300 SERIES USER GUIDE
26
Tachometer Scaling
As delivered, a +/-10V reference signal will drive the motor to a speed at which the
tachometer produces a -/+8V signal, regardless of RPM. Tachometer scaling refers to the
selection of components on the J17 header so that a particular signal at the reference
inputs will drive a motor to a particular RPM. Before you begin, you must know the
RPM of the motor when your control system is delivering its' maximum signal to the
amplifier. In addition, you must know the voltage coefficient of the tachometer (usually
in volts per thousand RPM or V/KRPM) and the amplitude of the signal the control
system puts out at maximum. The figure below shows the relation of the components that
are used in tachometer scaling:
R1, R2, and R11 are found on the
J17 component header.
Stock values are:
R1 = 10K
R2 = 40K
R11 = 50K
Fig. 8
We can treat R1 and R2 as a single resistor, and compute the value of this resistor while
keeping R11 at the stock value of 50K. Let V
tach
be the tachometer voltage at
maximum RPM, and let V
in
be the reference voltage at maximum RPM. The value of
R1+R2 will now be seen as:
()
.
RR
K Ktach KRPM
Vref
12
50
08
+=
××
×
Fig. 9
Suppose that you have a motor that is to run at 4000 RPM ( 4 KRPM) with a reference
signal of +/-8V, and a tachometer with Ktach = 7 V/KRPM. We can now compute
(R1+R2) as:
()
/
.
RR
K V KRPM KRPM
V
12
50 7 4
08 8
+=
××
×
Fig. 10
This will give us 218.8K as the value of R1+R2. This is close to 220K, a stock value.
Now R2 = 40K, so this leaves R1 = 180K, also a stock value. Change the component R1
on header J17 to a 180K resistor to complete the procedure.
Note that tachometer scaling is a DC gain setting process. Additional compensation may
be required (i.e. lead networks in the tachometer input circuit) to optimize AC operation
with a tachometer. Component C3 on the J17 header can be used in conjunction with R1
and R2 to create such networks. Calculation of these values is beyond the scope of this
manual (see Reference section in appendix).
TACH
REF
INPUTS
R1
R2
R11
TO
PWM
STAGE
SERVO
PREAMP
X0.8
GAIN
-
+
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