Let’s describe how the nanodac can be used to control the dosing level of a liquid or gas using the Feedforward feature of the PID algorithm to achieve an output proportional to the flow rate of the fluid.

Data Recorder with PID Control
The nanodac recorder/controller provides combined recording and control in a single ¼ DIN package.

The Nanodac is ideal for use on any application requiring up to four real universal inputs. In addition, fourteen inputs can also be written to over communications effectively making an 18 channel data logger. It also contains 2 PID control loops, which can be configured to control temperature, pressure, level, and process variables such as flow, etc,.

Additionaly, the nanodac recorder provides powerful logging and secure archiving of the data. It can store information in either or a secure (UHH), or open CSV format, or a check summed format to protect data integrity.

Also available is live trending, a simple menu allows any selected portion of the recorder history to be archived, either to a ‘memory stick’ plugged into the USB port at the rear of the recorder (Local Archiving) or to a computer or server, by means of the FTP protocol (Remote Archiving). This archived data remains in the flash (50MB) memory of the instrument and can be reviewed directly on the instrument display.

Archive period between the last hour, last day, last week, last month, can be chosen. Also one can archive everything in the recorders history or archive all files created since or updated since the last archive.

The archive status is displayed on the nanodac recorder/controller, which shows when data is being transferred or is complete. The archive data includes actual values from real or communications channels (PV), Alarm Messages, and Operator Input Messages. All of this information is accurately dated and time stamped from the on-board real time clock.

Residual control (Dosing)
Dosing or residual control, as it is also known, is the nomenclature given to the technique for controlling the addition of a substance to a flowing liquid or gas. Typically a dosing system is designed so that the required dosing level can be achieved by maintaining the control signal as a proportion of the fluid flow rate. The residual (dosing) level is used by the control algorithm to trim the output signal by the required amount. Any industry that mixes substances into flowing fluids potentially has the requirement for this type of control. These include the water, distilling, brewing, concrete, china clay, paper and soft drink industries or any process requiring disinfection in cooling towers, drinking water, food and beverage, swimming pools and wastewater.

Dosing Control Example

The figure above shows the general arrangement of the dosing control system which in this example uses chlorine as the dosing additive. The main fluid flow is measured upstream from the dosing pump, while the chlorine level is measured downstream. The feed forward feature in the control loop is used to achieve an output proportional to the fluid flow rate. This means that the dosing rate immediately tracks any changes in the flow rate and so avoids under or over dosing.

The dosing concentration, measured downstream, can be used to provide closed loop PID control action to trim the feed forward output. When the flow rate varies so will the transport delay of the control loop. To compensate for this, the integral time of the controller should be increased as the fluid flow rate reduces. This is achieved by using the Gain Scheduling feature of the nanodac recorder/controller. Three sets of control parameters are available, the active set selected according to the flow rate.

In some cases the dosing level to be controlled is further downstream from the dosing pump. This maybe by as much as fifteen to thirty minutes at normal flow rates. In these cases, providing the dosing level can also be measured much closer to the pump then the second loop in the nanodac recorder/controller can be used in cascade. The master loop controls the dosing level at the point further downstream. Process feedforward can still be used to compensate for flow rate changes.

Internal ‘Soft’ Wiring
In the above example, dosing level is measured using channel 1 configured for 4-20mA input. It is connected to the main PV input of the control loop. 

The flow rate is measured upstream using channel 2 configured for 4-20mA input, which is connected to the Feedforward input of the control loop.

The Control output is connected via Output Module 3. This is assumed to be 4-20mA in this example, but other outputs (such as on/off) may be used to match the type of pump in use.

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