Compressor unit and control device used thereby

In the inlet pipe (7) of the compressor element (1) is provided a pneumatically controlled throttle valve (9), whereas the motor (3) of the compressor element (1) has a pneumatically controlled speed regulator (6). This speed regulator (6) and the throttle valve (9) are both connected to the compressed air receiver (14) via a compressed air pipe (26) and a control device (18). This control device (18) contains an electropneumatic valve (19) in the compressed air pipe (26) which is coupled to an electronic control (20), whereas a pressure sensor (21) is connected to the compressed air receiver (14) and a pressure sensor (22) is erected in the compressed air pipe (26) between the valve (19) and the speed regulator (6) and the throttle valve (9). The control (20) is connected to both pressure sensor (21 and 22) and contains means to control the electropneumatic valve (19) as a function of the measured air receiver pressure and the measured regulating pressure which has been fed back, as well as an electronically adjusted nominal pressure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention concerns a compressor unit containing a compressor 
element driven by a motor which is provided with an outlet pipe and an 
inlet pipe, and a compressed air receiver onto which the outlet pipe is 
connected, whereby a pneumatically controlled throttle valve is provided 
in the inlet pipe, whereas the motor has a pneumatically controlled speed 
regulator and both this speed regulation and the throttle valve are 
connected to the compressed air receiver via a compressed air pipe and a 
control device with a control valve in the compressed air pipe. 
2. Description of the Related Art 
With known compressor units of the above type, the control device contains 
two valves erected in parallel, namely a pneumatic control valve and an 
electromechanical load valve. The pipe which is connected to the 
compressed air receiver via these two valves is connected to the 
connecting pipe between the speed regulator and the throttle. Onto this 
connecting pipe are connected branches which are provided with small air 
holes. 
The output of the compressor element depends on the rotational speed of the 
motor and thus of the speed regulator and the throttle in the inlet pipe. 
The rotational speed and the throttle are adjusted by means of the 
regulating pressure which is built up by the pneumatic control valve on 
the basis of the pressure in the compressed air receiver. 
The nominal pressure, i.e. the operating pressure under full load, is 
adjusted manually by means of the control valve. If the air receiver 
pressure is equal to the nominal pressure while load-running, the 
regulating pressure is zero, the throttle valve is entirely open and the 
rotational speed of the motor is maximal. 
If however, the air receiver pressure is higher, in particular maximal, for 
example 2 bar above the nominal pressure, the rotational speed is minimal 
and the throttle valve is entirely closed. The regulating pressure is 
proportional to the difference between the air receiver pressure and the 
nominal pressure. 
Between no regulating pressure and the maximum regulating pressure, any 
output can be set between the maximum and zero respectively. 
Since the pneumatic control valve only lets air through in one direction, 
the above-mentioned blow-off holes are necessary. By letting air escape 
via these blow-off holes, it is possible for the regulating pressure to 
drop when the air receiver pressure is lowered. 
By means of pipe restrictions and volumes to be filled, the regulating 
pressure dynamically approaches a first-order process. With a lowering and 
rising load, the variation of the air receiver pressure will be retarded. 
This results in an overshoot (air receiver pressure too high) when the 
load diminishes, and in an undershoot (air receiver pressure too low) when 
the load increases. 
The load valve is required in order to be able to start under no-load 
conditions, with a minimal rotational speed and a closed throttle valve. 
This load valve, which bridges the regulating valve, is opened when 
starting, so that the air receiver pressure can act directly on the 
throttle valve and the speed regulation. The air receiver pressure then 
amounts to for example 2 bar. 
When the compressor element is loaded, the load valve is shut and the 
regulating pressure is blown off via the blow-off holes, after which the 
above-described adjustment under load takes place. 
SUMMARY OF THE INVENTION 
The present invention provides a compressor unit which does not have the 
above-mentioned and other disadvantages, and which allows for a better 
adjustment, in particular with less or no deviation between the nominal 
pressure and the air receiver pressure under different loads, whereby the 
air receiver pressure does not rise so much when the load is lowered 
(smaller overshoot). 
