Brake control method for CNC machine tools - Solutions - Huaqiang

In CNC milling machines and inclined bed lathes, it's essential to control the servo motor's brake to prevent the table from sliding down when it's connected. Typically, servo motors are equipped with brakes. However, in practice, issues like occasional slippage during power on/off or servo alarms may occur. These problems often stem from incorrect wiring, improper parameter settings, or unsuitable motor selection (e.g., insufficient braking torque). While some of these issues can be resolved through adjustments, a more reliable approach involves analyzing the system’s principle and implementing an improved wiring method.

Brake Control Requirements

Under normal operation, the servo motor’s brake should be released. When the machine is powered off or a servo alarm occurs, the brake must engage to hold the worktable securely. Additionally, there should be no momentary slippage during power on, power off, or servo activation. This requires precise timing for the following scenarios:

Power On: The servo motor should be energized first, followed by a delay before releasing the brake.

Alarm During Operation or Power Failure: The motor should decelerate gradually (to avoid damage to the brake or motor), then lock and turn off excitation after a delay.

Power Off: First, the motor should be locked, and only after a delay should the excitation be turned off.

Principle Analysis

Power Failure Brake

The SJT servo motor with a power-off brake uses the Intorq BFK457 series brake, which is normally closed. It engages when power is lost and releases when energized, operating at DC 24V with an action time under 500ms. When the coil is energized, it attracts the armature disc, releasing the brake. When de-energized, the spring applies pressure to generate friction-based braking. This type of brake is suitable for holding the table but not for stopping movement, as it cannot force the motor to stop abruptly.

DA98B Signal Timing Analysis

CNC machines typically do not experience slippage when powered on or off. However, situations such as power-on, operational alarms, or sudden power failures require careful analysis. For example:

Power-On Process: After DA98B is powered on, it first outputs an alarm signal. Once the DC bus voltage stabilizes (about 500ms), the alarm is cleared. Then, after the external SON signal is valid, the motor is energized, and the SRDY signal is output. Finally, the hold signal is activated.

During Alarm or Enable Disconnection: The motor decelerates to a set speed within a specified time, then the hold signal is turned off, followed by a delay before disconnecting the motor and turning off the SRDY signal.

Power-Off Process: After power is cut, the DA98B detects the voltage drop, triggers an alarm, turns off the hold signal, and then disconnects the motor after a delay.

Parameter Descriptions

PA51: Maximum deceleration time before the brake can be engaged. Range: 0–5000 ms.

PA52: Servo lock delay time. Range: 0–5000 ms.

PA53: Motor speed at which the brake can be activated. Range: 0–3000 rpm.

Wiring Method

Using the configuration of GSK980TD + DA98B + SJT motor with a power-off brake as an example, the wiring diagram shows how to control the brake. The hold, ALM, and related signals are highlighted, while other signals like SON should be connected according to the manual. To avoid power supply issues, the DC24V power should not be sourced from the GSK980TD box, as it must be provided separately by the user.

Conclusion

Practical applications have demonstrated that this method effectively prevents worktable slippage during braking operations, offering a simple and reliable solution for CNC machine control systems.