This document provides a system block diagram and requirements for a NIST radiation pressure sensor. The block diagram shows the key components including a laser, spring, amplifier, and microcontroller used to measure small forces on a scale. The closed-loop requirements specify the controller must have a rise time less than 100ms, zero steady-state error to balance the bridge, and reject disturbances below 1kHz to prevent spring vibration. The microcontroller samples at 1.25us to control the system and meet the closed-loop performance goals.

1. ‐
NIST Radiation Pressure Sensor
System Block Diagram
Non‐Inverting
Amplifier
VCTRL Δd
Capacitor
FCTRL Fnet
Flaser
Fvib,grav
Spring
Voltage to Force
Equation
Bridge
C
INA Lock‐In
V VDIFF
DACC(s)ADC
Low‐Pass
Filter
Convert
Flaser
r
e
y
Signals
 r – DC voltage output of the lock‐in amplifier when the bridge is balanced (as well as possible) and the preload voltage has been applied, with no
additional load applied to the scale.
 y – Actual DC output of the lock‐in amplifier.
 C(s) – Controller implemented by the microcontroller.
Closed‐Loop Requirements
 Rise Time < 100 ms (Probably actually means Settling Time < 100 ms)
 Zero Steady‐State Error (Null Approach)
 Good Disturbance Rejection (Cannot allow spring to vibrate at frequencies below the frequency at which the loop is closed (~1kHz))
Microcontroller
(Teensy 3.2)
Display
(~1Hz)
Design this
in Simulink
Calibrate
ts = 1.25us (80 kHz)
Clock = 8 MHz
Δd
d
d0
Laplace DNE
Record When
Balancing Bridge
Measure m,k
Find TF
Experimentally
(Performance
varies with
settings)
Find TF with
LTSpice,
Simulink, or
Analytically
Find TF with
LTSpice,
Simulink,
Analytically, or
Experimentally
Find TF with
LTSpice,
Simulink, or
Experimentally
Reject
Noise
Ask
Shalom
Ask
Shalom