MODELS > Examples > Models Calculus

1. Laplace

This example illustrate the usage of Laplace transforms in MODELS. The unknown variable y(t) is calculated by specifying the Laplace transformed ratio between y(s) and a known variable x(s). Only polynoms are allowed y(s)/x(s) = (n0+n1s+n2s^2+n3s^3+...)/(d0+d1s+n2s^2+d3s^3+...). The history of x(t) must be specified. The usage of the resident functions deriv() and integral() is also illustrated. Note that the integral function must be initialized in HISTORY.

MODEL lap1 -- illustrates the use of Laplace in MODELS

VAR y[1..5], x

HISTORY y[1..5] {dflt:[0,0,0,0,0]} -- required to set history of integrals and denominators of laplacian transfer functions

x {dflt:0}

integral(x) {dflt:0}

EXEC

x:=t>0 -- logical function. x=0;t<timestep and x=1;t>=timestep

Laplace(y[1]/x) := 1|s0 / 1|s1 -- y[1](s)/x(s) = 1/s

y[2] := integral(x) -- y[2](t)=y[1](t)

Laplace(y[3]/x) := 1|s1 / 1|s0 -- y[3](s)/x(s) = s

y[4] := deriv(x) -- different approximation for y[4] and y[3]

Laplace(y[5]/y[2]) := 1|s1 / 1|s0 -- y[5]=x

ENDEXEC

ENDMODEL

2. Differential equation

This example compares lapace transforms and differential equations. When using the keywords claplace and cdiffeq the coefficients in the polynoms are not recalculated in the execution loop but remain constant.

MODEL lapdif -- illustrates the use of Laplace and diffeq in MODELS

VAR a1, a2

b1 -- approximation to a pure delay of size=timestep, using Laplace

b2 -- same as b1, using constant coefficients, using claplace

b3 -- 1/(1+s) with step input =10, using Laplace

b4 -- same as b3, equivalent notation using diffeq

b5 -- same as b4, with constant coefficients, using cdiffeq

INIT

k:=10

a1:=k

histdef(a2):=t -- privately assign a history function to a2

-- also possible with definition HISTORY a2 {DFLT:t}

ENDINIT

EXEC

IF t=0 THEN -- initialize at t=0. Also possible under INIT

b1:=0; b2:=0; b3:=0; b4:=0; b5:=0

ELSE

a2:=t

-- b1(t)=a2(t-ts) => b1(s)/a2(s)=exp(-ts*s)=exp(-s*ts/2)/exp(s*ts/2) "="

-- (1-s*ts/2)/(1+s*ts/2) series expansion valid for small timesteps ts

Laplace(b1/a2) := (1|s0 -timestep/2|s1)/(1|s0 +timestep/2|s1)

-- b2(t)=a2(t-ts) with constant coefficients (1 and ts/2). claplace used with constant coefficients, faster

claplace(b2/a2) := (1|s0 -timestep/2|s1)/(1|s0 +timestep/2|s1)

-- b3(t)+2*d/dt*b3(t)=k => b3(s)/k(s)=1/(1+s)

-- when b3(t=0)=0 and d/dt is the time derivative operand

Laplace(b3/k) := 1|/(1|s0 +2|s1)

-- b4(t)+2*d/dt*b4(t) = a1

diffeq(1|d0+2|d1)|b4 := a1

-- b5(t)+2*d/dt*b5(t) = k with constant coeffisients (1 and 2)

cdiffeq(1|d0+2|d1)|b5 := k

ENDIF

ENDEXEC

ENDMODEL

3. Combine

This example illustates how to solve mutually coupled linear or differential equations. The keyword sum must be used to specify a linear equation polynom.

MODEL comb2 -- illustrates COMBINE groups in MODELS

VAR y1, y2, y3, x

HISTORY y1 {dflt: sin(t)}

y2 {dflt: cos(t)}

y3 {dflt: -sin(t)}

integral(y2) {dflt: sin(t)}

integral(y3) {dflt: cos(t)}

x {dflt: cos(t)}

EXEC

COMBINE AS first_group

y1:=integral(y2)

y2:=integral(y3)

y3:=sum(-1|y1)

ENDCOMBINE