CRONOS®

You probably know that Process Instruments offers a cloud based remote access service for our CRIUS^® analyser, but did you know that…

… nearly everything you can do in front of the analyser you can do remotely?
… we have been working closely with our customers to bring you an even better service?
… a lot of our customers have seen the number of engineer callouts drastically reduce, saving them time and money?

Pi’s sleek and modern Remote Access solution

Pi has always been a leader in developing new technology and have had a remote access system for over 10 years! The latest iteration of remote access is now a key part of our solution toolbox. Here are some of the features of our remote access:

User interface – The interface mimics the analyser, making the data and other features easy to find. Anyone already familiar with the CRIUS^® analyser will already be familiar with remote access. Anyone requiring training can now be trained remotely using remote access and Skype.

User control – Each user has their own customisable access rights for what they can view and change on the analysers. This is set by the ‘parent’ user.

Speed – The service now runs around 50 times faster than its predecessor, allowing users to connect and download the current sensor values and settings in under 30 seconds (test performed on CoagSense coagulation controller with 27 different devices such as sensors!).

Reduced data costs – By making the connection between the server and analyser more efficient, Pi has reduced the amount of data required for nearly all tasks thereby significantly reducing costs.

Customer branding – On request, Pi can brand the remote access theme to promote a customer’s brand.

Data analysis – The graphical interface is a powerful and flexible tool for viewing your sensor data. The way we handle your analyser’s data means that every single data point is stored in the cloud, allowing a user to download 3 months’ worth of data with a single click.

Password reset – If a password has been lost or forgotten, a user can enter their email address into the password reset function which will then email a list of instructions on how to reset it.

Data limit – Users can now limit how much data each analyser can use per day, meaning no more unexpected data costs.

So what can Remote Access do for you?

The remote access service has always been designed to save you time and money by allowing you access to the analyser and its data from anywhere in the world. Now remote access can assist as a tool in doing a lot more:

• Remotely checking and changing settings for customers who may not be confident in doing so or who may not have access to the analyser.
• Allowing Pi’s service team to temporarily remotely connect to the analyser if customers have an issue and are unsure about how to solve it.
• Automatically sending daily/weekly reports of data logs and/or status logs to users from the server (no mobile data costs from the analyser).
• Easily view when maintenance is required or due on analysers, allowing users to efficiently plan which sites to visit and when.
• Know when alarms activate on the unit within seconds by enabling text and/or email alerts direct from the analyser.
• Tune control values to optimise dosing without the need to visit the site.
• Analyse stored data to investigate incidents, explain issues or validate the process.
• Save money by reducing the number of callouts and the number of miles your engineers travel!

But what about security?

When the analyser and remote access server communicate, Pi uses a ‘Pre-Shared Key’. This creates a secure connection between the server and analyser. The key acts as a password and without it the analyser will not respond. The key is randomly generated in every analyser making it unique to each, and if required the key can be re-generated from the analyser. The analyser can also be connected directly to a Virtual Private Network (VPN) which blocks unwanted traffic and connections automatically and will only transfer information between the analyser and server.

How does user access work?

Each user has their own username and password. This means that what they see and what they can do can be customised so only authorised users can make changes.

How are the analysers and customers managed?

If a user creates a group, then they become that group’s ‘parent’. The ‘parent’ of a group creates the analysers and users for that group. If a user is a ‘parent’ to more than one group, they will also have the ability to create users that can access multiple groups from a single user account.

Did you know…

… that PID can save you money by offering better process control?
… that PID can help you maintain a setpoint, even with a variable process?
… that the days of over complex and confusing PID are over?
… that Pi can tailor a PID system to your exact requirements? You may never have to touch those settings again!

In this Focus On, Pi would like to introduce you to PID if you haven’t come across it, and discuss some of the useful advanced features of modern PID systems, like on Pi’s CRONOS^® and CRIUS^® models for those more familiar with PID.

What is PID?

PID is a mathematical tool created by engineers and is used in controllers. It is a feature often found in industrial controllers and is available in Pi’s controllers, as an inexpensive upgrade.

What is PID for?

The best way to explain what PID does, is to take an example. Most people have been to a swimming pool at some point in their lives, so this is the example we shall use. PID is also applicable in a huge variety of other processes. If you are not sure, you can always contact us to discuss your application.

