear. Its main function is to reduce the tracking error to improve the dynamic response characteristics and improve the position control accuracy. Because most of the machine tools do not have this function, this article does not elaborate, just to note that the feedforward must be added after the above three control loops are optimally debugged.