eNotes: Mechatronics and Controls

36.1 SPECIAL I/O MODULES

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Counters,

each card will have 1 to 16 counters generally.

typical sample speeds 200KHz

often allow count up/down

the counter can be set to zero, or up/down, or gating can occur with an external input.

High Speed Counter - When pulses are too fast to be counted during normal PLC ladder scans, a special counter can be used that will keep track of the pulses.

Position controller - A card that will drive a motor (servo motor or stepper motor), and use feedback of the motor position to increase accuracy (feedback is optional with stepper motors).

Voice Recognition/Speech - In some cases verbal I/O can be useful. Speech recognition methods are still very limited, the user must control their speech, and background noise causes problems.

Thermocouple - Thermocouples can be used to measure temperature, but these low voltage devices require sensitive electronics to get accurate temperature readings.

Analog Input/Output - These cards measure voltages in various ranges, and allow monitoring of continuous processes. These cards can also output analog voltages to help control external processes, etc.

PID modules

There are 2 types of PID modules. In the first the CPU does the calculation, in the second, a second controller card does the calculation.

- when the CPU does the calculation the PID loop is slower.

- when a specialized card controls the PID loop, it is faster, but it costs less.

Typical applications - positioning workpieces.

Programmers - There are a few basic types of programmers in use. These tend to fall into 3 categories,

1. Hand held units (or integrated) - Allow programming of PLC using a calculator type interface. Often done using mnemonics.

2. Specialized programming units - Effectively a portable computer that allows graphical editing of the ladder logic, and fast uploading/downloading/monitoring of the PLC.

3. PLC Software for Personal Computers - Similar to the specialized programming units, but the software runs on a multi-use, user supplied computer. This approach is typically preferred over 2.

Man Machine Interface - A-B/Siemens/Omron/Modicon/etc offer human interface systems. The user can use,

touch screens

screen and buttons

LCD/LED and buttons

keypad to talk to PLC

PLC CPU's - A wide variety of CPU's are available, and can often be used interchangeably in the rack systems. the basic formula is price/performance. The table below compares a few CPU units in various criteria.

Specialty cards for IBM PC interface.

- Siemens/Allen-Bradley/ etc have cards that fit into IBM buses, and will communicate with PLC's.

IBM PC computer cards - An IBM compatible computer card that plugs into a PLC bus, and allows use of common software

Siemens CP580 the Simatic AT

- 1 com port (RS-232C)

- 1 serial port (?)

- 1 RS-422 serial port

- RGB monitor driver (VGA)

- keyboard

- 3.5" disk

- TTY interface

- 9 pin RS-232C mouse

Diagnostic Modules

Plug in and all they do is watch for trouble.

Short distance RF transmitters,

e.g., Omron V600/V620 ID system

a basic method for transmission of a text based message

tags on parts carry message

transceivers receive and transmit changes.

ID Tags - Special "tags" can be attached to products, and as they pass within range of pickup sensors, they transmit an ID number, or a packet of data. This data can then be used, updated, and rewritten to the tags by the PLC

Ethernet/modem cards for communication with remote computers. This is now an option on many CPUs

Thumb Wheel Module - this allows numbers to be dialed in on wheels with digits from 0 to 9. The values are read as BCD.

BCD input/output module - allows numbers to be output/input in BCD.

TTL input/outputs - when dealing with lower TTL voltages (0-5Vdc) most input cards will not recognize these. These cards allow switching of these voltages.

Encoder counter module - this module will take inputs from an encoder and track position. This allows encoder changes that are much faster than the PLC.

BASIC module - allows the user to write programs in the BASIC programming language.

ASCII module - adds an serial port for communicating with standard serial ports RS-232/422.

Stepper motor - Allows control of a stepper motor from a PLC rack.

Servo control module - has an encoder and amplifier pair built in to the card.

Communications - this allows communications or networks protocols in addition to what is available on the PLC. This includes DH+, etc.