diff --git a/case_study/assertion.py b/case_study/assertion.py
new file mode 100644
index 0000000..bf4a072
--- /dev/null
+++ b/case_study/assertion.py
@@ -0,0 +1,131 @@
+from sympy import Symbol, sympify, FiniteSet
+
+class Assertion:
+    """Define Assertion objects for checking expectations with respect to simulation results.
+
+    The class uses sympy, where the symbols are expected to be results variables,
+    as defined in the variable definition section of Cases.
+    These can then be combined to boolean expressions and be checked against
+    single points of a data series (see `assert_single()` or against a whole series (see `assert_series()`).
+
+    The symbols used in the expression are accessible as `.symbols` (dict of `name : symbol`).
+    All symbols used by all defined Assertion objects are accessible as Assertion.ns
+
+    Args:
+        expr (str): The boolean expression definition as string.
+            Any unknown symbol within the expression is defined as sympy.Symbol and is expected to match a variable.
+    """
+    ns = {}
+    
+    def __init__(self, expr:str):
+        self._expr = Assertion.do_sympify( expr)
+        self._symbols = self.get_symbols()
+        Assertion.update_namespace( self._symbols)
+
+    @property
+    def expr(self):
+        return self._expr
+    
+    @property
+    def symbols(self):
+        return self._symbols
+    
+    def symbol(self, name:str):
+        try:
+            return self._symbols[name]
+        except KeyError as err:
+            return None
+
+    @staticmethod
+    def do_sympify( _expr):
+        """Evaluate the initial expression as sympy expression.
+        Return the sympified expression or throw an error if sympification is not possible.
+        """
+        if '==' in _expr:
+            raise ValueError("'==' cannot be used to check equivalence. Use 'a-b' and check against 0") from None
+        try:
+            expr = sympify( _expr)
+        except ValueError as err:
+            raise Exception(f"Something wrong with expression {_expr}: {err}|. Cannot sympify.") from None
+        return expr
+    
+    def get_symbols(self):
+        """Get the atom symbols used in the expression. Return the symbols as dict of `name : symbol`"""
+        syms = self._expr.atoms(Symbol)
+        return { s.name : s for s in syms}
+    
+    @staticmethod
+    def reset():
+        """Reset the global dictionary of symbols used by all Assertions"""
+        Assertion.ns = {}
+    
+    @staticmethod
+    def update_namespace( sym: dict):
+        """Ensure that the symbols of this expression are registered in the global namespace `ns`"""
+        for n,s in sym.items():
+            if n not in Assertion.ns:
+                Assertion.ns.update({n:s})
+
+
+    def assert_single(self, subs:list[tuple]):
+        """Perform assertion on a single data point.
+
+        Args:
+            subs (list): list of tuples of `(variable-name, value)`,
+                where the independent variable (normally the time) shall be listed first.
+                All required variables for the evaluation shall be listed.
+                The variable-name provided as string is translated to its symbol before evaluation.
+        Results:
+            (bool) result of assertion
+        """
+        _subs = [ (self._symbols[s[0]], s[1]) for s in subs]
+        return self._expr.subs( _subs)
+
+
+    def assert_series(self, subs:list[tuple], ret:str='bool'):
+        """Perform assertion on a (time) series.
+
+        Args:
+            subs (list): list of tuples of `(variable-symbol, list-of-values)`,
+                where the independent variable (normally the time) shall be listed first.
+                All required variables for the evaluation shall be listed
+                The variable-name provided as string is translated to its symbol before evaluation.
+            ret (str)='bool': Determines how to return the result of the assertion:
+            
+                `bool` : True if any element of the assertion of the series is evaluated to True
+                `bool-list` : List of True/False for each data point in the series
+                `interval` : tuple of interval of indices for which the assertion is True
+                `count` : Count the number of points where the assertion is True
+        Results:
+            bool, list[bool], tuple[int] or int, depending on `ret` parameter.
+            Default: True/False on whether at least one record is found where the assertion is True.