When a person enters a swimming pool, they create a chlorine demand. They do this by introducing sweat, bacteria, organic molecules and other substances into the pool water. Chlorine reacts with these substances, which results in chlorine being used up and the chlorine level dropping. The chlorine level in this example, is often called the process variable or PV in the context of PID.

In order to maintain a concentration or level of chlorine, more chlorine needs to be dosed. If you dosed the same amount of chlorine per bather, the level would not be stable as all bathers create a different chlorine demand (e.g. swimming for fitness produces more sweat than swimming recreationally). Dosing manually brings in the issue of human error, and how operators approximate or calculate the amount of chlorine to dose based on current levels. Another issue with manual dosing is that it is not a continuous process, meaning it is unlikely a stable level will ever be reached.

What does PID do?

PID takes the measured level of chlorine or the PV and compares it to the desired level or set-point. This comparison gives the error which PID interprets and then calculates an output. The output is an electrical signal which controls the dosing of the appropriate chemical. The output can control heaters, dosing pumps and many other mechanisms that can be used to change the PV.

How is it used?

PID is made up of three parts, proportional, integral and derivative. Understanding what each part does helps operators choose the level of control best suited to them.

Proportional – Is the most commonly used for portion PID and suits most applications. When using proportional control, the further away the measured value is from the setpoint, the larger the output will be from the controller. This is an appropriate level of control for most processes, and users can gain a lot of control from a purely proportional system.

In some systems where the PV is lost to the process, e.g. chlorine from a pool, heat from a boiler etc. the proportional control never quite catches up with the setpoint. Users can see that although the process approaches the setpoint it rarely, if ever, gets to it. This is known as ‘droop’. The user can compensate for droop if the removal of the PV is fairly constant, simply by raising the setpoint, e.g. evaporation of chlorine from an empty pool. If droop changes often, (e.g. bather load or chlorine demand) then to eradicate the ‘droop’ then the integral part of PID can be applied to the signal to correct it.

Integral – The output from the integral term is determined by both the magnitude and the duration of the error. A small error over a long period of time will trigger a larger response than from a purely proportional system. This helps the elimination of the ‘droop’ seen in processes with continuous loss, and also serves to help reach the setpoint quicker.

Derivative – Derivative gain is rarely used and is generally set up only by expert engineers. Derivative gain uses the rate of change in the PV to try to predict future errors. This type of control is particularly susceptible to overcompensating, especially if there is even a small amount of signal noise (usually seen as spikes in the PV). Derivative gain is generally a tweak used by engineers to improve an already tight control, and is almost never used as an essential part of control.

What are the benefits of PID?

When properly set up, PID can lead to far tighter process control, which in turn can save you time and money. As an example, pool managers want to keep chlorine levels low, to improve the bathing experience and also save on chemicals. AquaSense is a chlorine analyser system that responds quickly and appropriately to a change in bather load (also known as chlorine demand). This means pool operators can save money whilst maintaining the safety of the pool. PID can also help reduce the risk of overshooting the desired setpoint, reducing the risk of dangerous overdosing of the chemicals.

Advanced Features and Safeguards

Whilst maintaining a setpoint with a PID loop is a huge advance over using threshold relays to maintain an upper and lower limit, it is sensible to control the loop with extra safeguards, such as:

• Maximum and minimum pump outputs. This is mainly used to prevent the controller from employing too aggressive a control, which can lead to overdosing. A minimum output can also be used in a system where the measured parameter is lost over time, to prevent the controller ever turning the dosing off.
• Ramp rate is a proportional control that allows users to choose how quickly or slowly the controller doses, in order to reach the setpoint. It is especially useful on startup, and can prevent the controller dosing too quickly.
• Wind up protection is an integral control, which limits the duration aspect of the control. This puts a limit on how much previous error can accumulate. Without wind up protection, there could be a very large integral value, if the process ever reaches zero or on startup.

These are all standard features in all Pi PID controllers.

Conclusion

In summary, PID is a very useful tool when used correctly, and can result in significant chemical savings, not to mention reduced pump wear and tear and lower electricity costs.

Process Instruments (Pi) are increasingly seeing water engineers and technicians relying on their PLC’s PID auto-tune feature. Many are becoming frustrated that the auto-tuners are unreliable when it comes to water processes. Sometimes the auto-tuned PID settings work, but then something changes in the process and the PID control no longer functions correctly.In this Focus On, Pi aims to outline how PID auto-tuners work and why they are often unsuitable for complex water treatment processes. We also give a brief introduction to some of the principles of tuning a PID manually.

