Using Microstar's PC-based control boards, engineers have introduced a new refinement to the venerable PID command control.
PID command control is a time-honored tool used by many modern industrial facilities. These days, loop tuning setups rely on optimization software to help deliver consistent results.
However, PID can sometimes perform poorly for a variety of reasons, from use in non-linear systems to processes that change over time. Thus, it is sometimes hard to apply the PID approach and its extensions without continual tuning: what was a good tuning degrades into one that tends to oscillate, overshoot, or respond slowly.
Microstar Laboratories, Bellevue, Wash., which makes data acquisition processor (DAP) boards for PC-based applications, may now have a solution for users who encounter this problem. Using off-the-shelf components, developers can build a more reliable and less aggravating control system for hard-to-control processes.
The key is that this new system is self-tunable. Every DAP board runs a real-time operating system, DAPL, controlled by PC software. A new DAPL command-PIDZST, a self-tuning PID control algorithm with zero-shaping-can keep systems in full control during the self-tuning cycles, never interfering with feedback processing.
The company now also has control software for systems that need frequent loop-tuning adjustments to track command-level changes consistently, to avoid poorly damped oscillations, and to maintain a good response time as conditions change. Ordinary derivative gain can become less effective or in some cases aggravate a tendency to oscillate when a control loop is poorly tuned. Applying a "zero shaping" gain on the command input-as done in the PIDZ command for a DAP board-can reduce tracking errors but can slow response if this adjustment becomes unnecessary.
System control with DAP
PID control action and system response after automatic tuning by PIDZST. Pictured is the disturbance level of about 100 units compared to the operating level of about 16,000 units. Image: Microstar Laboratories
"For a very long time, our products have had an architecture that is intended for data acquisition, which means it has the ability to collect data and because it is not tied to a host computer to drive it, it acts independently," says Larry Trammell, a senior software engineer at Microstar.
At some point, the company realized it had all of the ingredients for a control system. Even though DAP boards were not intended as controllers, they have the ability to collect data in a timely manner, have independent processors on board, and are able to respond quickly.
The advantage of using this platform for a system controller, says Trammell, is that "it's difficult to find a platform to build control systems where you don't have to start from scratch.?"
Though PID was common well before Trammell arrived at Microstar, the company's products eventually outgrew the algorithm. However, Microstar noted that a lot of people were still using it, and that it was working well for most users. "However, there's 5% where PID isn't quite what you want. But do you want to take that steep climb up the 'build-it-yourself' curve" asks Trammell, who can, using modern DAP boards, run 16 channels of independent PID loops at 4 msec updates. A lot of extra capacity is available for self-tuning measures like PIDZST.
The anatomy of PIDZST
PID commands have two different characteristics. They have a transfer (T) characteristic that says 'How do you respond when somebody sets you to a new level' that signals a certain transient shape they go through. The second characteristic is sensitivity (S) response, or what happens when output shifts.
To get good response on one characteristic, operators might have to compromise on the other. What the Z-term does is allow users to split gains on the two signal pads, says Trammell. If the controller system responds too much to changes on input path, users can drop down the gain on that path and that cuts down overshoot response to changes in system levels without compromising performances for disturbances, or S response.
Another advantage of the extra Z-gain in the PID loop is that it can render pre-filtering unnecessary.
The new PIDZST processing command includes the extensions of the PIDZ command, including proportional gain, integral gain, and derivative gain. But it also optimizes controller gain parameters in real time to allow for plant characteristics that may change while the process is running.
The problem most often occurs, says Trammell, when "what used to be a good tuning for the integral and derivative gains turns out to be less of an advantage. With a system that has too much integral and derivative correction, it will tend to not settle well. It used to be fine, but now that system has changed so that it's not fine anywhere."
Operators are often required to come and make changes to the system. "It becomes a real economic drag to have someone watch these loops all the time. If they're not really changing drastically, sometimes they're sluggish, sometimes they're not. If the change isn't fast, these things couldn't track it," says Trammell.
PIDZST is best suited for applications in which natural disturbances are relatively rare and not sufficient to provide adequate loop tuning information. PIDZST provides a self-scheduling, low-level injection test plus analysis processing that runs in real time and does not interfere with control loop operation.
Every DAP board from Microstar includes DAPL that users configure and monitor from PC software. Users can specify the real-time behavior of the DAP board by downloading commands for DAPL to execute at runtime.
Easy to integrate and open to experimentation
Users can make use of off-the-shelf components to build a control system that tunes itself. Because DAPL is the standard operating program, any DAP board can execute the PIDZST command.
An application based on a single board using a basic PID algorithm and optimized for multiple channel operation can be updated at time intervals as short as 0.05 msec on every channel, without missing an update. DAP boards work well as multiples in a synchronized network.
What users will often find is that they do not always need this processing power in their application, but it is there for when they do need it.
PIDZST does have some limitations, mostly due to signal injection. PIDZST operates at a stable level most of the time, so the users can't tell whether they're approaching instability because there's nothing to see. If a disturbance comes along, the user will get oscillations. At that point, it's too late to do anything about the tuning, says Trammell.?"This might be a dead end, but it's an option for people. I was surprised by how well it worked. It's a very good tool for a lot of people to use. The usual way we would support this type of thing is we would develop a toolkit or we would provide the source code so they could build their own system," he says.
Users can download DAPstudio, a Windows application used to control DAPL on any DAP board, and the PIDZST command is available for download to try it out in any proof-of-concept experiment.
PIDZST, then, has the potential to free users from the ongoing operating costs of stubborn control loops that tend to oscillate or overshoot and that require continual hands-on tuning. And they can do this without building a custom system.
"It's not really adding new capabilities to the PID control, it's just adding the administrative ability to not have do anything about (the system) all the time," says Trammell.
Hardware can be ordered for evaluation at no charge.
