As you know: The necessary condition for oscillation is G(s)*H(s)=1
Thus, in your case: K=s*(..)(..)(..).
Step1: solve the right side to a polynom N(s) of third degree;
Step2: Set s=jw and solve for Im(N(jw))=0 because the left side of the equation is real (K);
this gives the frequency of oscillation;
Step 3: Solve for Re(N(jw))=K; so you get the required value for K.
I didn't check the values; they may be OK.
However, if the given function G*H is the loop gain, the starting equation must be 1-G*H=0 and therefore k must be negative.
I didn't check the values; they may be OK.
However, if the given function G*H is the loop gain, the starting equation must be 1-G*H=0 and therefore k must be negative.
Eduardo, of course you are right.
However, when somebody tells me that a function H(s) equals the "open loop transfer function" (as houly did) I presume that the minus sign is incorporated since the loop gain (for low frequencies) always must be negative for a stable operating point.