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Electronics in the Mobile Industry As the mobile industry becomes more
sophisticated and automated in its equipment the
need for electronic control of the hydraulic systems
is growing. More and more OEMs are integrating
electronic and electrohydraulic equipment into their
machines at a rapidly increasing rate.
The one major drawback to the acceptance of
electronics on mobile equipment has been
overcome.
The reliability and ruggedness of
electronic components has improved to the point
that they can withstand millions of cycles and the
harsh outdoor environments that mobile equipment
is exposed to.
It has become increasingly important for the
Hydraulic Application Engineer and the Hydraulic
Component Salesman to become familiar with the
many electronic terms that are used and products
that are being integrated into the OEMs machine.
The compatibility of electrohydraulic valves and
the many electronic valve drivers and PLC
controllers is becoming an important issue for both
the OEM and the hydraulic component supplier. It
is necessary that one has an understanding of the
terminology and principles of operation of the
electronic components being used.
Many modern electrohydraulic devices, such as
proportional pressure, flow and directional valves
require relatively high-power electrical signals for
control and/or positioning. Typically, however, the
source of the command or control signal is a device
capable of delivering only low- power signals.
Because of this it is necessary to raise or amplify the
electrical level of the signal (in terms of voltage or
current, or both) before it is capable of operating the
particular proportional valve.
Because of heat handling and physical size
limitations, it is not practical to increase the capacity
or output of the originating device in order to deliver
higher power signals. Fortunately, when a low power
signal is applied to the input of an electronic circuit
known as an amplifier, it can produce the required
command signal for electrohydraulic devices at its
output.
Amplifier definition
Why Pulse Width Modulation
An amplifier is a relatively simple circuit used to
raise the level (or increase the amplitude) of an
electronic signal.
The amplified output is
proportional to the input signal. Various types of
amplifiers are used in electrical equipment, but all of
them perform the same basic function
When voltage is applied to a valve coil the current
flowing through it creates a magnetic field which
provides the force to shift the spool or poppet. The
input voltage divided by the resistance of the coil
equals the current draw. This is very straight forward
when used with on/off valves, but proportional
valves are only useful if one can control the spool
position by varying the input current.
A simple potentiometer can be used to add
resistance in series with the coil to vary the current.
This simple technique is very inefficient and not
practical when high currents are required.
Therefore it is necessary to use a form of
amplification of the input signal to obtain the desired
current level to drive the proportional valve.
When an infinitely variable DC signal is used to
operate a proportional valve solenoid, the output
transistor of the amplifier acts like a variable resistor.
It has to drop the power supply voltage down to that
required by the solenoid coil at any given time. Also
the full coil current, which may be several amps, has
to pass through this output transistor. The result of
high current draw and large voltage drops is high
heat created in the transistor which requires a
relatively large heat sink to dissipate.
Pulse Width Modulation (PWM) is a technique
used in amplifiers to overcome this problem. In this
case the output transistor is used as an on/off switch
and feeds the solenoid coil with a series of on/off
pulses at a constant voltage. The pulses are set at
constant frequency (typically 400-5000+ Hz) and the
signal level is determined by varying the duration of
the on pulse relative to each off pulse.
Electronics in the Mobile Industry The advantage of this technique is that during
each off pulse, the output transistor is not passing
any current and during each on pulse, there is
virtually no voltage drop across the transistor and
therefore very little heat is created. In practice of
course, there will be a small voltage drop across the
transistor during the on pulses, and it takes a finite
amount of time to switch on and off, so a small
amount of heat will still be created. The amount of
heat will be considerably smaller than that
produced by a conventional DC output signal.
Pulse Width Modulation has become the
standard for all valve amplifiers in order to reduce
amplifier size and power waste. No modifications
are required to the valve solenoid in order to use
this technique.
Pulse width modulation (PWM) is an efficient
technique to control current for driving a
proportional valve coil that allows the use of
electronics for current regulation, dither, ramping,
short circuit protection and dead band elimination.
A PWM signal is not constant, it is simply an
electronic switch. The signal is on for a period of
time and off for the rest (fig-1).
How PWM works
1hz = 1 cycle per second
1hz
1hz
1hz
50% on
25% on
75% on
0
12
D
is the characteristic of a circuit that
makes itself evident by opposing the starting,
stopping, or changing of current flow.
Electrical inductance is like mechanical inertia,
and the growth of current in an inductive circuit can
be compared to the acceleration of an automobile
on the surface of a road. The car begins to move at
the instant a constant force is applied to it. At this
instant its rate of change of speed (acceleration) is
greatest, and all applied force is used to overcome
the inertia (inductance of the coil) of the car. After
awhile the speed of the car increases and the
acceleration becomes zero. The applied force is
now used up in overcoming the friction of the road
(resistance of the coil) against the tires and the
inertia effect disappears . Conversely when power
is removed the cars inertia tends to keep it going
until the friction of the road slows the car to a stop
and the inertia effect again disappears.
Inductance
A simplified explanation of coil inductance is
required to clarify the behavior of the current in a
PWM signal.
Because of the configuration of a coil it has in
addition to its resistance, a certain inductance.
When a voltage is applied across an inductive
element (a coil), the current produced does not
immediately jump up to its constant value, but
gradually rises to its maximum over a period of time.
Conversely the current does not disappear
instantaneously, even if voltage is removed abruptly
(fig-2).
1hz = 1 cycle per second
1hz
0
12
50%
Fig 2
The duty cycle D refers to the percentage of the
cycle for which the signal is on. The duty cycle can
be anywhere from 0, the signal is always off, to 1,
the signal is constantly on. A 50% D results in a
perfect square wave.
The PWM signal frequency can be low
frequency (100-400 Hz) or high frequency (above
5000 Hz). High frequency PWM is more desirable
in that it produces a more constant ripple free
amperage output.
Fig-1
D
Coil Inductance Dither (current ripple)
Stiction and hysterisis can make controlling a
hydraulic valve seem erratic and unpredictable.
Stiction can keep the valve spool from moving
when input signal changes are small. The spool
then tends to over shoot when the signal becomes
large enough to free it. The force required to get the
spool to move initially is more than the force
required to keep it moving to the desired position.
Friction of a sliding spool causes a reduction in the
distance moved.
Hysterisis can cause the spool shift to be
different for the same command signal input,
depending on whether the change is increasing or
decreasing.
Dither is a rapid, small movement of the spool
about the desired position. It is intended to keep the
spool moving to avoid stiction and average out
hysterisis. Dither amplitude must be large enough
and the frequency slow enough to enable the spool
to respond and yet small and fast enough not to
cause a resulting pulsation in the hydraulic output.
These requirements sometimes are in conflict and
the goal is to provide just enough dither to
overcome the problem without creating other
issues.
This is further complicated by different valve
designs having a different response to the same
dither frequency and amplitude.
Changing the
PWM frequency will allow adjusting the dither, but
the amplitude and frequency of the dither can not be
set independently as may be required by various
valve designs.
1h = 1 cycle per second
200 to 300 h
1h
0
12
Inherent ripple
Fig-4
Fig-3
PWM current characteristics
At 25% signal(fig-3), D is shorter than the
time it takes the current to reach its maximum value.
This will result in a reduced current output to the
valve coil and therefore a reduced hydraulic output.
Also if the PWM frequency is low enough the
current will go back to zero during the off period and
the circuit will be said to have discontinuous current.
0
12
25%
1 cycle
D
Low frequency PWM, typically less than 400 Hz,
generates dither (current ripple) as a by-product of
the PWM process (fig-4). The PWM frequency is lo