A four-wheel antilock automobile braking system uses electronic feedback to control automatically the brake force on each wheel [15]. A block diagram model of a brake control system is shown in Figure E2.9, where Ff (s) and FR(s) are the braking force of the front and rear wheels, respectively, and R(s) is the desired automobile response on an icy road. Find Ff (s)/R(s).

R(s) + H(s) G(s) Fr(s) G(s) G3(5) FR(S) H(s) FIGURE E2.9 Brake control system.