Wednesday, 13 May 2015

Andy Preston, UK Drives Product Manager, ABB writes: I thought long and hard about explaining how PID (proportional integral derivative) control and especially in relation to variable-speed drives. But then I realised that there are far better folk out there than I who have done a pretty good job.

Even good old Wikipedia does a valiant effort.

So, I bottled it and borrowed a “simple” explanation from Wiki-world:

The PID controller algorithm involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D. Simply put, these values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change.  The weighted sum of these three actions is used to adjust the process via a control element such as the position of a control valve, a damper, or the power supplied to a heating element.

Now, why does a variable-speed drive have PID control?

Personally I would stick with the driving analogy; the explanation behind this link is rather long winded, but is very good. Note that in reality we usually only utilise the P and I terms for electrically controlled systems, the D is not used due to the inertia we tend to have in the mechanics of the motor and load.

There we have it. While PID control is a tricky subject to understand; full of complex mathematical concepts and equations, good news is that from an end-user’s perspective you don’t have to know all the theory.

PID control is a method of automating systems, eliminating the need for human input whilst delivering increased efficiency and substantial energy savings. By utilising a feedback loop, a machine can regulate itself using any variable. This could include pressure, flow, height, thickness or strain, and PID controllers are particularly capable in temperature-affected systems. Using these variables the variable-speed drive can be tasked to control a motor locally, using only the energy that it needs at any given time.

It allows maximum accuracy in ensuring a system is operating at its optimum capacity, with a far quicker response time to both expected and unexpected variables than is possible either under manual operation, or with non-PID controlled drives.

The ABB drive includes two independent PID loops, meaning there is no need for an expensive external PID controller or for a PLC, which can be complex.

Having two PID loops allows them to be run in a cascade arrangement, where one loop’s output drives the set point of another, meaning the system responds quicker and is less affected by disturbances.

If for instance your systems are affected by temperature change, ABB’s PID controlled drives ensure that they are run optimally all year round. This could mean running a heating system at reduced capacity to allow bright sunlight to naturally heat a building, or a water cooling system making the most of wintry temperatures helping it out.

Alternatively, a drive could be tasked to undertake accurate control of pressure from a compressor, using only the energy required to develop the necessary pressure for the application.

Whilst temperature and pressure are two examples of possible PID control variables, they can operate using any almost measurable variable, making them exceptionally versatile for use in a range of industrial and commercial applications.

Ultimately by automating the control process ABB’s ACS880 variable-speed drives can help to make your machinery more productive and safe, whilst at the same time saving you energy and money from the moment it’s installed.

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