Galil DMC-1510, User Manual

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DMC-1500 Chapter 10 Theory of Operation
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143
G(s) = P + sD
The next step is to combine all the system elements, with the exception of G(s), into one function,
L(s).
L(s) = M(s) Ka Kd Kf H(s) = 0.3175*107/[s2(s+2000)]
Then the open loop transfer function, A(s), is
A(s) = L(s) G(s)
Now, determine the magnitude and phase of L(s) at the frequency
ω
c = 500.
L(j500) = 0.3175*107/[(j500)2 (j500+2000)]
This function has a magnitude of
|L(j500)| = 0.00625
and a phase
Arg[L(j500)] = -180
°
- tan-1(500/2000) = -194
°
G(s) is selected so that A(s) has a crossover frequency of 500 rad/s and a phase margin of 45 degrees.
This requires that
|A(j500)| = 1
Arg [A(j500)] = -135
°
However, since
A(s) = L(s) G(s)
then it follows that G(s) must have magnitude of
|G(j500)| = |A(j500)/L(j500)| = 160
and a phase
Arg [G(j500)] = Arg [A(j500)] - Arg [L(j500)] = -135
°
+ 194
°
= 59
°
In other words, we need to select a filter function G(s) of the form
G(s) = P + sD
so that at the frequency
ω
c =500, the function would have a magnitude of 160 and a phase lead of 59
degrees.
These requirements may be expressed as:
|G(j500)| = |P + (j500D)| = 160
and
Arg [G(j500)] = tan-1[500D/P] = 59
°
The solution of these equations leads to:
P = 40cos 59
°
= 82.4
500D = 40sin 59
°
= 137.2
Therefore,
D = 0.2744
and
G = 82.4 + 0.2744s
The function G is equivalent to a digital filter of the form: