MTI’s Digital Accumeasure makes an effective digital position control sensor. Since the output of the Digital Accumeasure is Ethernet it can be located far from the controller and multiple sensors can be networked with no signal degradation as long as the positions can be read within the controller’s loop control cycle. In the image below Fig 11 we break out the latency of the MTI Digital Accumeasure (DACC) as a single element. In the scope position we see the commanded position signal (yellow) and the response of the capacitance probe carrier (blue sinusoidal signal). We measured to the carrier as the carrier represents the Capacitance probe response without actually looking at the gap capacitance because this is impossible without affecting the measurement. The violet signal is representative of the DACC’s digital output. The violet signal is actually the DACC analog output which is almost instantaneous with the DACC Digital output but lacks the latency of the Ethernet IP. We can see the latency of the DACC is on the order of 2.14 ms.
Next we look at the output response of the Digital Accumeasure (Y to PV) . With the filter set to the highest frequency the DACC response (latency ) is about 2.14 ms.
So the DACC latency is only a small portion of the overall loop response (2.14 ms out of 50ms total ) . The total 50 ms closed loop response is the cumulative effect or sum of the control loop timing cycle (500us), the Piezo stage (~ 10 ms), the Digital Accumeasure latency (2.14ms) , the Ethernet communications latency (~ 800us per packet) and the integration effect of the PID loop which takes multiple cycles to stabilize. Remember a control loop cycle in this application is 500us. To decrease the loop response time it’s possible to decrease the controller cycle and also use a faster piezo stage driver amplifier. Additionally it’s also possible to improve the PID tuning for a faster settling time.