JVL MIS23 Series, User Manual

Page: 22 / 347

20 JVL Industri Elektronik A/S - User Manual - Integrated Stepper Motors MIS23x, 34x, 43x
2.3 Analogue Inputs
2.3.2 Analogue input filters
The Intelligent MIS motors have 8 general-purpose I/Os, that can be used as both digital
inputs, digital outputs and analogue inputs. When an I/O is configured to be an input, it
simultaneously has both a digital value (high or low) and an analogue value in the range
0.00 to 5.00 Volts. Input voltages higher than 5.0 Volts will be internally limited and read
as 5.00 Volts.
The inputs use a resolution of 10 bits, which means that in the raw motor units a value
of 5.00 Volts reads out as the value 1023. This gives a resolution of 5.00/1023 = 4.8876
mV per count.
The eight values from the analogue inputs are maintained by the SMC75 firmware in the
registers 89...96 as raw, unfiltered values with the fastest possible update frequency, and
additionally in the registers 81...88 as filtered values. The firmware does not use any of
the values for dedicated functions. It is always up to the program in the motor to read
and use the values.
The analogue filtered values are typically used to suppress general noise or to define how
quickly the input value is allowed to change, or in some cases to limit the input voltage
range. A typical example is an analogue input that is connected to a manually controlled
potentiometer, so an operator can regulate the speed of the machine by turning a knob.
In many environments, this setup is subject to noise, which could make the motor run
unevenly, and cause too sharp accelerations or decelerations when the knob is turned.
The filter functions supported in the SMC75 firmware always use three different steps.
Confidence check
First the raw input value is compared to two Confidence limits: Confidence Min and
Confidence Max. If the new value is either smaller than the Confidence Min limit or larger
then the Confidence Max limit, it is simply discarded (not used at all), and the value in its
associated register is unchanged. This is done to eliminate noise spikes. Confidence limits
can only be used if not all of the measurement range is used. Values of 0 for Confidence
Min and 1023 for Confidence Max will effectively disable the confidence limits.
Slope limitation
After a new sample has passed the Confidence limit checks, its value is compared with
the last filtered value in its associated register. If the difference between the old and the
new value is larger than the Max Slope Limit, the new value is modified to be exactly the
old value plus or minus the Max Slope Limit. This limits the speed of change on the signal.
Since the samples come at fixed intervals of 10 mS, it is easy to determine the number of
Volts per millisecond. A value of 1023 will effectively disable slope limitation.
Filtering
After a new sample has both passed the confidence limits checks and has been validated
with respect to the slope limitation, it is combined with the last filtered value by taking a
part of the new sample and a part of the old filtered value, adding them together and
writing the result back to the final destination register – one of the registers 81...88. For
instance a filter value of 14 would take 14/64 of the new sample plus 50/64 of the old
value. A filter of 64 would simply copy the new sample to the rule, thus disabling the fil-
tering. This completes the filtering of the analogue inputs.