- Don Welch, Director of Engineering and Business Development
- Cory Bufi, Software Engineer
- Isaac Abbott,Product Innovation Engineer – PBS Group
How To Create a Digital Closed Loop Feedback System using Piezo Positioning
Today’s electronic designs often call for reasonably fast and precise positioning of miniature components for process deposition elements such as ink jet nozzles , 3D printing elements etc. And as process control moves from mm positioning to micron to sub-micron positioning it becomes necessary to monitor and control positioning actuators such as piezo flexure stage and voice coil motors. Open loop control of the positioning stage may have been adequate in non-precision applications but piezo electric stages and voice coil motors don’t have a super accurate open loop linear response. Open loop control voltage or motor current input is simply not precise enough for these applications. Feedback is necessary for accuracy using capacitance sensors, strain gauges or laser interferometers.
Capacitance sensors are far less expensive than laser interferometers but have the stability and precision of laser interferometers. They are also more accurate than strain gauges and other contact displacement gauges such as LVDT. Capacitance sensors also don’t touch the piezo stage or voice coil motor so there are no mechanical loading effects. Digital feedback control is gaining popularity as the desired method rather than older all-analog techniques.
This application note describes how to use a MTI digital displacement sensor as a position sensor in the feedback control system with a precision piezo electric positioning stage. It describes how to implement a complete digital closed loop control using an example with a Piezo System Jena stage. It shows the components needed for a real world implementation and lists some of the latency and delay parameters encountered. It was not our intent to make the closed loop response as fast as possible but rather have a reasonably fast closed loop response and to determine the latency of the MTI Digital capacitance sensor.
Although analog capacitance sensors have been used for decades only recently have they been available as digital sensors. A typical closed loop control system diagram is shown in Fig 1.
Figure 6: Open-loop single-step test at 200us loop time, SP is the command setpoint, PV is the measured value of the stage position , CO is the controller output