Robotics Automation - Proportional-Integral-Derivative (PID)

Accurate measurements and control is essential for every process to be run in a plant. It becomes necessary as it helps in ensuring the processes such as production, distribution and treatment processes which are carried out under the right conditions for the right amount of time and in the right quantities, thus control devices are a crucial part of every industrial process.

Historically, many traditional control processes relied on either mechanical or solid state electronic technology, with recent developments in pneumatics being used to produce changes in the process. With various advances in computerized control technology, there are now a list of new possibilities for the way that processes can be monitored and controlled. These include the use of complex algorithms capable not only of reacting to changes in process conditions but also increasingly predicting them as well, enabling corrective action to be taken automatically.

The wide variety of industrial processes in use today means there are many different types of control systems, each of which are geared towards their own particular application. The process are: Discrete processes, Batch processes, Continuous processes.

To run these different processes efficiently and effectively, two types of control systems are used, namely

  • Open-loop control systems - Open loop controllers commonly known as 'non-feedback controllers'. These controllers compute their input into a system based on a specific model / known set of conditions. These controllers do not use data from the process to change their output and are unable to make up for any disturbances in process conditions. If there is any variations in process conditions which is required to be achieved by an operator manually adjusting the final control element. Thus, open loop control systems are best suited to processes where there is a relatively stable set of operating conditions or where these conditions can be governed by a known relationship.
  • Closed-loop control systems - Modern control theory is grounded on feedback - i.e. signals from a process that can be used to control it more effectively. A closed-loop controller uses feedback to control states or outputs of a system or process. The term 'closed-loop' comes from the information path in the system - process inputs to a system have an effect on the process outputs, which is measured with sensors and processed by the controller. The result (the control signal) is used as an input to the process, closing the loop.

Fluid power has been used since ages but recent developments have seen an increasing demand for safer, quieter pneumatic systems and leak-proof hydraulics. Recently developed feedback control valves have travelled a long way from the days of early fluid power systems, which were either on with full flow, or off with no flow. Later came along the proportional servo valves which controlled the flow using inherent mechanical features of the valve.

A fundamental technological discovery was with the use of electrical control in the form of the solenoid. This started, the sophistication and complexity of valves which evolved at a modifying pace leading to the most recent proportional integral derivative (PID) electronic feedback devices and computer based systems of valve islands linked by fieldbus networking.

Proportional-integral-derivative (PID) control is the most common control algorithm used in industry. PID controllers being well known can be assigned to their strength in various operating conditions, their simple functionality and easy operability by engineers using current computer technology.

Engineers today use control to either modify plant behavior to keep system output stable and improve response time (the time the plant takes to go from its current state to the state defined with the new input) or minimize the energy the plant uses to transition between the different states. To attain this, engineers operate controllers themselves or use mechanical devices or PC or PC-related technology. Thanks to the evolution of computer-based systems, engineers can now use PCs and appropriate hardware to interact with plants and read their output and input signals.

Usually a proportional-integral-derivative (PID) controller is a device which is often used to control electronic devices and systems. Mathematical principles are typically applied by this device to process a signal which inturn triggers a response in the electronics it is connected to. Digital PID controllers often function as same as to analog ones, but may include microprocessors, programmable logic controls, as well as specialized software.

Digital PID controllers are sometimes used to manage single devices, but can also be included in an entire system. They are used to adjust the output signals correctly in systems such as temperature controllers, based on some level of feedback. These devices generally use mathematical calculations called algorithms, which can allow them to activate when programmed thresholds are reached. Conditions can also be monitored consistently at specific times; this function is called as the sample rate.

PID algorithms are used in a variety of control systems in industry. In robotics and for FRC specifically, one can commonly see PID implemented for motion control, either for drive train motors or for other servo actuators. Most robotics systems are weakened so that they never overshoot their setpoint. The goal of tuning such systems, then, is decreasing the rise-time and steady-state error to achieve the best possible performance. PID control is only effective for linear systems. If there is a system with non-linear response, the PID gains that are effective in tuning the system for one section of the response curve will not be valid for other parts of the response curve, resulting in erratic and uncontrolled behavior of the system.

Error signals help drive the function of digital PID controllers. The proportional term refers to the mathematically reducing of the error, while the integral function typically aims to make the error as small as possible. The three elements of proportional-integral-derivative controllers are often designed by experts in mathematical theory as well as by software programmers. A control interface, a computer program, can help people manage digital PID controllers without advanced expertise.

Sometimes software code is needed to tune a digital PID controller, while debugging may be needed to adjust the variables managed by the device. Digital PID controllers can also learn the times needed to heat up a room or cool it down. They are usually able to keep the temperatures steady in a room as well.

Most digital PID controllers function through fixed values. Different PID controllers can be connected to manage the performance of a large system. Another benefit of digital PID controllers is that the sampling time can be a small fraction of how long it takes for a parameter to be adjusted, so accuracy and effectiveness are typically maximized.