Electromagnet drop time detection method

Electromagnetic actuator release characteristics are evaluated by sampling at high frequency the voltage waveform resulting when the magnet coil is deenergized. The samples are converted to a digitally coded form and stored in sequence. The stored values are then examined in reverse sequence to identify the first examined sampling interval in which the sample value exceeds a predetermined characteristic release level for the electromagnetic actuator. Finally, the release time is determined by calculating the time after energization corresponding to the sampling interval that has been identified as the release interval.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The subject invention relates generally to diagnostic equipment for 
determining the quality of electromagnets and, more specifically, to 
diagnostic equipment for determining drop time for the armature of an 
electromagnetic actuator. 
2. Art Discussion 
In devices such as printers that are increasingly making use of electronic 
logic components, actuation decisions represented by electrical signals 
are often translated to mechanical motions by armatures driven by 
electromagnets. Where high speed operation is desired, electromagnet 
response characteristics become critical to correct operation, yet visual 
or tactile detection of out of specification performance is often 
impossible. Detection becomes especially difficult if a quality assessment 
is desired without removal from a larger mechanism, such as a printer. 
To detect out of specification characteristics for electromagnetic 
actuators, it is known to use motion detectors relying on light beams or 
physical contact, but such detectors typically require removal of the 
actuator from a larger assembly. Measurement of the drive waveform for an 
electromagnet is known for determining drop time, but, typically, the 
overall waveform is either filtered to remove effects of oscillations or a 
point well beyond the oscillatory portion of the waveform is studied in 
detecting some measure of performance not directly correlated to drop 
time. 
BRIEF SUMMARY OF THE INVENTION 
The waveform produced on deenergization of an electromagnetic actuator is 
analyzed in reverse time order to identify a voltage level corresponding 
to armature release or drop. Initially, the waveform is repeatedly sampled 
over a period extending beyond an empirical worst case drop time, and the 
samples are stored in sequence. By accessing and analyzing the samples in 
reverse sequential order, the ringing that tends to mask the drop point is 
avoided. This avoidance is achieved without any need to closely estimate 
the time when ringing stops, and the well behaved decay associated with 
armature release has started. 
As a further aspect of the invention, the magnet voltage is scaled and 
clipped to remain within the effective range of an analog-to-digital 
converter that is employed for sampling. The clip level is selected to 
remove only voltage spikes that occur during ringing and do not contain 
useful information regarding drop time.

DETAILED DESCRIPTION REGARDING A PRESENTLY PREFERRED IMPLEMENTATION FOR THE 
INVENTION 
Referring to FIG. 1, a voltage waveform for deenergization of an 
electromagnetic actuator (see element 50, FIG. 2, discussed below) 
typically exhibits a period of ringing (transient oscillations) during 
which the voltage level oscillates unpredictably with a smooth decay 
occurring in the range where the armature (element 54, FIG. 2) separates 
from a core (element 52, FIG. 2). It has been found that for a specific 
design of electromagnetic actuator 50, the voltage at the instant the 
separation (or drop) occurs is essentially consistent, even the waveforms 
may vary widely and the voltage level (DLEVEL) for the drop may be 
achieved repeatedly during the transient oscillations that occur incident 
to deenergization. (A representative waveform is indicated in FIG. 1, but 
note that more or less severe ringing may occur and the tendency for 
ringing varies from device to device even assumming the same design and 
careful manufacturing.) 
According to the invention, samples of the voltage waveform are repeatedly 
taken at high sampling frequency. No analysis is undertaken during 
sampling to avoid delay that would increase the sampling period (denoted 
TINC). Sampling occurs over a period of time that would, for a worst case, 
include the drop point (the time of armature separation). The samples, as 
is described below, are then analyzed in reverse time order to identify 
the drop point. 
Referring to FIG. 2, a preferred diagnostic system for implementing the 
invention includes a processor 10 with a connected read/write storage 
section (RAM) 12. Connected to the processor 10 through data, address and 
control busses 14, 16 and 18, respectively, are an interfacing device 20 
and a read-only-storage (ROS) 22 that is structured to include recallable 
processor instructions and constant data. 
The interface device 20 serves to translate signals from a form compatable 
to the processor 10 to forms useful for various other devices (discussed 
below). Within the interface device 20 is a data register 23, an address 
decoder 24 and a set of latches including latches 260, 262, 264 and 266. 
The latch 260 is associated with a particular address A(1) by the decoder 
24 and is connected to enable a magnet driver 30. 
From the latch 262, signals are applied to the read/convert terminal of an 
analog-to-digital (A/D) converter 32. Data from the A/D converter 32 is 
applied to a data bus 36, in response to a pulse signal at the 
read/convert terminal, for transmission to the data register 23, this in 
response to an address signal A(2) at the decoder 24. The magnet driver 30 
is connected to ground, when enabled, one side of a coil 48 of the 
electromagnetic actuator 50 to cause the armature 54 to move against the 
core 52 (as shown in phantom). From a node 31 connecting the magnet driver 
30 to the coil 48, a magnet voltage signal S.sub.M is applied to an 
attenuator 34. 
