FEATURE SIGNAL CONDITIONING
Jim Vaughan, MD of Kistler Instruments, examines the development of the piezoelectric sensor and looks into the introduction of a new signal conditioning platform designed to make use of the special characteristics of today’s piezoelectric sensors
SIGNAL CONDITIONING for piezoelectric sensors
T
he piezoelectric effect was discovered in 1880 by French physicist brothers,
Jacques and Pierre Curie, who found that some crystal materials, mainly quartz, Rochelle salt and tourmaline, produced a surface charge in response to a mechanical stress. It was not until 1917, however, that the converse piezoelectric effect was used to produce an ultrasonic submarine detector, which led to the effect being applied to the development of a wide range of uses – from microphones to megacycle resonators and from ultrasonic transducers to gramophone pick-ups. In 1944, two Swiss engineers, Walter P
Kistler and Hans Conrad Sonderegger, working at the Swiss Locomotive and Machine Works in Winterthur, recognised that the piezoelectric effect could be used to measure pressure curves and peak pressures during combustion in engine cylinders. Within six years, Kistler had invented and patented the first charge amplifier (charge/voltage converter), leading to the development of the first miniature piezoelectric pressure sensor and, in 1959, Kistler Instrumente was established. Without the charge amplifier, piezoelectric sensors would not have become the basis of the laboratory and industrial measuring chains used today.
DEVELOPMENTS The key to the practical use of the piezoelectric effect is the ability to convert the charge on the surface of the crystal to a voltage signal with precision and stability. The first amplifiers introduced were analogue instruments, but in 1988 the first microprocessor controlled charge amplifier was launched. This formed the basis for the extensive
range of amplifiers now available, covering single channel to multi-channel solutions to meet the needs of applications ranging from automobile engine development to industrial quality assurance and aerospace design. The widespread use of the National Instruments (NI) CompactRIO reconfigurable embedded control and acquisition system prompted Kistler to design a modular charge amplifier to provide an easy to use interface with the NI CompactRIO system. Previously, piezoelectric sensors required external amplifiers, needed a separate power supply and had to be configured individually. The Type 5171A charge amplifier not only simplifies wiring and configuration, but also leads to an enhanced signal quality. Measured values of up to ±1,000,000 pC can be acquired at high resolution with a usable frequency range up to 20 kHz and a sample rate of up to 52,73 kSps. Available with either one or four channels, the charge amplifiers can be used individually or in multiples to suit the system requirements.
A SIGNAL CONDITIONER In the field of Engine Combustion Analysis, the application demands a flexible, multi-channel signal conditioning platform. Kistler introduced the Signal Conditioning Platform (SCP) to make use of the special characteristics of the current range of piezoelectric sensor, such as the Kistler PiezoSmart sensors, which have all the relevant data of the sensor stored on a TEDS (Transducer Electronic Data Sheet) available for automatic setting of parameters and adjustments. This simplifies setting up and, importantly, virtually eliminates the risk of error. Different amplifier modules are
Kistler charge amplifier, Type 5070A
30 JULY/AUGUST 2016 | INSTRUMENTATION
also available, supporting a variety of different sensors (piezoelectric, piezoresistive, strain-gauge, hall- effect). In addition, the SCP is fully compatible with all commercially
Type 5171A modular charge amplifier
available combustion analysers. The latest version of the SCP is
provided with an Ethernet interface for remote control via PC and allows for potential networking of a number of systems so that all can be operated from one central PC. When used with PiezoSmart sensors, the system reads the data from the TEDS for automatic parameterization of the measurement modules, so eliminating the possibility of error due to manual data input, avoiding user-error, and improving data quality and
Kistler’s SCP slim signal conditioning platform
process reliability. The use of PiezoSmart sensors also enables facility-wide sensor management to be implemented. Recalibration requirements can be tracked, based on the number of working cycles and/or peak cylinder pressures with a histogram of the working cycles and peak pressure now available for each sensor at the click of a button. An integrated CAN-bus interface
provides real-time monitoring of cylinder peak pressures without the need for a dedicated combustion analysis system. The CAN-bus also provides information on the operating temperature of low pressure piezoresistive sensors, allowing users to immediately identify potentially damaging engine or equipment events and take corrective action, thereby avoiding unsafe operating conditions. Two variants are available: the SCP-slim with a 2-slot chassis, (scalable for up to 16 channels) and a 19” bench or rack-mounted version with 8-slots (scalable for up to 32 channels).
Kistler Instruments
www.kistler.com
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