Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Theme: «HYDRAULIC DRIVE ELEMENTS OF MECHATRONIC
SYSTEMS»
General concept
...
Thus, the hydrostatic pressure is the basis of the
hydraulic drive, and the movement of the working bodies (hydraulic motors) – the
result of moving the volume of liquid under pressure in a confined space
...
If the hydraulic transmission
consists of devices, structurally designed in one body, it is called the hydraulic
transmission of undivided execution
...

According to the source of supply of working fluid hydraulic drives are divided into
pump, pump-free, battery and trunk
...

Non-pump hydraulic drive – hydraulic drive, working on the principle of
communicating vessels and actuated mechanically
...

Battery hydraulic drive – hydraulic drive, in which the working fluid is supplied to
the hydraulic motor from a hydraulic accumulator, pre-charged from an external
source, not part of the drive
...

In the controlled machines of mechatronic systems, accumulator and main hydraulic
drives are not used, only hydraulic drives are used, which include hydraulic
accumulators
...

When possible, the hydraulic drives are divided into regulated and unregulated
hydraulic drives
...

(In unregulated hydraulic drives, the speed of movement of the working body always
remains unchanged
...
Step control
is carried out by means of step change of supply or flow rate of the working fluid in
the driving or driven link
...
By changing the
number of connected pumps with the same or different flows, as well as turning on
or off a different number of hydraulic motors, you can quite smoothly move from
one speed to another slave link
...

According to the circulation of the working medium, hydraulic drives are divided
into hydraulic drives with closed and open circuits of the working fluid circulation
...

Hydraulic drive with open circuit circulation-hydraulic drive, in which the working
fluid from the hydraulic motor enters the hydraulic tank
...

The hydraulic drive has a number of advantages:
– smaller weight and dimensions of the hydraulic drive compared to the weight and
dimensions of the mechanical and electrical drives, due to the absence or reduction

of heavy loaded shafts, reducers, couplings, frictions, ropes, generators with
electrical equipment, etc
...
The use
of swivel joints and high pressure hoses in conjunction with hydraulic lines allows
you to create a more perfect layout of hydraulic drive units on machines
...
The cycle time is reduced and the machine's
productivity is increased;
– stepless speed control of working movements, allowing to increase the utilization
of the drive motor, simplify the automation of the drive and improve the working
conditions of the driver;
– ease of control, reducing the energy costs of the driver, regardless of the drive
power;
– possibility of power branching
...

The hydraulic drive allows simple means to carry out any combination of these
movements, and to combine the movements in most cases, several pumps are
installed
...
This is achieved by
appropriate selection of working volumes of the pump and hydraulic motor
...
Using a hydraulic cylinder as a
driven link, it is possible to convert the rotational motion of the leading link (pump)
into the reciprocating motion of the driven link (hydraulic cylinder piston)
...
This provides an independent location

of the master and slave link and, in addition, achieved a reliable fixation of the slave
link in any given position and the ability to control the speed of its movement;
– reliable protection against overloads of the drive motor, metal structures and
working bodies
...
) can reduce the cost of the
drive and facilitate its operation and repair
...
e
...

The disadvantages of the hydraulic drive include:
– dependence of the characteristics of the hydraulic drive on the viscosity of the
working fluid, which varies with temperature; in automatic hydraulic systems,
viscosity instability disrupts the operation of automatic devices;
– the solubility of air in the working fluid, which also disrupts the operation of the
hydraulic drive and especially automatic devices;
– internal and external leakage of the working fluid, which reduce the efficiency of
the system
...

In general, there is an analogy between the electrical circuit and the hydraulic circuit:

Q

p1

p

I

φ1

R

U
φ2

p2
Q = p

Throttle
(hydraulic
resistance)

1
Kr

I =U

1
R

Here p = p1 − p2 is the pressure drop between the pressure line and the drain line;
Q is the flow rate of the working fluid; K r – hydraulic resistance
...


 m3 
 or liters
The flow of working fluid Q is measured in cubic meters per second 
 s 


 lit 
 lit 
per minute 
 or liters per second  
...

 N 
Fluid pressure is measured in Newtons per square meter  2 
...

And: 1
2
m
In mechatronic and automated systems, pressure has a pressure of 6·106 Pa and
more
...


IMPORTANT! The pressure in the hydraulic system is not determined by the
characteristic of the pump, but by the external load acting on the hydraulic motor
shaft or hydraulic cylinder rod
...

Pumps are adjustable and unregulated, reversible and non-reversible
...

Piston cavity of the
hydraulic cylinder

Stock cavity of the
hydraulic cylinder

Hydraulic cylinder
To ensure the rotational motion of the actuator in the hydraulic drive, hydraulic
motors are used (in electric drives, the analog is an electric motor)
...


Hydraulic
motor
unregulated
non-reversible

Hydraulic
motor
regulable
non-reversible

Unregulated
reversible
hydraulic motor

Regulable
reversible
hydraulic
motor

To create resistance in the hydraulic system used hydrothrottles
...

Hydrothrottles allow you to limit the speed of the actuators
...
The electric analog of the back valve is a diode
...
The thyristor is the electric analog of a one-way hydraulic lock
...
Filters are
coarse and fine cleaning
...
The safety valve opens when the pressure in the
hydraulic system rises above a certain value, and the working fluid from the pressure
line merges into the hydraulic tank
...


Hydropneumatic
accumulator
The hydro-pneumatic accumulator is also used in cases when there is a decrease in
hydraulic pressure to vacuum at a high speed actuators
...


To change the direction of fluid flow, the direction of movement of the cylinder or
the direction of rotation of the hydraulic motor is designed control valve (directional
valve)
...
The most common is a
three-position four-line control valve
...

The designations of such control valves are shown below
...
On line B the working fluid
is fed into the hydraulic tank
...

The following figure shows the hydraulic control valve:
p pump

Q
+

pC

Q pump

pD

Sliding valve

+
Q
-

Given that the working fluid tends to shrink under the action of pressure (external
load), then for precise fixing of the working body use double-sided hydraulic locks:

Double-sided hydraulic lock

Such devices are mounted near the hydraulic cylinder or hydraulic motor
...

Thus, they provide a stationary state of the actuator (hydraulic motor or hydraulic
cylinder) or its movement at a constant speed in both directions
...
To do this, you can use an regulable pump or hydraulic motor
...

To control the speed of the actuators in mechatronics and automation using
proportional hydraulic control valve or, as they are called, two-stage hydraulic
control valve or hydraulic control valve of the "nozzle- shutter" type
...

Polarized
Electromagnet

U
Nozzle 1

Sliding shutter



Nozzle 2

−U


Mechanical feedback

h

p2

p1

x
Sliding valve

pdrain

Q pump

+U

In that case, if the control signal U is zero, then α = 0 and the shutter is in a neutral
vertical position
...
In this
case, the movable slide valve is in the neutral position, and the flow through the
control valve is zero: Q = 0
...
The gap between the nozzle 1 and the shutter becomes
smaller, and the gap between the nozzle 2 and the shutter becomes larger
...
Together with the sliding valve,
the mechanical feedback rod begins to move, one end of which is connected to the
sliding valve, and the other end is connected to shutter
...

Therefore, the condition will be met: each control signal U will correspond to a
certain angle α of rotation of the shutter, a certain movement of the sliding valve x,
a certain flow rate of the working fluid Q and a certain speed of the actuator υ (for
forward motion) or ω (for rotational motion)

---