The signal attenuator 34 (see FIG. 4) is preferably a resistor voltage 
divider including a resistor 500 which receives the signal S.sub.M and a 
resistor 502 which is connected to ground. At the node 504 between 
resistors 500 and 502, the attenuated signal S'.sub.M is produced. A diode 
506 connected to a voltage source clamps the voltage at the node 504. The 
effect of the attenuator 34 is to scale the voltage S.sub.M to the 
conversion range of the A/D converter 32. By clamping the voltage S'.sub.M 
with the diode 506, voltage excursions during ringing that extend well 
above the drop voltage level are clipped for the attenuated voltage 
S'.sub.M. The clip level is preferably chosen to be slightly above the 
value for the drop level DLEVEL as adjusted by the scaling factor imposed 
by the resistors 500 and 502. The effect of such attenuation is to 
increase voltage conversion accuracy without a loss of significant 
information. 
The signal S'.sub.M after passing through the buffer amplifier 37 is 
applied to the A/D converter 32 which is connected to a data bus 36. 
Latches 264 and 266 are associated by the decoder 24 with specific 
addresses A.sub.4 and A.sub.5 and are connected to energize "good" and 
"bad" indicators 38 and 40, respectively. It is preferable for a printer 
environment, however, to merely store respective messages in the RAM 12 
for selective automatic printout, as is a well known technique for 
providing diagnostic information. 
To provide for an operator initiated signal to start a test sequence, a 
switch 42 is connected to control bus 18 through a buffer 44. In a 
printer, this switch 42 might be a keyboard switch or switch combination. 
To determine the quality of individual electromagnetic actuators 50, the 
expected drop time voltage level DLEVEL is established by physical 
measurements on sample devices, for example, using optical detectors or 
mechanical detectors (not shown). For such benchmark tests, a lab 
environment may be used, but, it should be appreciated, that a special 
environment would not generally be required for the diagnostic tests 
according to the invention. Moreover, the lower limit (LVAL) on drop time 
and the upper limit (HVAL) which serve to specify the range of acceptable 
performance are predefined according to host system requirements prior to 
the diagnostic testing. A worst value for the drop time (TMAX) is also 
identified and is used in conjunction with the sampling period (TINC) 
established by the processor 10 to determine the number of samples (NSAMP) 
to be stored. A portion 46 of the RAM 12 that has sequential addresses 
identifiable with the index N is used for storing the samples. 
Referring to FIG. 3, logic for effecting the method of the invention is 
incorporated in the structures of the ROS 14 and the processor 10. The 
logic is entered (block 99), for example, from a polling loop as is a well 
known approach to prioritizing service requests to a processor such as the 
processor 10. 
An initial decision construct (block 100) detects whether or not the switch 
42 is closed to request a test sequence. If so, the latch 260 is set 
(block 102) to cause energization of the electromagnetic actuator 50 by 
magnet driver 30. The processor then idles in a loop (block 104) a 
predetermined number (D) of processor cycles which allows a full magnetic 
field buildup. 
After the delay, the electromagnetic actuator 50 is deenergized by 
resetting (block 106) the latch 260. A sample index is set to 1 (block 
108) to prepare for the taking of voltage samples and, in a loop including 
blocks 110 to 116, a preselected number (NSAMP) of sampling cycles are 
performed. At block 110, the latch 262 is pulsed to cause the A/D 
converter 32 to convert the incoming signal to digital form and place the 
digital code corresponding to the last sample on the data bus 36. The data 
register 23 is then read to provide sample values which are stored 
sequentially in a section 46 of the RAM 12. Indirect referencing is 
indicated using the variable name SAMP subscripted by the indexing 
variable N. The indexing variable N is incremented (block 114) for each 
pass through the loop and an exit is achieved when a predetermined number 
(NSAMP) of passes is completed (block 116). 
With the sampling completed, a further indexing variable (INDEX) is 
initialized (block 118) relative to the number of samples and a loop is 
entered (blocks 120 and 122) for examining the samples in reverse sequence 
to locate the first to exceed the empirical drop voltage level (DLEVEL). 
The index variable INDEX, upon exiting the loop from block 122, indicates 
the sample for which the relationship relative to DLEVEL shifts and the 
test of block 122 is satisfied. 
The time (TIME) for the identified sample is determined by multiplying the 
index number for the identified sample by the sample period TINC (block 
124). The release time TIME is then compared (block 126) with a predefined 
set of acceptable range extremes (LVAL and HVAL) to determine whether or 
not the electromagnetic actuator 50 is out of specification. If out of 
specification, an operator perceivable indication is provided, for 
example, by actuating (block 128) the indicator 40. For a satisfactory 
test at block 126, the indicator 38 is actuated (block 130). As was 
mentioned above, for detection performed in a host printer (not shown), 
the printer is preferably activated by the test logic to indicate the test 
results for block 126 by selectively printing appropriate messages that 
are stored in the ROS 22 (FIG. 2). 
The invention has been described with reference to a presently preferred 
implementation thereof. It will be appreciated that variations and 
modifications within the scope of the claimed invention will be suggested 
to those skilled in the art.