+        """
+        _subs = [ (self._symbols[s[0]], s[1]) for s in subs]
+        length = len( subs[0][1])
+        result = [False]* length
+        
+        for i in range( length):
+            s = []
+            for k in range( len( _subs)): # number of variables in substitution
+                s.append( (_subs[k][0], _subs[k][1][i]))
+            res = self._expr.subs( s)
+            if res:
+                result[i] = True
+        if ret == 'bool':
+            return True in result
+        elif ret == 'bool-list':
+            return result
+        elif ret == 'interval':
+            if True in result:
+                idx0 = result.index(True)
+                if False in result[idx0:]:
+                    return (idx0, idx0+result[idx0:].index(False))
+                else:
+                    return (idx0, length)
+            else:
+                return None
+        elif ret == 'count':
+            return sum( x for x in result)
+        else:
+            raise ValueError(f"Unknown return type '{ret}'") from None
+
diff --git a/tests/test_assertion.py b/tests/test_assertion.py
new file mode 100644
index 0000000..144aa51
--- /dev/null
+++ b/tests/test_assertion.py
@@ -0,0 +1,67 @@
+from case_study.assertion import Assertion
+from math import sin, cos
+import matplotlib.pyplot as plt
+import pytest
+from sympy import symbols, sympify
+
+_t = [0.1*float(x) for x in range(100)]
+_x = [0.3*sin(t) for t in _t]
+_y = [1.0*cos(t) for t in _t]
+
+def show_data():
+    fig, ax = plt.subplots()
+    ax.plot(_x, _y)
+    plt.title("Data (_x, _y)", loc="left")
+    plt.show()
+
+def test_init():
+    Assertion.reset()
+    t,x,y = symbols("t x y")
+    ass = Assertion( "t>8")
+    assert ass.symbols['t'] == t
+    assert Assertion.ns == {'t':t}
+    ass = Assertion("(t>8) & (x>0.1)")
+    assert ass.symbols == {'t':t,'x':x}
+    assert Assertion.ns == {'t':t, 'x':x}
+    ass = Assertion( "(y<=4) & (y>=4)")
+    assert ass.symbols == {'y':y}
+    assert Assertion.ns == {'t':t, 'x':x, 'y':y}
+    
+
+def test_assertion():
+    t,x,y = symbols("t x y")
+    # show_data()print("Analyze", analyze( "t>8 & x>0.1"))
+    Assertion.reset()
+    ass = Assertion( "t>8")
+    assert ass.assert_single( [('t', 9.0)])
+    assert not ass.assert_single( [('t', 7)] )
+    res = ass.assert_series( [('t',_t)], 'bool-list')
+    assert True in res, "There is at least one point where the assertion is True"
+    assert res.index(True) == 81, f"Element {res.index(True)} is True"
+    assert all( res[i] for i in range(81,100)), "Assertion remains True"
+    assert ass.assert_series( [('t',_t)], 'bool'), "There is at least one point where the assertion is True"
+    assert ass.assert_series( [('t',_t)], 'interval') == (81, 100), "Index-interval where the assertion is True"
+    ass = Assertion("(t>8) & (x>0.1)")
+    res = ass.assert_series([('t',_t), ('x',_x)])
+    assert res, "True at some point"
+    assert ass.assert_series([('t',_t), ('x',_x)], 'interval') == (81,91)
+    assert ass.assert_series([('t',_t), ('x',_x)], 'count') == 10
+    with pytest.raises(ValueError, match="Unknown return type 'Hello'") as err:
+        ass.assert_series([('t',_t), ('x',_x)], 'Hello')
+    # Checking equivalence. '==' does not work
+    ass = Assertion( "(y<=4) & (y>=4)")
+    assert ass.symbols == {'y' : y}
+    assert Assertion.ns == {'t': t, 'x': x, 'y': y}
+    assert ass.assert_single( [('y', 4)])
+    assert not ass.assert_series( [('y', _y)], ret='bool')
+    with pytest.raises(ValueError, match="'==' cannot be used to check equivalence. Use 'a-b' and check against 0") as err:
