Suspected issue with negative inputs to integrators / processes

[rossmacdonald98]

Description

When combining several inputs to an integrator or process block, negative inputs do not seem to be handled correctly. Using an adder block to combine inputs before the integrator restores expected behaviour.

Steps to Reproduce

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""
PathSim Simulation
==================

Generated by PathView on 2026-01-22 14:43:32
https://view.pathsim.org

PathSim documentation: https://docs.pathsim.org
"""

# ────────────────────────────────────────────────────────────────────────────
# IMPORTS
# ────────────────────────────────────────────────────────────────────────────

import numpy as np
import matplotlib.pyplot as plt

from pathsim import Simulation, Connection
from pathsim.blocks import (
    Adder,
    Amplifier,
    Delay,
    Integrator,
    PulseSource,
    Scope
)
from pathsim.solvers import SSPRK22

# ────────────────────────────────────────────────────────────────────────────
# BLOCKS
# ────────────────────────────────────────────────────────────────────────────

# Sources
pulsesource = PulseSource(
    amplitude=1,
    T=1
)

# Dynamic
integrator = Integrator()
block_2 = Integrator()
delay = Delay(
    tau=0.5
)

# Algebraic
x_10 = Amplifier(
    gain=10
)
x_5 = Amplifier(
    gain=-10
)
adder = Adder()

# Recording
scope = Scope()

blocks = [
    pulsesource,
    integrator,
    block_2,
    delay,
    x_10,
    x_5,
    adder,
    scope,
]

# ────────────────────────────────────────────────────────────────────────────
# CONNECTIONS
# ────────────────────────────────────────────────────────────────────────────

connections = [
    Connection(pulsesource[0], x_10[0], delay[0]),
    Connection(x_10[0], integrator[0], adder[0]),
    Connection(x_5[0], integrator[1], adder[1]),
    Connection(integrator[0], scope[0]),
    Connection(adder[0], block_2[0]),
    Connection(block_2[0], scope[1]),
    Connection(delay[0], x_5[0]),
]

# ────────────────────────────────────────────────────────────────────────────
# SIMULATION
# ────────────────────────────────────────────────────────────────────────────

sim = Simulation(
    blocks,
    connections,
    Solver=SSPRK22,
    dt=0.01,
    dt_min=1e-16,
    tolerance_lte_rel=0.0001,
    tolerance_lte_abs=1e-08,
    tolerance_fpi=1e-10,
)

# ────────────────────────────────────────────────────────────────────────────
# MAIN
# ────────────────────────────────────────────────────────────────────────────

if __name__ == '__main__':

    # Run simulation
    sim.run(duration=10.0)

    # Plot results
    sim.plot()
    plt.show()

Expected Behavior

No response

Version

https://view.pathsim.org/

Browser

Chrome

Additional Context

No response

[milanofthe]

@rossmacdonald98 integrators in pathsim are mimo, so two inputs are integrated as a vector and result in two parallel integration paths with two outputs. You have to add a second output port to the integrator, there you will find the second integrated input.

I see that this might be a bit confusing UX wise. Adding inputs should probably auto sync the outputs and vice versa.

[rossmacdonald98]

@milanofthe Ahh makes sense! Thank you for the clarification, I guess this means that using an adder block to combine inputs before an integrator is the correct procedure.