fix parameter usage

Fixes #594

Author: ChrisRackauckas

src/systems/control/controlsystem.jl

@@ -140,7 +140,7 @@ function runge_kutta_discretize(sys::ControlSystem,dt,tspan;
     states_timeseries = [getindex.(timed_vars,j) for j in 1:n+1]
     k_timeseries = [[getindex.(k_vars,i,j) for i in 1:m] for j in 1:n]
     control_timeseries = [[[Variable(x.name,i,j)(sys.iv()) for x in controls(sys)] for i in 1:m] for j in 1:n]
-    ps = parameters(sys)
+    ps = [p() for p in parameters(sys)]
     iv = sys.iv()
 
     # Calculate all of the update and stage equations

test/controlsystem.jl

@@ -1,15 +1,16 @@
 using ModelingToolkit
 
 @variables t x(t) v(t) u(t)
+@parameters p
 @derivatives D'~t
 
 loss = (4-x)^2 + 2v^2 + u^2
 eqs = [
     D(x) ~ v
-    D(v) ~ u^3
+    D(v) ~ p*u^3
 ]
 
-sys = ControlSystem(loss,eqs,t,[x,v],[u],[])
+sys = ControlSystem(loss,eqs,t,[x,v],[u],[p])
 dt = 0.1
 tspan = (0.0,1.0)
 runge_kutta_discretize(sys,dt,tspan)