Delta RMC151 User Manual download pdf (Page 81)

3 Controller Features

Good Practice

Do not install limit sensors such that they become active close to the end of travel, but

then become inactive at the end of travel, as shown below. In this case, commands in the

wrong direction will be allowed.

Poor Practice

3.2.7. Feedback Resolution

Feedback Resolution specifies the smallest increment that can be measured by the feedback

device (such as a position transducer or encoder). With analog feedback the resolution can

also be a function of the input circuit (Analog-to-Digital converter) on the controller.

Sometimes Resolution is referred to as Granularity.

Why is Feedback Resolution Important?

There are two main reasons why feedback resolution is important:

a. Positioning Accuracy

The controller cannot hold a position if it can’t accurately determine how close it is to

the desired position. It is generally necessary to have resolution that is several times

better than the desired accuracy. Notice, however, that high resolution is a

requirement for, but is not equivalent to high accuracy.

b. Quantization Noise

A less obvious, but equally important reason is quantization noise. Since velocity is the

change in position per unit of time, the velocity resolution is dependent on the position

resolution and the controller loop time. If the controller has a 1 millisecond loop time,

the velocity resolution will be 1000 times worse than the position resolution. The

acceleration measurement will be 1000 times worse than the velocity measurement.

On many systems the velocity measurement is used (with the differential gain) to

increase the system stability. Excessive quantizing noise on the velocity measurement

limits its effectiveness.

Maximum Feedback Resolution for the RMC

The maximum resolution available on the RMC for various feedback types is listed below:

• MDT

For MDT Start/Stop or PWM feedback with a typical transducer having a gradient (or

calibration constant) of 9 µs/in, the resolution obtained by the RMC70

MA Module is

0.0005 inch with 1 recirculation. The resolution obtained by the RMC150 MDT Module

is 0.001 inch with 1 recirculation.

• SSI

The SSI transducer sends the position information digitally, so the only limit is in the

transducer or encoder. Five micron resolution is common for linear SSI transducers,

and 8192 counts per turn is common for rotary encoders. Other resolutions are readily

available.

The RMC70 MA Module and RMC150 Universal I/O Module support 8 to 32 SSI bits.

The RMC150 SSI Module supports 8 to 31 bits. However, Delta recommends that

the SSI Counts value should not exceed 24 bits (16,777,216). See the Exceeding

24 Bits section below.

• Analog

The RMC70 AA, A2, and AP2 modules and the RMC150 H, G, and Universal I/O

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