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# Non-contact Spindle Resistance Measurement

IP.com Disclosure Number: IPCOM000108801D
Original Publication Date: 1992-Jun-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 6 page(s) / 221K

IBM

## Related People

Cossette, LA: AUTHOR [+2]

## Abstract

The electrical resistance of a rotating spindle can be measured without making physical contact to the rotating portion of the assembly. This is accomplished by completing the measurement circuit with a capacitance formed between the rotating spindle and a simple probe. Once this complete measurement circuit has been formed, there are a number of ways to determine the spindle resistance.

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This is the abbreviated version, containing approximately 34% of the total text.

Non-contact Spindle Resistance Measurement

The electrical resistance of a rotating spindle can be
measured without making physical contact to the rotating portion of
the assembly.  This is accomplished by completing the measurement
circuit with a capacitance formed between the rotating spindle and a
simple probe.  Once this complete measurement circuit has been
formed, there are a number of ways to determine the spindle
resistance.

The electrical resistance of a rotating spindle can be measured
without making physical contact to the rotating portion of the
assembly.  The non-contact characteristic is achieved via the
formation of a capacitance between the common surfaces of the
rotating portion of the spindle and a stationary probe.  In the
example presented in Fig. 1, the probe is a metal comb in close (but
not contacting) proximity to the disks of a rotating disk
pack/spindle assembly.  The electrical equivalent model for Fig. 1 is
shown in Fig. 2.

In Fig. 2, the unknown spindle resistance is R, and the probe/
spindle capacitance is C1 (which can be easily characterized).  C2 is
an unknown internal capacitance of the spindle, and r is a sense
resistor that can be used in solving for the unknown resistance R.  A
transfer function can be analytically computed to determine the
resistance R of the spindle.  Several different circuits/approaches
can be used to determine R since a measurement circuit is completed
by forming C1, but only one is presented here.

For the circuit detailed in Fig. 2, a sinusoidal input of the
following form can be chosen:
Equation 1:  V in(t) = A sin(wt + theta)
where A is the signal amplitude, w is the radian frequency, and theta
is the input phase.  This signal drives the circuitry and is the
voltage source.  The spindle/probe impedance Z (see Fig. 2) can be
expressed as:
Equation 2:  Z = 1/sC1 + R/(1+sRC2) = {1 + sR(C1+C2) / sC1(1+sRC2)}
The transfer function that relates the Laplace transforms of the
output voltage across the sense resistor r and the source voltage of
Equation 1 is as follows:
Equation 3:
{V out(s) / V in(s} = {r/(r+Z)} =
{srC1(1+sRC2) / srC1(1+sRC2)+1+sR(C1+C2)} =
{s**2 + s(1/RC2) / s**2 + s(rC1+RC1+RC2) / rRC1C2 + 1/rRC1C2}
From Equation 3, R can be computed by measuring the phase, gain, or
both phase and gain values of the transfer function.

It is important that a spindle ground path exists for the
in-hub spindle motors that are used in our current small form factor
direct-access storage device (DASD) products. Since the presence of
such a ground path is necessary for the proper function and long-term
reliability of these devices, it is important to have a process to
measure this parameter in a manufacturing environment.

In a manufacturing line, the luxury of lab equipment and time
does not exist.  Further, it is critical for the measurement process
not to damage or contaminate the devices under test.  The cu...