This aim is reached according to the invention in that the regulating valve 
is an electropneumatic valve which is coupled to an electronic control, 
whereas a pressure gauge is connected to the compressed air receiver which 
transforms the pressure in the compressed air receiver in an electric 
signal, and in that a pressure sensor is installed in the compressed air 
pipe between the electropneumatic valve and the speed regulation and the 
throttle valve in order to feed back the regulating pressure exerted on 
this speed regulation and the throttle valve and to transform it in an 
electric signal, whereby the control is electrically connected to both 
pressure sensors and contains means to control the electropneumatic valve 
as a function of the measured air receiver pressure and the measured 
regulating pressure which has been fed back, as well as an electronically 
adjusted nominal pressure. 
Preferably, the control contains means to compare the measured air receiver 
pressure with the electronically adjusted nominal pressure, means to 
determine the required regulating pressure on the basis of the deviation 
of the air receiver pressure in relation to the nominal pressure, and 
means to compare this required regulating pressure with the measured 
regulating pressure, and to transmit a signal as a function of the result 
of this comparison for the control of the electropneumatic valve. 
The present invention also concerns a control device which is clearly 
designed to be used in a compressor unit according to any of the preceding 
embodiments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The compressor unit which is represented in FIG. 1 contains a compressor 
element 1 which is driven by a motor 3 via a transmission 2. 
This motor 3 is a combustion engine whose fuel supply 4 is connected to a 
pneumatic speed regulator 6 via a mechanical clutch 5. 
Onto the compressor element 1 is connected an inlet pipe 7 which opens into 
the environment via one or several filters 8. In this inlet pipe 7 is 
provided a pneumatically controlled throttle valve 9. 
This throttle valve 9 contains a housing 10, a part of which forms part of 
the inlet pipe 7, and a valve element 11 which can be shifted in said 
housing 10. 
This valve element 11 is pushed open by a spring 12. 
On the other side of the spring 12, between the valve element 11 and the 
housing 10, is formed a closed chamber 13 whose volume can vary. 
Naturally, the above-mentioned valve may also be of another type, and it 
may for example be a butterfly valve, whereby the valve element 11 is then 
rotatable instead of slidable. 
The compressor unit also contains a compressed air receiver 14 which 
simultaneously functions as an oil separator and which is connected to the 
compressor element 1 via the outlet pipe 15. The compressed air receiver 
14 is equipped with an outlet pipe 16 itself, in which is provided a valve 
17. 
The compressor unit further contains a control device 18 to control the 
speed regulator 6 and the throttle valve 9. 
This control device 18 mainly consists of an electropneumatic valve 19, an 
electronic control 20 connected onto it and two pressure sensors 21 and 22 
which measure a pressure and transform it in an electric signal and which 
are electrically connected to the electronic control 20 via lines 23 and 
24. An electronic signal can be added to the control 20, established or 
adjusted manually in an operating panel 25a. The value of this electronic 
signal corresponds to the nominal pressure. 
The electropneumatic valve 19 is provided in a compressed air pipe 26 which 
is connected to the compressed air receiver 14 on the one hand and which 
splits in two on the other hand and is connected to the chamber 13 of the 
throttle valve 9 and the cylinder of the suction mechanism which forms the 
speed regulator 6. 
The pressure sensor 22 is also provided in the compressed air pipe 26, 
between the electropneumatic valve 19 and the bifurcation of this 
compressed air pipe 26. 
The pressure sensor 21 is connected to the compressed air receiver 14 via a 
pipe 27. 
In the housing 10, downstream of the throttle valve 9, a blow-off valve 28 
has also been built in which is connected to the pipe 26 in the vicinity 
of the compressed air receiver 14 by means of a blow-off pipe 29. 
As is represented in FIG. 2, the electronic control 20 is a PLC 
(programmable logic controller) containing a comparing means 30 for 
comparing the pressure in the air receiver 14 to an adjusted nominal 
pressure. 
The pressure in the air receiver 14 measured by the pressure sensor 21 and 
the measured air receiver pressure is converted to an electronic signal 
and sent along line 23 to the comparing means 30 in the electronic control 
20. 
The equivalent electronic signal for the nominal pressure, adjusted 
manually by the means 25a, is conveyed through line 25 to the comparing 
means 30 in the electronic control 20. 
Comparing means 30 then compares the measured pressure in the air receiver 
14 with the adjusted nominal pressure so that a first difference in 
pressure signal is output to a transforming means 31. 
