Sensor fundamentals 101

No matter how much data processing speeds continue to advance, the fundamental embedded systems requirement is for sensors to measure and convert real-world data into a digitized format for PC/104 systems to process.

This PC/104 Embedded Solutions Fundamentals 101 column focuses on some of the more exciting and unique sensors that are readily available on the commercial market. It briefly presents some of the basic but important aspects of A/D converters and reviews some of the common pitfalls to avoid when selecting a PC/104 data acquisition board with onboard A/D converters for a particular PC/104 embedded system design.

LVDT
A Linear Variable Differential Transformer (LVDT) is a type of displacement transducer. It measures the displacement of a mechanical moving object in actual applications ranging from jet engines to robotics. For example, hydraulics and mechanical assemblies utilize LVDTs.

Figure 1 depicts a transformer with a primary winding and two secondary windings connected in opposition with a movable core. The dots at each transformer winding indicate the polarity of the induced voltage. The movable core of an LVDT is part of a shaft that extends out of the LVDT and attaches to any movable object. As the object moves, causing the shaft or core to move within the LVDT, the LVDT accurately measures the displacement of the object.

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Figure 1: a transformer with a primary winding and two secondary windings connected in opposition with a movable core. The dots at each transformer winding indicate the polarity of the induced voltage.

The excitation provided to an LVDT is usually a sine wave measuring several volts RMS and is typically between 1 kHz to 20 kHz. The output of an LVDT is based upon the relative displacement of the magnetic core. When the magnetic core is centered, with respect to the two secondary windings, the output summation of both secondary windings is zero or null. As the core moves toward one of the secondary windings, the net summation output increases in amplitude and produces a non-zero differential AC voltage output. The phase of the summation signal will be in-phase with the primary or 180° out-of-phase with the primary, depending on which secondary winding the core moves toward.

Some of the advantages of an LVDT are:

  • They are readily available and very economical
  • They are very reliable in terms of service life due to a magnetic-based sensor.
  • The core does not contact the transformer.

A variation of an LVDT is the Rotary Variable Differential Transformer (RVDT). The RVDT is based upon the same principles and produces the same type of output as the LVDT. The magnetic core of an RVDT moves in a rotary motion with respect to the primary and each secondary. This is useful for 0º-360° rotational motion, as opposed to the linear motion of an LVDT. Table 1 lists a number of sensor applications.

Sensor applications

LVDT

Used to measure the displacement of mechanical moving objects

Accelerometers

Used to measure acceleration, angle of tilt, collision, gravity, and rotation

Hall effect sensor

Used to measure changes in a magnetic field

Table 1

Accelerometers
Acceleration is the measure of how quickly speed changes, and an accelerometer is a sensor that measures acceleration. Accelerometers have made the most remarkable advancements in the last five to 10 years. They used to be very large, power-hungry, and expensive devices. Some of the newer accelerometer technology enables measuring the angle of tilt of the sensor itself.

The incorporation of accelerometers in embedded systems has been, and continues to be, very diverse. Accelerometers can monitor acceleration, angle of tilt, collision, gravity, and rotation. Such diverse applications as automobile collision sensors, monitoring the pitch and roll of unmanned aerial vehicles, and the thumb joystick found in many handheld electronic devices utilize accelerometers.

Many of today’s accelerometers either incorporate a heated gas bubble located on the silicon or a spring-suspended capacitive-based system. Thermal sensors surround the gas bubble system and detect the movement of the gas bubble in much the same way that the bubble in a carpenter’s level works. The suspended, capacitive-based accelerometer includes a spring suspended plate above the silicon surface. A differential capacitor created by the suspended plate and a fixed plate located on the silicon surface measures deflections in the plate due to external acceleration.

Hall effect sensor
The Hall effect sensor is simply a magnetic field sensor. It is especially useful if the targeted component incorporates or can incorporate a magnetic field. Hall effect sensors operate in applications such as anti-lock brake systems, gear rotation monitoring, and solid-state switch applications. Hall effect sensors are advantageous because they have no moving parts, are solid state, are available in a broad temperature range, and are very reliable.

Semiconductor current flow is not orthogonal. When a constant DC current is present in a semiconductor in one direction, no voltage (potential) is created in a perpendicular direction. But when a magnetic field is placed at a right angle to the semiconductor material, the current passing through the semiconductor is disturbed. As a result, a DC voltage will be present. The DC voltage measured is proportional to the strength of the magnetic field. This principle is the Hall effect (see Figure 2).

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Figure 2

More than 125 years ago Dr. Edwin Hall discovered the Hall effect while using conductors. However, no one applied the Hall effect to actual applications until semiconducting materials were invented in the 1950s. In addition, no one utilized the Hall effect sensors in the electronics industry until the last several decades when the advancement of semiconducting materials made the availability of the sensors more common.

