export some conversion functions

Author: baggepinnen

lib/ControlSystemsBase/src/pid_design.jl

@@ -1,4 +1,5 @@
 export pid, pid_tf, pid_ss, pid_2dof, pid_ss_2dof, pidplots, leadlink, laglink, leadlinkat, leadlinkcurve, stabregionPID, loopshapingPI, placePI, loopshapingPID
+export convert_pidparams_from_parallel, convert_pidparams_from_standard, convert_pidparams_from_to, convert_pidparams_to_parallel, convert_pidparams_to_standard
 
 """
     C = pid(param_p, param_i, [param_d]; form=:standard, state_space=false, [Tf], [Ts], filter_order=2, d=1/√(2))
@@ -739,6 +740,13 @@ end
 
 """
     convert_pidparams_from_to(kp, ki, kd, from::Symbol, to::Symbol)
+
+Convert PID parameters from `from` form to `to` form.
+
+The `from` and `to` forms can be chosen as one of the following
+* `:standard` - ``K_p(1 + 1/(T_i s) + T_d s)``
+* `:series` - ``K_c(1 + 1/(τ_i s))(τ_d s + 1)``
+* `:parallel` - ``K_p + K_i/s + K_d s``
 """
 function convert_pidparams_from_to(kp, ki, kd, from::Symbol, to::Symbol)
     Kp, Ki, Kd = convert_pidparams_to_parallel(kp, ki, kd, from)