+        ass = Assertion("y==4")
+    ass = Assertion( "y-4")
+    assert 0==ass.assert_single( [('y', 4)])
+    
+
+if __name__ == "__main__":
+#    retcode = pytest.main(["-rA","-v", __file__])
+#    assert retcode == 0, f"Non-zero return code {retcode}"
+    test_init()
+    test_assertion()
\ No newline at end of file
diff --git a/tests/test_bouncing_ball_3d.py b/tests/test_bouncing_ball_3d.py
new file mode 100644
index 0000000..eceb489
--- /dev/null
+++ b/tests/test_bouncing_ball_3d.py
@@ -0,0 +1,82 @@
+from math import sqrt
+from pathlib import Path
+
+import pytest
+from component_model.example_models.bouncing_ball_3d import BouncingBall3D
+from component_model.model import Model
+from fmpy import plot_result, simulate_fmu
+from shutil import copy
+
+def nearly_equal(res: tuple, expected: tuple, eps=1e-7):
+    assert len(res) == len(
+        expected
+    ), f"Tuples of different lengths cannot be equal. Found {len(res)} != {len(expected)}"
+    for i, (x, y) in enumerate(zip(res, expected, strict=False)):
+        assert abs(x - y) < eps, f"Element {i} not nearly equal in {x}, {y}"
+
+def test_make_fmu():#chdir):
+    fmu_path = Model.build(
+        str(Path(__file__).parent / "data" / "BouncingBall3D" / "bouncing_ball_3d.py"),
+        dest = Path( Path.cwd()))
+    copy( fmu_path, Path(__file__).parent / "data" / "BouncingBall3D")
+
+
+def test_run_fmpy(show):
+    """Test and validate the basic BouncingBall using fmpy and not using OSP or case_study."""
+    path = Path("BouncingBall3D.fmu")
+    assert path.exists(), f"File {path} does not exist"
+    dt = 0.01
+    result = simulate_fmu(
+        path,
+        start_time=0.0,
+        stop_time=3.0,
+        step_size=dt,
+        validate=True,
+        solver="Euler",
+        debug_logging=False,
+        visible=True,
+        logger=print,  # fmi_call_logger=print,
+        start_values={
+            "e": 0.71,
+            "g": 9.81,
+        },
+    )
+    if show:
+        plot_result(result)
+    t_bounce = sqrt(2*10*0.0254 / 9.81)
+    v_bounce = 9.81 * t_bounce # speed in z-direction
+    x_bounce = t_bounce/1.0 # x-position where it bounces in m
+    # Note: default values are reported at time 0!
+    nearly_equal(result[0], (0, 0, 0, 10, 1, 0, 0, sqrt(2*10/9.81), 0, 0)) # time,pos-3, speed-3, p_bounce-3
+    print(result[1])
+    arrays_equal(result(bb), (0.01,
+                      0.01, 0, (10*0.0254-0.5*9.81*0.01**2)/0.0254,
+                      1, 0, -9.81*0.01, sqrt(2*10*0.0254/9.81), 0, 0))
+    t_before = int(sqrt(2 / 9.81) / dt) * dt  # just before bounce
+    print("BEFORE", t_before, result[int(t_before / dt)])
+    nearly_equal(
+        result[int(t_before / dt)],
+        (t_before, 1*t_before, 0, 1.0 - 0.5 * 9.81 * t_before * t_before, 1, 0, -9.81 * t_before, x_bounce, 0, 0),
+        eps=0.003,
+    )
+    nearly_equal(
+        result[int(t_before / dt) + 1],
+        (
+            t_before + dt,
+            v_bounce * 0.71 * (t_before + dt - t_bounce) - 0.5 * 9.81 * (t_before + dt - t_bounce) ** 2,
+            v_bounce * 0.71 - 9.81 * (t_before + dt - t_bounce),
+        ),
+        eps=0.03,
+    )
+    nearly_equal(result[int(2.5 / dt)], (2.5, 0, 0), eps=0.4)
+    nearly_equal(result[int(3 / dt)], (3, 0, 0))
+    print("RESULT", result[int(t_before / dt) + 1])
+
+
+if __name__ == "__main__":
+#    retcode = pytest.main(["-rA", "-v", __file__, "--show", "True"])
+#    assert retcode == 0, f"Non-zero return code {retcode}"
+    import os
+    os.chdir(Path(__file__).parent.absolute() / "test_working_directory")
+    test_make_fmu()
+    test_run_fmpy( show=True)
\ No newline at end of file