In fact, a vast array of sensors available on the market today ranging from temperature sensors, pressure sensors, and position sensors incorporate a Hall effect sensor. These sensors internally include a magnet and Hall effect sensor, which enable the desired physical element (temperature, pressure, or position) to alter the physical distance of the magnetic element to the Hall effect sensor.

A/D selection

Careful selection of the resolution of the A/D is very important. Utilize a bit resolution that produces an accuracy that is satisfactory to the accuracy required by the embedded system design. The system sensor’s data sheet will determine the maximum bit resolution that is practical to use based upon the minimum accuracy the output of the sensor itself can provide.

For example, suppose a data sheet of a sensor lists a minimum accuracy output of 2.5 mV per something (gauss, PSI, temperature) and has an output range of 0-10 VDC. An A/D converter with a 12-bit resolution and input analog range 0-10 VDC would be able to accurately measure the maximum accuracy of the sensor (12-bit = 4,096 binary numbers, 10 V/4,096 = 2.4m V).

It is also more important to determine if it is necessary to have that level of resolution. If the end application of the PC/104 system dictates only an accuracy required by the sensor that is a value greater than the resolution capabilities of the sensor itself, then utilize an A/D with a lower bit resolution that meets the system requirements of the embedded design. The PC/104 embedded system design requirements should dictate the accuracy level of the required A/D converter.

A/D converters
The most fundamental requirement for any type of analog-based sensor is to convert the analog signal to a digital format for processing by a CPU, DSP, or FPGA located within the PC/104 stack. The A/D converter is the IC that performs this process. The two main parameters that determine how accurately it converts the analog input to a digital signal are the bit resolution of the A/D converter and how fast the A/D converter samples the input analog signal.

Analog sensor output will have a fixed voltage range that it uses to represent the total range of the signal that it is measuring. For example, a pressure sensor may be able to measure a range of 0-100 psi and outputs an analog signal range of 0-5 VDC, which proportionally represents the pressure it is measuring at any given point in time. Alternatively, an LVDT sensor may have an analog AC output range of ±10 VAC.

The analog input range that the A/D converter can accept should match with the output voltage range of the sensor. A variety of A/D converters are available with many different input ranges and are typically dependent upon the supply voltages used to power the A/D converter itself. Analog sensor output typically has to be scaled through analog circuitry to match the input range of the A/D converter.

The A/D converter takes discrete samples of the incoming analog signal and converts each sample to a digital number. This digital number is proportional to the full-scale input analog range that the A/D converter can accept as an analog input. An 8-bit A/D converter can digitally represent 28 steps. The higher the bit resolution of the A/D converter, the more steps that can be represented. The bit resolution, in conjunction with the analog input range of the A/D converter, determines the overall resolution of the digital output. A 10-bit A/D converter with a 0-5 V analog input range can digitally represent the analog input signal in 4.9 mV increments or steps. (10-bit = 1,024 binary numbers, 5 V/1,024 = 4.9 mV). The bit resolution, in addition to how fast the A/D converter can perform the conversion process, determines the digital accuracy of the measured analog signal. Nyquist Theorem dictates that the sample frequency needs to be at least twice the highest frequency found in the input analog signal to the A/D converter. If an LVDT sensor has a 10 kHz output, the A/D converter needs to have a sample rate of at least 20 kHz or take a sample every 0.1 ms or smaller to avoid anti-aliasing.

The technology of A/D converters continues to advance along with the entire electronics industry. Many of today’s A/D converters have integrated mux systems, self-calibrating circuitry, integrated error compensation, and additional features that have advanced the accuracy, reliability, and capability of A/D converters as a whole. The important thing is to select a PC/104 data acquisition board that has an A/D converter that possesses the functional parameters and capabilities that meet the needs of the external sensors and the requirements of the PC/104 embedded system design itself.

PC/104 meets the physical world
The true advancement of PC/104 systems is not necessary at the silicon level with smaller and faster processors and memory, but rather at the real-world interface level. When the embedded engineer identifies new ways of utilizing existing sensors to gather data from the physical world in new and creative ways, that is when the embedded engineer pushes the envelope of PC/104 technology and embedded systems as a whole.

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Additional information, references, and Web links about sensors and A/D converters are available at www.jacyltechnology.com.

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Table 2: LVDT and RVDT resources
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Table 3: Accelerometer resources
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Table 4: Hall effect sensor resources
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Table 5: A/D converter resources

Joel Huebner is president of Jacyl Technology, Inc. He holds two degrees with honors from Purdue University in electrical engineering and computer engineering. Joel has more than 15 years’ experience as an electrical design engineer in the military aerospace industry and in the custom electronic design R&D industry.

For further information, contact Joel at:

Jacyl Technology, Inc.
PO Box 350
Leo, IN 46765
Tel: 800-590-6067
E-mail: jhuebner@jacyl.com
Website: www.jacyltechnology.com