Irrigation Canal Water Level Control Irrigation water is channeled through two m

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               Irrigation Canal Water Level Control

                Irrigation water is channeled through two motorized vertical gates from an upstream to a downstream agriculture area. Motorized gates are used to regulate the water flow and thus the downstream level.   Fully closed vertical gates will produce continuous reduction in the downstream water level. Low downstream water level will not allow irrigation in the downstream area. Fully raised vertical gates will result in excessive irrigation in the downstream area and thus much wasted water. Adequate regulation of the vertical gates position can maintain desired downstream water levels at different times and thus support required irrigation cycles and preserve water resources at the same time.

Two identical constant speed motors derive the two vertical gates. The motors can move the gates up or down but only one motor can run at any time. This is done in order to reduce the overall power requirements of the regulator site.   The lowest gate should be selected for a raise operation and the highest gate must be selected for a lowering command. Both gates are equipped with position sensors, fully closed limit switches, and fully opened limit switches.

Two downstream level sensors provide redundant measurements at three points downstream. These sensors must be validated as they should provide similar readings consistent with previous measurements. We will assume two validated sensors input value for this project. Upstream flooding limit switch is used to provide the regulator an indication of possible flooding upstream and thus command the system to raise both gates until the flooding condition is removed and downstream level is close to the upstream level.

Each motor is equipped with an overload alarm switch, which is used to trigger any unusual conditions such as over temperature or load.    The motors provide an input discrete signal within 5 seconds indicating if the motor is running or not. If a motor fails to start then the other motor is selected and an alarm is issued. Motors can also start by activating the Push Button located on the local panel if the AUTO/MAN switch is in manual and the LOCAL / REMOTE switch is on LOCAL.

A selected motor is scheduled to run for 15 seconds.   This is followed by an idle period of 10 minutes. No motor is allowed to run during the idle time. This is done to prevent repetitive activation of the motors during downstream water level transient. It is important to keep the gates at close positions in order to minimize the loading on both gates structure. An emergency shutdown switch (ESD) is available to the operator to shut down the system in addition to the START and the STOP push button switches.

System I / O Map

The first step in the design of a PLC control application is the translation of the process specification to actual input / output resources. This is known as the PLC Input / Output (I/O) map.   This important step lists all I/O tags, assigned PLC addresses, and description. Figure1 lists the irrigation control system discrete inputs, while Figure2 shows the corresponding PLC input tags. Figure 3 and Figure 4 repeat the same process for the discrete outputs. Notice that none of the analog inputs / outputs for this control process is listed. We only limited our case study to ON / OFF control based on water level analog real time measurements relative to user defined set point for the downstream water level.

Tag Name

Address Number

Comments

AUTO

I:2/1

Auto / Manual Selector Switch

REMOTE

I:2/2

Remote/ Local Selector Switch

VG1_ROL

I:2/3

Vertical Gat1 Running On Line

VG2_ROL

I:2/4

VG2 fully Running On Line

VG1_LS_RAISED

I:2/5

VG1 Limit Switch fully Raised

VG2_LS_RAISED

I:2/6

VG2 Limit Switch fully Raised

ESD

I:2/7

Emergency Shutdown

VG1_LS_LOWERED

I:2/8

VG1 Limit Switch fully Lowered

VG2_LS_LOWERED

I:2/9

VG1 Limit Switch fully Lowered

Figure1 Irrigation System Input

Tag Name

Address Number

Comments

VG1_ Raised

O:3/0

Vertical Gate1 raise one step

VG2_ Raised

O:3/1

Vertical Gate2 raise one step

VG1_Lowered

O:3/2

Vertical Gate1 lower one step

VG2 _ Lowered

O:3/3

Vertical Gate2 Lower one step

Tag Name

Address Number

Comments

AUTO

I:2/1

Auto / Manual Selector Switch

REMOTE

I:2/2

Remote/ Local Selector Switch

VG1_ROL

I:2/3

Vertical Gat1 Running On Line

VG2_ROL

I:2/4

VG2 fully Running On Line

VG1_LS_RAISED

I:2/5

VG1 Limit Switch fully Raised

VG2_LS_RAISED

I:2/6

VG2 Limit Switch fully Raised

ESD

I:2/7

Emergency Shutdown

VG1_LS_LOWERED

I:2/8

VG1 Limit Switch fully Lowered

VG2_LS_LOWERED

I:2/9

VG1 Limit Switch fully Lowered

Explanation / Answer

Logic diagrams are recommended for sound documentation and as means of transitioning to the actual ladder programming implementation. This step, as will be demonstrated, makes ladder programming much easier. The logic diagram in Fig. 5 shows the implementation for the vertical gate 1 raise operation. Other operations logic diagrams can be constructed in a similar way for the vertical gate 1 lowering operation ( Fig. 6) and both the raise and lowering for vertical gate 2 ( Fig. 7).

The logic diagram is segmented into four parts. Part 1 ( Fig. 5) derives VG1 selection for gate 1 control up raise operation. The control up raise command requires that the desired set point is greater than or equal to the downstream aver age water level, downstream average level is outside the dead band, and VG1 is the next gate to go up. VG1 is next to go up if it

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