Delta Tau GEO MACRO DRIVE User Manual download pdf (Page 81)

Geo MACRO Drive User and Reference Manual

Setting Up Primary Feedback 69

Setting up Sinusoidal Encoders

The Geo Drive with the Interpolator option accepts inputs from two sinusoidal or quasi-sinusoidal

encoders and provide encoder position data to the motion processor. This interpolator creates 4,096 steps

per sine-wave cycle.

The Geo MACRO drive so as to read the sinusoidal encoders needs the device control variable

MS<node>, MI101 (for the first channel #1) or MS<node>, MI102 (for the second channel #2) equal to 6.

Also the user can reverse the direction of the sinusoidal encoder by setting the MS<node>, MI910 equal

to 3 (ClockWise) or 7 (CounterClockWise)

If the direction decode variable is changed, the user must save the setting, MSSAVE{node} and reset the

card MS$$${node} before the fractional direction sense matches.

Note

Home Capture with high resolution feedback requires bit 11 of Ixx24 on the Turbo

PMAC to be set to one (value $800 or 2,048). Bit 12 of Ixx24 enables the Sub-

count Capture while the Geo MACRO’s: MS<node>, MI7mn9 is set to one.

Principle of PMAC Interpolation Operation

Analog

Photo

Current

Encoder

Controller

Differential

Amplifier

Sin / Cos

Signals

Comparator

1 - Bit A/D

n-bit

A/D

n-bit

A/D

Decoder /

Counter

The sine and cosine signals from the encoder are processed in two ways in the Geo Drive board (see

above diagram). First, they are sent through comparators that square up the signals into digital quadrature

and sent into the quadrature decoding and counting circuit of the Servo IC on the Geo Drive. The units of

the hardware counter, which are called hardware counts, are thus ¼ of a line. For most users, this fact is

an intermediate value, an internal detail that does not concern them. However, this is important in two

cases. First, if the sinusoidal encoder is used for PMAC-based brushless-motor commutation, the

hardware counter, not the fully interpolated position value, will be used for the commutation position

feedback. Therefore, the units of Ixx71 will be hardware counts.

Second, if the hardware position-compare circuits in the Servo IC are used, the units of the compare

scaling issues are handled automatically through the move-until-trigger constructs).

The second, parallel, processing of the sine and cosine signals is through analog-to-digital converters,

which produce numbers proportional to the input voltages. These numbers are used to calculate

mathematically an arctangent value that represents the location within a single line. This is calculated to

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Delta Tau GEO MACRO DRIVE User Manual Page 81

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