Parameter manipulation while solving ODE

New to Julia
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The differential equations library allows you to define parameters of any type, so functions should also be fine afaik.
Assuming your control parameter only depends on quantities that are known at each step of the integration (i.e time and the state), the following should do the trick:

function controlActionTheta(du,u,t)
   Theta = (maximum(u),t,0) #or whatever you want to define there is
   return Theta
end

function lorenz(du, u, p, t)
    theta = p(du,u,t)
    du[1] = 10.0 * (u[2] - u[1]) + theta[1]
    du[2] = u[1] * (28.0 - (u[3])) - u[2] + theta[2]
    du[3] = u[1] * u[2] - (8/3) * (u[3]) + theta[3]
end

prob = ODEProblem(lorenz,u0,tspan,controlActionTheta)

In case you want to do something different, you might also want to checkout the callback library of DifferentialEquations.jl:
https://diffeq.sciml.ai/stable/features/callback_library/

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