Consider the spring-mass-dashpot system mounted on a massless cart as shown in Figure 3-3. Let us obtain mathematical models of this system by assuming that the cart is standing still for 1 < 0 and the spring-mass-dashpot system on the cart is also standing still for t< 0. In this system, u(t) is the displacement of the cart and is the input to the system. Att = 0, the cart is moved at a constant speed, or ú = constant. The displacement y(1) of the mass is the output. (The displacement is relative to the ground.) In this system, m denotes the mass, b denotes the viscous-friction coefficient, and k de- notes the spring constant. We assume that the friction force of the dashpot is proportional to ý - ů and that the spring is a linear spring; that is, the spring force is proportional to y – u. For translational systems, Newton's second law states that

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Consider the spring-mass-dashpot system mounted on a massless cart as shown in Figure 3-3. Let
us obtain mathematical models of this system by assuming that the cart is standing still for t < 0and
the spring-mass-dashpot system on the cart is also standing still for t < 0. In this system, u(t) is the
displacement of the cart and is the input to the system. Att = 0, the cart is moved at a constant speed,
or ú = constant. The displacement y(t) of the mass is the output. (The displacement is relative to
the ground.) In this system, m denotes the mass, b denotes the viscous-friction coefficient, and k de-
notes the spring constant. We assume that the friction force of the dashpot is proportional to y - ů
and that the spring is a linear spring; that is, the spring force is proportional to y – u.
For translational systems, Newton's second law states that
ma = SF
where m is a mass, a is the acceleration of the mass, and EF is the sum of the forces acting on the
mass in the direction of the acceleration a. Applying Newton's second law to the present system
and noting that the cart is massless, we obtain
d'y
dy
du
- k(y - u)
%3D
dr?
dt
or
d'y
dy
du
+ ku
dt
+ b
+ky = b
m
dr?
dt
This equation represents a mathematical model of the system considered. Taking the Laplace
transform of this last equation, assuming zero initial condition, gives
(ms? + bs + k)Y(s) = (bs + k)U (s)
Taking the ratio of Y(s) to U(s), we find the transfer function of the system to be
bs + k
Y(s)
Transfer function = G(s) =
U(s)
ms? + bs + k
Such a transfer-function representation of a mathematical model is used very frequently in
control engineering.
Massless cart
ww
Transcribed Image Text:Consider the spring-mass-dashpot system mounted on a massless cart as shown in Figure 3-3. Let us obtain mathematical models of this system by assuming that the cart is standing still for t < 0and the spring-mass-dashpot system on the cart is also standing still for t < 0. In this system, u(t) is the displacement of the cart and is the input to the system. Att = 0, the cart is moved at a constant speed, or ú = constant. The displacement y(t) of the mass is the output. (The displacement is relative to the ground.) In this system, m denotes the mass, b denotes the viscous-friction coefficient, and k de- notes the spring constant. We assume that the friction force of the dashpot is proportional to y - ů and that the spring is a linear spring; that is, the spring force is proportional to y – u. For translational systems, Newton's second law states that ma = SF where m is a mass, a is the acceleration of the mass, and EF is the sum of the forces acting on the mass in the direction of the acceleration a. Applying Newton's second law to the present system and noting that the cart is massless, we obtain d'y dy du - k(y - u) %3D dr? dt or d'y dy du + ku dt + b +ky = b m dr? dt This equation represents a mathematical model of the system considered. Taking the Laplace transform of this last equation, assuming zero initial condition, gives (ms? + bs + k)Y(s) = (bs + k)U (s) Taking the ratio of Y(s) to U(s), we find the transfer function of the system to be bs + k Y(s) Transfer function = G(s) = U(s) ms? + bs + k Such a transfer-function representation of a mathematical model is used very frequently in control engineering. Massless cart ww
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