FANUC α iS servo HRV calibration practice

2.3 HRV4: The Ultimate Choice for Pursuing Extreme Accuracy (30i/31i Series)

For machine tools that require nanoscale interpolation and ultra-low latency, HRV4 is the only answer.

Features: Adopting a 62.5 μ s current control cycle and enabling extended HRV function (No.2300 # 0=1). It further squeezes DSP performance, but limits the number of axes that each servo card can control (usually reduced to 1 axis/DSP).

Setting: HRV4 and HRV3 cannot coexist. Set 2014 # 0=1 and ensure that the connected amplifier and detector (such as α i pulse encoder, RCN727, AT553) support high-speed communication.

Practical experience: In HRV4 mode, the observer parameters POK1 and POK2 need to be readjusted. For example, POK1 (No.2050) needs to be reduced from 956 to 264, and POK2 (No.2051) needs to be reduced from 510 to 35.

Systematic elimination scheme for mechanical vibration and resonance

Machine tool vibration is the main bottleneck that restricts spindle speed and machining surface quality. FANUC servo provides multi-stage filters to effectively solve resonance at the software level.

3.1 Micro vibration treatment in stopped state

High frequency micro vibrations occur when the motor stops, usually due to excessive proportional gain of the speed loop or mechanical backlash.

High frequency management function of speed loop (2017 # 7=1): Increase the calculation frequency of speed loop ratio and raise the oscillation threshold. However, please note that this feature will disable the observer function.

Variable proportional gain when stopped (2016 # 3=1): When the position error is less than the threshold (No.2119), the proportional gain of the speed loop in the stopped state is automatically reduced to 75% or 50% of the set value (in conjunction with No.2207 # 3).

N-pulse suppression (2003 # 4=1): For the "crawling" vibration caused by small friction, the No.2099 parameter is set to ignore the small feedback pulses in the proportional term of the speed loop.

3.2 Resonance elimination strategy during operation

Choose the appropriate filter based on the different resonance frequencies:

Mid low frequency resonance (50-150Hz): Use a disturbance cancellation filter (No.2223 # 0=1). Adjust No.2318 (gain) and No.2320 (inverse model gain), observe the frequency characteristics of the velocity loop, and suppress the amplitude.

High frequency resonance (200-400Hz): First, try the torque command filter (No.2067). The smaller the setting value, the lower the cutoff frequency (such as setting 2185 to correspond to 100Hz).

High frequency resonance (>400Hz): A resonance cancellation filter (No.2113, etc.) must be used. The manual provides four levels of filters, which can be set to center frequency (No.2113), attenuation bandwidth (No.2177), and damping (No.2359) respectively. Advanced technique: Activate the active resonance elimination filter (No.2270 # 3=1), and the system will automatically track resonance peaks that drift due to temperature or wear.

Improvement of contour accuracy: all-round compensation from arc to square angle

The ultimate goal of servo optimization is to make the actual trajectory infinitely approach the programming path. This requires a precise combination of feedforward and backlash acceleration.

4.1 Feedforward adjustment: Eliminating errors in arc radius

Traditional servos rely on position error drive, which can result in theoretical radius errors during arc interpolation.

Advanced Preview Feedforward (No.2092): The set value is usually 9700-10000 (corresponding to 97% -100%). Enable in conjunction with No. 2005 # 1=1. In theory, 100% feedforward can eliminate tracking errors, but considering the delay in the speed loop, it is generally set at 98% -99%.

Speed feedforward (No.2069): used to compensate for lag during acceleration and deceleration. Adjustment method: Observe through the 1/4 circular square shape of R5/F4000. If there is overcutting when entering the arc, increase the speed feedforward; If it protrudes, reduce the speed feedforward.

Feedforward timing adjustment (No.2095): When the arc appears elliptical, use this parameter to fine tune the phase. Positive values lead, negative values lag.

4.2 Quadrant protrusion compensation: two-stage reverse gap acceleration

When reversing the tangential axis of a circular arc, sharp peaks often appear due to friction and clearance.

First level acceleration (No.2048): Add a pulse to the speed command at the moment of commutation. Recommend an initial value of 100. Judgment criteria: If dents (overshoot) appear under low feed rate of F500, the value should be reduced; If there is still a protrusion, increase it.

Second level acceleration (No.2039): Used to compensate for the sustained frictional force after switching. Usually set to 100-500. Adjust the duration of action in conjunction with No.2082 (second level acceleration starting distance) and No.2089 (end magnification).

Adaptive function: Using No.2114 (acceleration ratio) and No.2338 (acceleration limit), the compensation amount is proportional to the arc acceleration. Optimization path: Adjust No.2048 at low feed rate; Adjust No.2114 at medium feed to eliminate medium speed bumps; Use No.2338 to limit the dents caused by overcompensation under high feed.