感谢zhan_hong
我后来咨询西门子技术支持，有另一种方法（硬件不需改动，但要牺牲PID功能）：

P2800=1 enable free function blocks 使能自由功能块
P2802.8=1 enable multiplier 使能乘法器

P2877.0=21 乘数1
P2877.1=21 乘数2

The square of the output frequency(always positive) is now available in r2878

P2264=2878 PID反馈信号
P2270=1 (square root) PID反馈功能选择器

The square root of r2878 is now available in r2272. This will now be the unsigned value of the output frequency(always positive).

P0771=2272 analogue output will now give the desired result.

Note:
The use of the PID mathematical function assumes the PID function is not already being used. i.e.
P2200=0