# MTK Control System with Current Not Working
**URL:** https://discourse.julialang.org/t/mtk-control-system-with-current-not-working/129558
**Category:** New to Julia
**Tags:** electricalcomponent, differentialequation, modelling, controlsystems, mtk
**Created:** [June 2, 2025, 1:00pm UTC](https://discourse.julialang.org/t/mtk-control-system-with-current-not-working/129558 "2025-06-02T13:00:04Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@madtorresga](https://discourse.julialang.org/u/madtorresga)
#### Post date: [June 2, 2025, 1:00pm UTC](https://discourse.julialang.org/t/mtk-control-system-with-current-not-working/129558/1 "2025-06-02T13:00:04Z")
Hello! I’m trying to implement this exact same [example](https://docs.sciml.ai/ModelingToolkitStandardLibrary/stable/tutorials/dc_motor_pi/), but controlling with the current that passes through the resistance instead of the speed in `speed_sensor.w`.
```julia
@equations begin
#...
#connect(current_sensor.i, :y, feedback.input2) #not working
feedback.input2 ~ current_sensor.i
connect(R1.n, current_sensor.p)
connect(current_sensor.n, L1.p)
#...
end
@named model = DCMotor()
sys = structural_simplify(model)
```
At `structural_simplify` it gives `ERROR: MethodError: no method matching -(::SymbolicUtils.BasicSymbolic{Real}, ::ODESystem) The function `-` exists, but no method is defined for this combination of argument types.`
Since I’ve had similar issues with other implementations, I figured it could be a beginner’s problem.
---
### Author:  [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)
#### Post date: [June 2, 2025, 1:17pm UTC](https://discourse.julialang.org/t/mtk-control-system-with-current-not-working/129558/2 "2025-06-02T13:17:26Z")
It’s because `feedback.input2` is a connector and not a variable. You would have to do
```julia
feedback.input2.u ~ current_sensor.i
```
instead.
Why does
```julia
connect(current_sensor.i, :y, feedback.input2)
```
not work?