Transforming means 31 transforms the first difference in pressure signal to 
a required pressure regulating signal and transmits the required pressure 
regulating signal to a second comparing means 32 which compares the 
required pressure regulating signal, which corresponds to a required 
pressure, with the actual or measured regulating pressure detected in 
pressure gauge 22 which signal has been sent to second comparing means 32 
via line 24. 
In the second comparing means 32, a second difference in pressure is 
calculated which is the difference between the required pressure input 
from transferring means 31 and the actual pressure input from pressure 
gauge 22 via line 24 so that a second difference in pressure signal is 
output to transmitting means 33 which transmits a signal to the 
electropneumatic valve 19 as a result of the second calculated difference. 
The means 31 and 33 may be PID(Proportional integral derivative) controls, 
as is schematically represented in FIG. 2, whereby the PID control forming 
the means 31 provides for the master control and whereby the other PID 
control is a slave control. Both operate according to the conventional PID 
algorithm: 
##EQU1## 
whereby: R, TI and TD are the parameters of the PID control; X is the 
difference between the adjusted nominal pressure and the measured air 
receiver pressure at the master control, and the difference between the 
required regulating pressure and the measured regulating pressure at the 
slave control; 
K is a constant which is -1 at the master control and +1 at the slave 
control. 
On the outlet of the slave control and thus of the means 33, an offset can 
be added in 34 which coincides with the voltage at which the 
electropneumatic valve 19 is shut, for example 5 Volt. 
According to a variant, the function of the second PID control or slave 
control can be limited to a reinforcement of the outgoing signal of the 
master control. 
The working of the compressor unit and the control device 18 is as follows. 
The electronic control device 18 determines what voltage is applied to the 
electropneumatic valve 19 and thus the pass section of this 
electropneumatic valve 19 by means of the air receiver pressure measured 
by the pressure gauge 21, the fed-back regulating pressure measured by the 
pressure sensor 22 and the nominal pressure which has been manually 
adjusted in 25. 
As soon as the pressure in the compressed air receiver 14 exceeds the 
nominal pressure, the means 30 will transmit a signal to the means 31, 
which will generate a required regulating pressure as a function of the 
measured difference, which is then compared with the actual fed-back 
regulating pressure exerted on the speed regulator 6 and the throttle 
valve 9 by the means 32. As a function of the latter difference, the 
control 20 applies a voltage to the electropneumatic valve 19 which 
further opens the compressed air pipe 26, such that the throttle valve 9 
shuts further and the rotational speed of the motor 3 is reduced. 
At a regulating pressure of two bar, the rotational speed is minimal and 
the throttle valve 9 is shut completely. 
In an analogous manner, when the pressure in the compressed air receiver 14 
is lower than the nominal pressure, the means 30 will also transmit a 
signal to the means 31, and, as a function of the difference between the 
required regulating pressure generated by these means 31 and the fed-back 
regulating pressure, the electropneumatic valve 19 will further shut the 
compressed air pipe 26 via the control 20, as a result of which the 
throttle valve 9 opens further and the speed of the motor 3 increases. 
When the regulating pressure is zero bar, which implies that the pressure 
in the compressed air receiver 14 and thus in the outlet pipe 15 is equal 
to the nominal pressure, the rotational speed is maximal and the throttle 
valve 9 is entirely open. 
When the throttle valve 9 is entirely closed, the valve element 11 pushes 
the blow-off valve 28 open, so that air can escape from the compressed air 
receiver 14 via the blow-off pipe 29. 
When running idle, the nominal pressure is equal to zero and the control 20 
will place the electropneumatic valve 19 in this position whereby the part 
of the pipe 26 which is connected to the speed regulator 6 and the 
throttle valve 9 is connected to the compressed air receiver. 
The above-described control device 18 is more efficient than a strictly 
pneumatic control device. The deviation of the air receiver pressure in 
relation to the nominal pressure under different loads is excluded. When 
the load diminishes, the surplus or the temporary excess pressure in the 
compressed air receiver is lower. Also the stability is better. 
If no air is blown off for a longer while, the air receiver pressure can be 
automatically set at a lower value, which will result in fuel savings. 
The electronic control 20 must not necessarily be composed as described 
above. Instead of applying the above-described master/slave principle, one 
can also apply other control strategies such as a fuzzy logic or 
model-based control system. 
The invention is by no means restricted to the above-described embodiment 
represented in the accompanying drawings; on the contrary, such a 
compressor unit and control device can be made in all sorts of variants 
while still remaining within the scope of the invention.