How to mount accelerometers
By Starlyn Aurit Motorola Austin, TX
Accelerometers measure both static and dynamic acceleration, and are typically mounted and soldered onto a pc-board (PCB) or similar type substrate. The new mechanical system that results accurately measures tilt, vibration, and shock, as long as appropriate mounting techniques are employed.
Placement, alignment, and structural stiffness are critical for accelerometer output accuracy. That's why achieving a high resonant frequency in the mechanical interface between the accelerometer and substrate is so critical. If it is not stiff enough, the accelerometer may measure false signals generated by the motion effects of the board, and result in accelerometer output errors.
Stiffness, accomplished by increasing board thickness, number of layers, and track density, aids transfer of acceleration to the sensor, eliminates false signals, keeps unwanted frequencies out of the signal band, and reduces output effects.
When placing the accelerometer on the substrate, alignment is also critical. With the sensing axis represented along the X axis, it is important to ensure placement accuracy to within ±3°.
To speak with a Motorola applications engineer, call (512) 895-2085.
How to suppress EMI in dc motors
By Richard Green, Senior Electrical Engineer, Pittman, Harleysville, PA
From medical-diagnostic applications to office-automation equipment, suppressing noise is critical. When faced with increasingly stringent electromagnetic compatibility (EMC) requirements, designers turn to a variety of active and passive approaches to reduce EMI in precision motion-control systems.
To reduce the effects of EMI, engineers add differential line drivers to dc motor and encoder combinations. Differential circuits commonly employ twisted-pair wiring, especially for longer transmission distances, and improve noise immunity by processing a signal that is the algebraic difference of two complementary signals at the input. The lines feed to a differential receiver, which reinverts one input and adds the voltage in the lines, effectively canceling EMI.
Other EMI-reduction options to filter noise include passive components such as capacitors and indicators. For low-frequency EMI (typically below 30 MHz), engineers install capacitors from the terminal to the ground and/or terminal to terminal. Ferrite beads placed over the wires' insulation reduce high-frequency noise (generally above 30 MHz) on wire and cable harnesses. For maximum noise suppression, engineers combine other passive components to develop a low-pass LC filter that is inductive-capacitive at low frequencies and dissipative at higher frequencies.
To speak with a Pittman applications engineer, call (215) 256-6601.