introduction
When braking a machine tool spindle motor, an energy-consumption braking method is usually used, which involves inputting DC power to the motor's AB phase.
The spindle motor is stopped using an energy-efficient braking method. When the spindle rotates at low speed in a low gear, it takes approximately 0.5 seconds, and when it rotates at high speed in a high gear, it takes approximately 2.5 seconds. Currently, the spindle motor braking method is shown in Figure 1: first, switch KM1 is disconnected, then switch KM2 is closed, thereby disconnecting the three-phase AC power supply and connecting the DC power supply. After a 2.5-second delay, the spindle motor is considered to have stopped rotating, and then tool changing or other operations are performed.
When the spindle rotates at different speeds, the time required to stop it using energy-efficient braking varies. Using the same energy-efficient braking time results in extended, meaningless machining auxiliary time, reducing machine tool efficiency. Furthermore, if switch KM2 fails to close reliably, or the DC power fuse trips and fails to provide DC power, the spindle motor can only decelerate under friction, requiring a long braking time. However, even after a 2.5-second delay, the machine tool's CNC system still recognizes the spindle motor as having stopped rotating. Performing tool changes or other actions at this point can easily lead to accidents.
Therefore, we have improved the design of the machine tool spindle braking control method. Instead of using a 2.5s delay to determine the spindle rotation status, we monitor the spindle rotation status in real time and immediately issue a spindle stop signal when the spindle rotation speed drops below a certain level.
2 Spindle speed monitoring solution
A strip of iron is mounted on the synchronous drive shaft of the electric motor, rotating synchronously with the motor. A proximity switch detects this strip, detecting two pulse signals per revolution. The rotational speed is determined by detecting these pulse signals. There are two methods for detecting the pulse signals.
The first approach involves reading the number of pulse signals N within a certain period Tp. A PLC is a sequential controller; its program executes step-by-step from beginning to end, with each execution constituting a scan cycle, which is then repeated cyclically from the beginning. If the program has 2000 steps, with each step having an execution time of 30μs, the scan cycle is approximately 60ms, and the scan frequency is approximately 16Hz. The accurately detectable pulse frequency should be below 8Hz. When the pulse frequency of the rotational speed is greater than 16Hz (i.e., speed n ≥ 480 r/min), the PLC, affected by its scan frequency, cannot accurately detect the number of pulses. In severe cases, at high speeds, very few pulses may be detected, leading to a misjudgment that the motor has essentially stopped and subsequent actions are performed, causing an accident. This approach is only suitable for monitoring the low-speed rotation of the spindle.
The second approach involves detecting the duration T of a pulse signal that is either "0" or "1". When n < 60 r/min, a signal indicating that the spindle has stopped is issued. Due to the delay in PLC program execution, by the time the CNC system receives the spindle stop signal and executes the following actions, the spindle has already completely stopped rotating. The duration T of the pulse signal being either "0" or "1" for n = 60 r/min is 0.25 s. Therefore, we set the timer for detecting the pulse signal to 0.25 s. Similarly, at high speeds, inaccurate pulse measurement may occur, but regardless of the severity, the "0" or "1" signal will change at least once within 0.25 s, thus accurately determining whether the spindle has stopped rotating. This approach is applicable to detecting the spindle braking state during high or low spindle rotation. In practical applications, we adopted this approach.
3. PLC-based control method for spindle energy consumption braking
In the PLC design program, X20.0 is the input of the speed pulse signal, M05 is the spindle stop signal, and Y50.0 is the spindle stop signal.
Two timers, TM1 and TM2, determine whether the X20.0 pulse signal has been continuously "0" or "1" for a set time. As long as one of the timers reaches the set time, that is, R100.1 or R100.2 becomes "1", which causes R100.3 to become "1". At this time, M05 is "1", which causes Y50.0 to output "1", thus issuing a signal that the spindle braking is complete, achieving accurate judgment of spindle stop.
4. Conclusion
We adopted an improved control method using PLC to control the braking of the machine tool spindle, which can reliably determine the rotation state of the machine tool spindle, avoid machine tool malfunctions, save auxiliary time for spindle machining, and make the machine tool's protection performance more perfect.
