Panel-mounted process controller for 4–20 mA flow, pressure and level loops

01

Count the analog inputs and outputs and check their loads

Yokogawa's YS1700 programmable indicating controller has, in its basic type, five 1 to 5 V DC analog inputs, one 4 to 20 mA output and two 1 to 5 V DC outputs (one convertible to 4 to 20 mA); the type with expandable I/O has eight analog inputs, four analog outputs and fourteen digital inputs and outputs. The 4 to 20 mA output accepts a load of 0 to 750 Ω, and the 1 to 5 V outputs at least 2 kΩ. [1]

Yokogawa states I/O conversion accuracy of ±0.1% of span for 1 to 5 V inputs and ±0.2% of span for analog current outputs, and gives the measurable range of the 4 to 20 mA output as 0.8 to 21.0 mA, with a tight-shutoff feature. [1] Editorial calculation: at 20 mA through the maximum 750 Ω load, the output must drive 0.020 × 750 = 15 V across the loop, so add up the resistance of the valve positioner, any isolator and the cable before checking it against that limit.

At the compact end, Maxwell's AU48, a 1/16 DIN (48 × 48 mm) universal-input controller, accepts 0–10 V, 4–20 mA, 0–5 V, 2–10 V and 0–20 mA as well as thermocouples and RTDs; it ships with a 250 Ω resistor to be fitted across two terminals when a mA input is used, and lists 0.25% F.S. accuracy and 0.001 maximum resolution for analog input. [2] Editorial calculation: 4–20 mA across 250 Ω gives 1–5 V, which is why mA and 1 to 5 V inputs are interchangeable through such a resistor.

Give the supplier each loop's transmitter signals (4–20 mA, 1–5 V, direct sensor), the final element and its input, the number of analog and digital inputs and outputs, the control functions, required control period, panel cutout and depth, power supply, communication protocol, and hazardous-area approvals.

02

Plan what the output does when the controller fails

Yokogawa describes the YS1700 as using two CPUs, one for control and one for display, so that display and manual operation remain available if either fails; a hard manual circuit independent of the digital circuits lets the operator adjust the controller output even if both CPUs fail. On a detected system failure the FAIL lamp lights, the FAIL contact opens and analog outputs are held. [1]

Yokogawa also lists the YS1700's front faceplate as dust- and splash-proof to IP54 for the basic type, optional FM and CSA nonincendive approvals, a 250 mm depth for the basic type, a weight of 1.6 kg, and operation from either 24 V DC or 100 V AC. [1] Editorial note: ask each supplier what the output does on power loss, CPU failure and input break, and whether a manual station is built in or must be added to the panel.

03

Choose the control functions and control period

In function-selection mode the YS1700 runs single-loop, cascade or selector control without programming at a 100 ms control period; in programmable mode the period is 50, 100 or 200 ms and the user program holds up to 1,000 text steps or 400 function-block modules, with more than a hundred calculation module types including temperature and pressure compensation. [1]

Add-on functions listed include self-tuning, non-linear PID, output limiters, alarm detection and a remote cascade setpoint input, with optional Modbus/TCP over Ethernet or RS485 communication. [1] Maxwell lists a 250 ms sampling rate for the AU48, with NPID, APID and on/off modes and auto-tuning, and power consumption below 2 W across a −30 to 75 °C ambient range. [2] For more loops in one unit, Maxwell's MF43HYC-652A runs up to four channels on a 4.3-inch touch screen, with relay, SSR drive or 4–20 mA output per channel and Ethernet, Wi-Fi and USB ports. [3]

04

Signal list, accuracy, control period and failure behaviour to request per model

Ask each supplier for the input and output signal list, accuracy as % of span, control period, failure behaviour and approvals of the model quoted.

05

Analog inputs, A/D and D/A, control CPU, outputs, hard manual backup and display

06

Supply, accuracy as % of span, control period and panel size for a 4–20 mA loop controller

Some makers state display resolution where you might expect A/D bits, so ask for both when comparing quotes. [2]

07

Send the loop, signal and failure data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Loops: [number; process variable; control function]
Inputs: [signals; count; transmitter power]
Outputs: [4–20 mA count; final element; loop resistance]
Control: [PID, cascade, ratio, selector; control period]
Failure: [manual backup; output on failure]
Panel: [front size; depth; cutout; IP rating]
Power and comms: [supply; protocol]
Approvals: [hazardous area; CE]

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