Did you know that…

…auto-tuners are not effective at tuning processes with long loop times?
…the CRONOS® and CRIUS® have many built-in PID safety features to make control easier and safer?
…PID loops can often be tuned remotely, meaning huge savings on site visits and travel time?

What is PID tuning?

A Proportional – Integral – Derivative (PID) controller is a control loop feedback mechanism widely used in industrial control systems. PID controllers allow a system to continuously and automatically modulate a control mechanism (e.g. a dosing pump, a valve, motor speed), to attempt to achieve a desired setpoint.

There are a myriad of ways that a PID controller can be ‘tuned’. In a very basic PID controller, the operator can choose how much of their control should be based on P, I and D. In reality, D is almost never required in water processes due to the nature of the control loop.

This is generally what ‘auto-tuners’ undertake. They make small changes to the ratio between P, I and D control, track the response, decide if the change is good or bad, and then change again.

What is the auto-tune doing?

So what the auto-tuner is doing is changing, measuring, and changing again; which can work perfectly on processes with a very short loop time (time between a change and a response from the system), the auto-tune making many changes in a short amount of time. The changes cumulate into a ‘tuned’ PID system.

Below is an example system of a PID auto-tune.

In the diagram, we can see that for the system to work, the box in red is a crucial part of the process. If the change brought on by the PID auto-tuner isn’t visible quickly enough, it may affect the way the auto-tuner categorises the change (good or bad), or won’t give the auto-tune the opportunity to make enough iterative changes to bring a complex water system under control.

The good news

There is good news for water technicians and engineers and that is that Pi are here to help. All our salespeople are application specialists and are all capable of helping you manually tune a PID controller for your processes. Our CRONOS® and CRIUS® controllers are both capable of delivering excellent control in many processes, and have many built-in PID features to help make your process more controlled, robust and safe.

Where to start with PID tuning

In many ways, the auto-tuners mimic plant engineers making adjustments and tuning their PID settings. They simply lack 2 key components;

• Patience – because the loop times are so long, it will take time to tune a PID loop.
• Context – no auto-tuner can account for blocked dosing lines, or 2 identical pumps that are improperly calibrated and output different volumes, or seasonal changes in measured parameter concentrations. Context is incredibly important in tuning a PID loop.

With this in mind, here is how our engineers approach PID tuning on our instruments;

1. Check the measurement and output method are all working properly. This includes any calibration on the probes and on the pumps as well as checking that the dosing lines are clear.
2. Consider the loop time and change the PID’s Update Delay. This changes how often the PID algorithm makes a change to the output.
3. Start small and ramp up. Start with the P control and slowly increase the P factor until it is changing the measurement in a satisfactory timeframe. If you can get a good level of control with just P then that’s excellent. There is often no need to overcomplicate things past this. The addition of I and D components is often unnecessary.
4. If you have wild or erratic control lower the P until the control is stable, even if it doesn’t reach your setpoint then add some I control.
5. Always aim to have I as low as you can get away with, as I can cause more problems with control than it solves.

Once you have the basic P and I ratio sorted, or if you are unable to get the P and I into control, it is worth considering these additional settings which will help make your control scheme more robust and safer.

• Min/Max Output – restricts the range that the PID can work within, which can stop potential over- and under-dosing. This setting may also reduce the ability of the PID loop to respond to changes in the water.
• Start Mode – allows the process to be dosed using a flow proportional value or a manual percentage value, during a predetermined start up period. This is often used to get the process up and running before going into PID control.
• Ramping – smooths the start-up of the process where the error between the measured value and setpoint may be very large (which can cause erratic overshoot and overcorrection).
• Bumpless Transfer – smooths out the process of switching between manual and automatic control.
• Integral Wind Up Protection – puts a limit on the effect of I because I looks at error over time. A very small error over a large amount of time can result in a very large I output resulting in erratic control.
• Overfeed Protection – protects against the failure of other equipment, such as failing pumps or blocked dosing lines. This puts the control into alarm if the controller is calling for dosing but is seeing no change.

How our products can help

Even armed with all this knowledge, PID tuning can be a bit of a dark art. What works on one site may not work on another and even experienced engineers sometimes call us for help.

One of the ways we can help is using Remote Access. Our Remote Access portal allows people with the appropriate security clearance to make changes to settings and observe measurements from their desk.
Sales Manager, Dr. Rob Paramore, recently described his experience of changing a customer’s PID settings (at their request), whilst talking them through what he was doing on the phone: