Control system for vibration testing apparatus

A load support servomechanism circuit controls the armature of a shaker apparatus. The circuit includes a position sensor, peak detectors connected to outputs of the position sensor, an A/D converter connected to outputs of the peak detectors through a switching bank, a power supply and a microprocessor which provides clock, read, write and ready signals and which automatically controls the position of the armature through solenoids operating on an exhaust valve and a pressure valve. Manual operation of the shaker armature is also provided.

TECHNICAL FIELD 
The present invention relates to a control system for regulating the 
position of the movable element of vibration testing apparatus. 
BACKGROUND ART 
There are a considerable number of vibration testing apparatus that are 
well-known in the prior art. Such apparatus is used to mechanically shake 
an item for the purpose of diagnostically testing responses to certain 
driving forces. The item is physically attached to a moving portion of the 
apparatus and when the apparatus is activated, the item is subjected to a 
variety of test conditions. The moving portion of the vibration testing 
apparatus is typically driven by a force which may be continuous, cyclical 
or impulsed. One class of such apparatus employs the use of an 
electromagnetic field between field and armature windings. Various driving 
signals are impressed across the armature winding to control the movement 
of the armature. For convenience, the movable member of the vibration 
testing apparatus may be referred to from time to time hereinafter simply 
as the armature. 
In many of the prior art vibration testing apparatus springs or static air 
pressure systems were relied upon to center the armature between its axial 
displacement limits. However, such systems are proving inadequate to meet 
the present demands for heavier pay loads, increased armature 
displacement, and unsymmetrical shock and random wave forms. 
DISCLOSURE OF INVENTION 
It is an object of this invention to provide a simple and reliable control 
system for automatically maintaining the neutral operating position of the 
moving member of a vibration testing apparatus. 
It is another objective of the present invention to allow an operator to 
preselect, or even change during operation, a neutral operating position 
of the moving member of the vibration testing apparatus. 
It is a further object of the invention to provide a closed loop control 
system utilizing an optical array displacement transducer and an active 
air system to establish the neutral position of the armature in the 
absence of spring flexors or the like utilized in the prior art. 
These and other objects of the invention will become apparent from the 
following description. 
The control system of the present invention comprises an armature position 
sensor. Preferably the position sensor includes two physically opposed 
rows of photo-transistors and infra-red light emitting diodes positioned 
such that an increasing number of photo-transistors are driven into 
conductive saturation as the armature of the vibration testing apparatus 
is raised. The current from the position sensor is converted to a voltage 
proportional to armature position. The output of this converter is applied 
to a pair of peak detectors operative to detect positive and negative 
armature displacement peaks respectively. Under control of a 
microprocessor, the voltage output from these detectors is applied via an 
analog switch for application to an analog to digital (A/D) converter for 
digital scaling. 
When a position correction is required, the microprocessor causes an 
appropriate valve to be opened by energizing respective air valve 
solenoids. The opening of a pressure valve operates to raise the armature 
while the opening of an exhaust valve causes the armature to be lowered. 
A suitable power supply means supplies both the AC and DC voltages 
necessary to operate the system.

BEST MODE FOR CARRYING OUT THE INVENTION 
The general method of operation of the load support control system of this 
invention can be readily understood by reference to FIG. 1. As shown in 
FIG. 1, the control system includes means 14 for sensing the position of 
the load support means which is connected with the movable member, such as 
the armature, of a vibration testing apparatus 10. Vibration testing 
apparatus 10 is provided with a pneumatic cylinder means 12 connected to 
the armature. Preferably, position sensor means 14 is an optical sensor 
means employing a plurality of photo-transistors and light emitting 
diodes. The output of position sensing means 14 is a current signal 
proportional to the position of the armature. 
The signal from the position sensor 14 is amplified, has noise removed and 
is converted from a current to a voltage by an amplifier/filter/converter 
means 16. The amplified, cleaned up and converted output of 
amplifier/filter/converter means 16 is applied to a lower peak detector 18 
and a higher peak detector 20. Lower peak detector 18 provides an output 
which is proportional to the peak of the lower excursion of the armature 
while higher peak detector 20 provides an output which is proportional to 
the peak of the high excursion of the armature. Accordingly, the two 
outputs of peak detectors 18 and 20 are voltages which represent the 
highest and lowest peak displacement of the armature. 
The outputs of the detectors 18 and 20 are conveyed to a suitable analog 
switch 22 the output of which is applied to an analog to digital (A/D) 
converter 24, which may be a conventional 8-bit converter. The A/D 
converter converts the analog voltage signal proportional to armature 
position to a digital number, which now represents the position of the 
armature in a form to be used by microprocessor 26, or other suitable 
computer means. 
Microprocessor 26 can be a generic microprocessor with RAM outboard. In the 
particular arrangement shown and described in FIG. 2, an 8-bit, single 
chip microprocessor is used. With the various algorithms, system time 
constants, control characteristics and the like maintained within the 
microprocessors internal ROM. During operation the microprocessor 26 
controls the operation of the control system, as well as, controlling 
access to the look-up table and making decisions about what to do with the 
information, and furnishing correct information is used to energize the 
solenoid coil of the proper valve (pressure or exhaust) to control the 
position of the armature. Thus, the control system operates to 
automatically provide timely and precise control of the armature position. 
The analog switch 22 is controlled by the microprocessor 26 to select 
either the higher or lower peak detector output to be routed to the A/D 
converter 24. For example, microprocessor 26 can read the lower peak 
detector by first turning on analog switch 22 to route that output to the 
A/D converter 24 which converts it to a number. This number is read by 
microprocessor 26 and assumes it to be the lower peak. The microprocessor 
will then cause switch 22 to select the output of higher peak detector 20 
to be routed to the A/D converter 24. The microprocessor then reads that 
number and assumes it to be the upper peak. Thus, the microprocessor has a 
number that represents the lower peak and a number which represents the 
higher or upper peak. It goes through a mathematical alogorithm to 
determine the average position based on the lower and higher peak 
excursions and compares that with a reference number, illustrated by block 
28, representative of the desired armature position. The reference number 
can be provided in any suitable manner such as manually, in firmware or in 
software. In the arrangement described in more detail herein the reference 
number is provided manually by a thumbwheel switch which allows the 
operator to change the reference as desired. 
The control system is also provided with a look-up table 30. Both the 
reference 28 and the look-up table 30 are shown as separate blocks, but 
either or both can be programmed in as firmware or in RAM as software. 
The microprocessor 26 provides a signal to energize coil 36 to open 
pressure valve 32 if it has determined that the armature is too low. 
Alternatively, microprocessor 26 provides a signal to energize coil 38 to 
open exhaust valve 34 if it has determined that the armature is too high. 
For example, when pressure valve 32 is opened the high pressure (80 psi) 
air is applied to the pneumatic cylinder means 12 which caused the 
armature to be raised. Similarly, when exhaust valve 34 is opened the 
pneumatic cylinder is exhausted and the armature is allowed to fall to a 
lower position. 
FIG. 2 is a schematic circuit diagram of one embodiment of the system of 
this invention. As shown in FIG. 2, position sensor 14 comprises a 
plurality of light emitting diodes (LED) 52, 53, 54, 55, etc. respectively 
connected to resistors 56, 57, 58, 59, etc. provide light to a series of 
photo-transistors 60, 61, 62, 63, 64, etc. such that an increasing number 
of photo-transistors are driven into conductive saturation as the armature 
of the vibration testing apparatus is raised. The aforementioned detector 
transistors basically run in a "digital mode," i.e., either "on" or "off," 
to avoid drift with temperature, supply voltage, etc. Fringing of light 
around the end of a sensor blade (not shown) causes a finite voltage 
transition and therefore a smoother output current transfer function than 
would be realized without it. The output of the position sensor 14 is 
essentially a current proportional to the armature displacement. This 
current passes through resistors 65, 66, 67, 68, 69, etc., appears on line 
71 and is conducted to the negative terminal of operational amplifier 72. 
Amplifier 72, together with its associated resistors 73, 74, 75, and 76 
acts as a buffer where the signal is converted to a voltage proportional 
to armature position. For example, an output voltage of about 5 volts can 
indicate a completely extended armature and nearly 0 volts can indicate a 
completely lowered armature. Potentiometer 77 may be used to establish an 
offset voltage of the amplifier. It may be adjusted, for example, to yield 
a voltage of 2.5.+-.0.1 volts DC when the armature is mechanically 
constrained to the center of its operating range within 0.032 inches. The 
output of amplifier 72 is then applied to the active filter formed by 
capacitor 79, resistor 80, operational amplifier 81 and capacitor 82. The 
active filter is used to reduce noise by operating as a unity pass band, 
low pass filter. Potentiometer 83 acts through resistor 84 to establish 
the correct system gain to connect the circuitry to the exact sensor used 
on the vibration testing apparatus. The voltage generated by the active 
filter can be measured at terminal 85. 
A pair of peak detectors 18 and 20 comprising operational amplifiers 86 and 
87 effect positive and negative armature displacement peaks, respectively. 
The positive input terminal of operational amplifier 86 receives a 
displacement signal from terminal 90 through resistor 88. Likewise, the 
positive terminals of both amplifiers 86 and 87 receive the signal from 
the output terminal of operational amplifier 81. The time constant for 
each of the respective operational amplifiers 86 and 87 is determined by 
resistor 92, 93, and capacitor 94; and by resistor 95, 96 and capacitor 
97, respectively. The peak voltages detected lie between 0 and 5 volts and 
are buffered by operational amplifiers 98 and 99. It should also be noted 
that the positive input terminal of operational amplifier 99 is connected 
to ground through resistor 101. Operational amplifier 98 is connected to 
ground through resistor 93 and also through capacitor 94. Terminal 102 
provides a positive 12 volt input to the operational amplifier 99. The 
respective output voltages of the operational amplifiers 98 and 99 are 
conducted to an analog switch 22 comprising switches 103 and 104. Switches 
103 and 104 have a common connection to ground through resistor 105. 
Under the direct control of microprocessor 26, the output voltage of 
switches 103 and 104 are individually switches to an A/D input buffer 
formed by operational amplifier 108 and transistor 109. The negative 
terminal of operational amplifier 108, as well as the emitter of 
transistor 109, are connected to ground through resistor 112. There is an 
additional ground connection for the operational amplifier 108 and also a 
connection to ground through capacitor 114. There is also a direct 12 
volts DC connection to operational amplifier 108. The output voltage from 
the aforementioned A/D input buffer is conveyed to an eight bit analog to 
digital converter 24. The A/D converter 24 has a connection to a 5 volt DC 
source and a connection to ground through capacitor 115. The input signal 
to analog to digital converter 24 is scaled to provide a digital signal 
between 0 and 5 volts. After the digital conversion, the data is 
transferred to the microprocessor 26 via an eight bit parallel bus 
indicated generally as lines 116. The A/C clock requirements are supplied 
by the microprocessor, and read, write, and ready functions are also 
directly wired from the microprocessor 26 to the A/D converter 24 as 
shown. 
When a position correction is required, the microprocessor 26 opens the 
appropriate valve 32 or 34 (exhaust valve 34 to lower the armature, 
pressure valve 32 to raise the armature). For example, by lowering the 
voltage on terminal E below one volt exhaust valve 34 is caused to open, 
while lowering the voltage on terminal P pressure valve 32 is caused to 
open. The air valve solenoid coils 36 and 38 operate at 5 volts to open 
the pressure or exhaust valves 32 or 34, respectively. The current 
required for solenoid coils 36 and 38 is respectively supplied by 
transistors 120 and 121, in conjunction with diode 122 and 123 and 
switching bus 125. Switching bus 125 also has a 115 volt AC input 126 as 
shown. 
The action of the control circuit is complex and involves long 
time-constant wait states. The basic operation achieves the objective of 
having the appropriate valve opened while the armature position is 
retreating from the desired operating point. When the retreat has been 
arrested, a correction is calculated and the appropriate valve is held 
open long enough to correct for the armature position error (25 
milliseconds to 1 second). This is followed by a relatively long (1-10 
seconds) wait state to allow for the armature to react to the changed air 
pressure through the amplifier damping. A switch 130 is connected to the 
valves 32 and 34 and controls the system by determining whether operation 
is to be automatic or manual. If the operation is to be automatic, a 16 
position push-wheel switch 135 located on the cover of the electrical box 
controls the armature position reference. Switch 135 has four digital 
outputs and thus is able to provide digital inputs representing 16 
positions to microprocessor 26. The connections to the microprocessor 26 
also have a DC input from terminal 136 through resistors 137-140. Relative 
operating ranges of the shaker are represented by the sixteen positions 
available on this switch and the control system will cause the armature to 
be moved to the position indicated on the switch. The positions are 
represented by hexidecimal digits (0-9, A-F) with the lower numbers 
representing lower armature positions. A switch 130 can be moved at any 
time during operation at the discretion of the operator to change the 
system from manual to automatic operation and vice versa. If the manual 
switch 130 is activated, the control system is disabled and the then 
current position of the armature is maintained but not controlled and 
therefore subject to drift due to temperature, load variation, etc. In 
this mode of operation, push buttons located on the air valve assembly can 
be manually actuated to raise or lower the armature. These push buttons 
can be manually activated at any time but will be countered by the control 
system if it is in the active or automatic mode. 
The foregoing described control system for vibration testing apparatus 
allows for preselection (as well as a change during operation) of the 
neutral operating position of the armature of the vibration testing 
apparatus. The armature may be readily biased to match any particular 
fixture height as well as to allow for unsymmetrical shock pulses or the 
like. That is, the system will automatically bias the armature to 
compensate for deflection due to increased or decreased load during 
operation as well as unsymmetrical waveform and the like, to maintain the 
peak excursions of the armature displacement within desired operating 
limits. 
Although a prferred embodiment has been disclosed and described in detail 
herein, it should be understood that the scope of this invention is to be 
determined by the appended claims.