# In MTK, how can I change a system's parameter's default values?
**URL:** https://discourse.julialang.org/t/in-mtk-how-can-i-change-a-systems-parameters-default-values/135092
**Category:** Modelling & Simulations
**Tags:** modelingtoolkit
**Created:** [January 16, 2026, 7:41am UTC](https://discourse.julialang.org/t/in-mtk-how-can-i-change-a-systems-parameters-default-values/135092 "2026-01-16T07:41:04Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@cstjean](https://discourse.julialang.org/u/cstjean)
#### Post date: [January 16, 2026, 7:41am UTC](https://discourse.julialang.org/t/in-mtk-how-can-i-change-a-systems-parameters-default-values/135092/1 "2026-01-16T07:41:04Z")
I have a `sys::System`, and I’d like to build a `sys2` that is like `sys`, but `sys2.some_param`’s default is `20`. Right now I use
```julia
"""
``jldoctest
julia> @named sys = MySystem();
julia> param = sys.some_param;
julia> initial_conditions(assign_parameters(sys, [param=>14]))[vol]
14
``
"""
assign_parameters(sys::System, new_params::AbstractDict) =
MTK.@set sys.initial_conditions = merge(initial_conditions(sys), new_params)
```
to achieve that, but this is ugly code that uses MTK internals. It also does not work if the Dict contains `some_param[2] => new_value`; I have to pass `some_param => new_array`.
I’m aware that “Don’t do this; pass the new parameters as the `ps` parameter” might be the official answer, but it’s not very satisfying. Given that `sys` has parameters, it feels natural that we could change them. `remake` does not seem defined for it.
For context, I want to have `sys2 = fit_parameters(sys, my_data)`. I could of course have it return the new parameters, but I’d prefer to return the new system.
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [January 16, 2026, 8:29am UTC](https://discourse.julialang.org/t/in-mtk-how-can-i-change-a-systems-parameters-default-values/135092/2 "2026-01-16T08:29:57Z")
Probably better as an issue to discuss adding such a function.
---
### Author:  [@Brad\_Carman](https://discourse.julialang.org/u/Brad_Carman)
#### Post date: [January 16, 2026, 3:59pm UTC](https://discourse.julialang.org/t/in-mtk-how-can-i-change-a-systems-parameters-default-values/135092/3 "2026-01-16T15:59:14Z")
I have a package I’m working on that can solve this problem: [bradcarman/ModelingToolkitParameters.jl: Parameter Management utility for ModelingToolkit models](https://github.com/bradcarman/ModelingToolkitParameters.jl/tree/main). You can see the initial docs here: [docs](https://github.com/bradcarman/ModelingToolkitParameters.jl/blob/main/docs/src/index.md) that explain the full concept.
For your example, the solutions would look like this…
```julia
# standard defaults
@named sys = MySystem()
# special defaults
motor_params = MotorParams(r=20)
@named sys2 = MySystem(; motor_params)
```
The full MWE…
```julia
using ModelingToolkit
using ModelingToolkit: D_nounits as D, t_nounits as t
using ModelingToolkitParameters
@kwdef mutable struct MotorParams <: Params
k::Float64 = 0.1
r::Float64 = 0.01
l::Float64 = 1e-3
end
@component function Motor(; name)
pars = @parameters begin
k
r
l
end
vars = @variables begin
v(t)
dphi(t) = 0
i(t) = 0
di(t)
end
eqs = Equation[
D(i) ~ di
v ~ i * r + l * di + dphi * k
D(dphi) ~ k * i
v ~ sin(t)
]
return ODESystem(eqs, t, vars, pars; name)
end
@component function MySystem(; name, motor_params = MotorParams())
systems = @named begin
motor = Motor()
end
defaults = motor => motor_params
return System(Equation[], t, [], []; name, defaults, systems)
end
# standard defaults
@named sys = MySystem()
# special defaults
motor_params = MotorParams(r=20)
@named sys2 = MySystem(; motor_params)
```