Result of the damp function for discrete time systems

Hi,

This is my first contribution on GitHub ;)

Problem

The method damp() seems to return a wrong result for discrete systems with negative poles

import control as ctl
H = ctl.tf([1],[1,0.2],1)
H.damp()

Result : RuntimeWarning: invalid value encountered in log

Investigation

The problem seems to come from the type of the poles variable in the dampmethod of the LTI object

if isdtime(self, strict=True):
     splane_poles = np.log(poles)/self.dt
else:
      splane_poles = poles
wn = absolute(splane_poles)
Z = -real(splane_poles)/wn
return wn, Z, poles

Solution

The poles variable must be cast to a numpy complex type as follows

if isdtime(self, strict=True)
      splane_poles = np.log(poles.astype(complex))/self.dt
...

Hi,

thanks for catching this!

This is my first contribution on GitHub ;)

Awesome! Would you try to make the next step and create a Pull Request with your proposed changes?

Bonus points for adding your example to test_damp() (using pytest.mark.parametrize()) :

python-control/control/tests/lti_test.py

Lines 54 to 71 in 6b7a9c7

    
           def test_damp(self): 
        
               # Test the continuous time case. 
        
               zeta = 0.1 
        
               wn = 42 
        
               p = -wn * zeta + 1j * wn * np.sqrt(1 - zeta**2) 
        
               sys = tf(1, [1, 2 * zeta * wn, wn**2]) 
        
               expected = ([wn, wn], [zeta, zeta], [p, p.conjugate()]) 
        
               np.testing.assert_allclose(sys.damp(), expected) 
        
               np.testing.assert_allclose(damp(sys), expected) 
        
               # Also test the discrete time case. 
        
               dt = 0.001 
        
               sys_dt = c2d(sys, dt, method='matched') 
        
               p_zplane = np.exp(p*dt) 
        
               expected_dt = ([wn, wn], [zeta, zeta], 
        
                              [p_zplane, p_zplane.conjugate()]) 
        
               np.testing.assert_almost_equal(sys_dt.damp(), expected_dt) 
        
               np.testing.assert_almost_equal(damp(sys_dt), expected_dt)

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Result of the damp function for discrete time systems #646

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	def test_damp(self):
	# Test the continuous time case.
	zeta = 0.1
	wn = 42
	p = -wn * zeta + 1j * wn * np.sqrt(1 - zeta**2)
	sys = tf(1, [1, 2 * zeta * wn, wn**2])
	expected = ([wn, wn], [zeta, zeta], [p, p.conjugate()])
	np.testing.assert_allclose(sys.damp(), expected)
	np.testing.assert_allclose(damp(sys), expected)

	# Also test the discrete time case.
	dt = 0.001
	sys_dt = c2d(sys, dt, method='matched')
	p_zplane = np.exp(p*dt)
	expected_dt = ([wn, wn], [zeta, zeta],
	[p_zplane, p_zplane.conjugate()])
	np.testing.assert_almost_equal(sys_dt.damp(), expected_dt)
	np.testing.assert_almost_equal(damp(sys_dt), expected_dt)

Result of the damp function for discrete time systems #646

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bnavigator commented on Aug 17, 2021

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