FOC: Compensation Control

[latex] [/latex] When it comes to the design of the field-oriented current control loop, the most common control structures make use of two simple PI-regulators. This is a good choice, because they are easy to implement and -correcty parameterized-, they fit our demands for the control system (see: Is PID control superior to PI-control?) Many application notes and control unit manufacturers are recommending to parameterize the PI-controller gains by the use of the compensation control scheme, a special case of the optimum magnitude criteria. The calcultion is based on the fundamental electrical equations in the dq-reference frame: \[u_d = R_s \, i_d + L_d\,\frac{d i_d}{d t} – L_q\,\Omega_{el}\,i_q\] \[u_q = R_s \, i_q + L_q\,\frac{d i_q}{d t} + L_d\,\Omega_{el}\,i_d + \Omega_{el}\,\Psi_m\] Now, let’s assume that we feedforward-compensated the coupling terms correctly and there are no distrubance terms (caused by harmonics, cross-couplings, interlocking-time…). This simplifies our equations to: \[u_d = R_s \, i_d + L_d\,\frac{d i_d}{d t}\] \[u_q = R_s \, i_q + L_q\,\frac{d i_q}{d t}\] It can be seen that the equation for both axis have the same structure, so as further example, we’ll stick with the equation for the d-axis. The Laplace tramsform of the feedfordward-compensated d-axis equation leeds us to: \[G_p(s) = \frac{1}{L_d\,s+R_s} = \frac{1/R_s}{(L_d/R_s)\,s+1}\] That’s a simple first-order lag with a steady state gain of \(G = 1/R_s\) and a time constant of \(\tau = L_d/R_s\). This plant structure makes it possible to design a PI-controller as a so called compensation controller. Using a PI-controller \[G_r(s)=k_p+\frac{k_i}{s}\] with the control gains \(k_p = L_d/T_w\) and \(k_i = R_s/T_w\) leads to the closed loop reference transfer function \[G_w(s)=\frac{L_d\,s+R_s}{L_d\,T_w\,s^2+(R_s\,T_w+L_d)s+R_s}=\frac{1}{T_w\,s+1}\] Notice that this compensation only takes place in the closed loop reference path. The disturbance the closed loop transfer function \[G_z(s)=-\frac{T_w\,s}{(T_w\,s+1)(L_d\,s+R_s)}\neq\frac{1}{T_w\,s+1}\] of the reference path leads to Using the compensation control scheme, the control gains are given as \[k_p = \frac{L_d}{T_w}=2\,\pi\,f_w\,L_d\qquad k_i = \frac{R_s}{T_w} = 2\,\pi\,f_w\,R_s\] whereas T_w is our desired time-constant for the closed loop system. Now, we can adjust the bandwidth of the closed loop system by adjusting T_w. But is there a catch?