Specialized sensors recording various physical parameters enable man and machine to make better decisions and to optimize processes. To this end, the quality and functional safety of the sensor technology used are equally important to achieving the desired results. In addition, conversion of the measured parameters should be as precise as necessary and involve easy-to-integrate electrical outputs.
Designers face a challenge to inexpensively amplify small signals—signals that usually have nonlinear and temperature-dependent characteristics—and to condition them in such a way as to enable them to be safely transmitted over long cables in a rough industrial environment. A designer also needs to decide if the signal transmission should be carried out digitally or as an analog signal. This article outlines possible approaches to this problem and describes the structure of a universal, integrated, programmable signal conditioner—designed for use with linear and rotary encoders, AMR sensors, and opto sensors with on-sensor preconditioned signals—that meets these requirements for industrial applications.
Signal Quality and Error Protection are Crucial
To optimally adapt and evaluate the various sensor elements used to measure temperature, force/pressure, acceleration, position or light intensity, for example, you need an instrumentation amplifier to provide the required amplification. The instrumentation amplifier is a differential device, amplifying both channels equally; it must be flexible, adjustable, and have a high impedance to deal with the very small sensor signals involved. It must also have the ability to compensate for manufacturing offsets. At the signal conditioning stage, you should (when possible) take into account the nonlinearity caused by temperature or temperature drift and also filter out, suppress, or avoid noise or interference induced at the sensor. Sensor bridge arrays (typically using a Wheatstone bridge) are particularly suitable for suppressing common-mode interference and can also supply a sufficient signal quality even with minor changes in voltage. When locating possible error sources in the signal path, consider the following possibilities:
- Detecting cable breaks or short circuits
- Interference induced on the sensor side or during signal transmission
- Disruptions in the voltage supply or loss of ground
- Exceeding the maximum operating temperature
A redundant signal path model could prove prudent in situations that demand a high degree of error protection but will double the costs for sensor wiring. A good compromise in detecting simple errors is to condition the sensor signals differentially and to combine this with an integrated temperature detector and a voltage and sensor function monitor to enable a variety of diagnostic functions, including identification of sensor or wiring/bonding failures and over-temperature monitoring. When transmitting sensor signals, one alternative is to digitize the values immediately following signal conditioning and then transmit them using safe digital protocols. To achieve higher measurement resolutions, however, you will need ADCs for each sensor and this will involve higher costs for complex fieldbus connection protocols.
Simple voltage (e.g., 0–10 V) or current interfaces (e.g., 4–20 mA) are fairly common but do not enable monitoring as they stand. System designers therefore opt for differential transmission of analog measurement values, enabling the sensor signal to be validated logically on the driver side and common-mode interference to be suppressed even with long connecting cables. With these issues in mind, iC-Haus conceived the iC-TW3, a differential, programmable signal conditioning setup with three channels, equipped with differential line drivers for closed 100–120 Ω lines.
A Universal Signal Conditioner
Figure 1 shows the differential signal path for the iC-TW3 universal signal conditioner. The device consists of a programmable input amplifier, an offset compensation stage, a dynamic filter, and a differential output amplifier. The input offset, gain, and low-pass filter frequencies can be set in the signal path. During amplification across all three stages an overall range of –6 to 57 dB can be set in intervals of 0.08 dB. An offset of ±1240 mV can be configured in multiples of 40 mV for the front-end amplifier. An offset compensation value of ±254 mV can be set in units of 2 mV downstream of the dynamic filter amplifier. The output amplifier also contains the differential line driver and boosts the conditioned signal, so that a low-impedance line termination (e.g., of 120 Ω) can also be used for direct transmission at a signal of 1 Vpp.
The input amplifier can also operate in single-ended mode. If this is required, the minus input of the amplifier is connected to VDD/2. As an additional option, cable breaks to the sensor element can be monitored by switching on the internal 2 MΩ pull-up resistors. In the event of error, the signal conditioner iC-TW3 indicates that there is a disconnected sensor by transmitting a LOW at the NERR output.
Automatic Temperature Compensation
Temperature errors are often not compensated for in the sensor but in the central computer, microcontroller, PLC, or drive. The temperature has to be measured directly at the sensor and transmitted as an additional parameter. Alternatively, the temperature can be measured at the sensor, with limit monitoring and compensation performed locally. This latter method is based on linear interpolation between two temperature measuring points. For this purpose the iC-TW3 permits a total of 16 freely selectable interpolating points across a range of 0–255, with the lowest value 0 and the upper 255. With the integrated temperature sensor, this is equivalent to a range of about –50°C to 150°C. The distance between two points on a sensor temperature curve is thus freely selectable and can be adjusted to suit any type of curve. Using these interpolating points that are stored in a look-up table, the iC-TW3 automatically interpolates the gain and offset of channels A and B as well as an offset for channel Z. A total of five 8-bit values for the 16 interpolating points possible are thus stored in a table in the connected I2C EEPROM. The example shown in Figure 2 has seven defined interpolating points for temperature compensation to correct the nonlinear offset and gain of a connected sensor.
Figure 2. Temperature compensation of the gain and offset by interpolation
An external temperature sensor can also be connected to the iC-TW3, should the sensor be physically separated from the electronics and thus subjected to other ambient temperatures. An 8-bit value between –50°C and 150°C is used to define a selectable threshold temperature that triggers an alarm. This alarm is displayed as a LOW at the iC-TW3's NERR output; it can also be used to drive a general error LED.
Conditioning by Microcontroller or on a PC
A bidirectional, pulse-width-modulated, 1-wire interface on the iC-TW3 enables R/W access to all registers as well as to the connected parameter storage device (a standard I2C EEPROM). In application it can be used for direct calibration via a microcontroller port. The connection can also be configured as an optical write-only link, if the compensation of encapsulated sensors is to be 'wireless', i.e., performed via a light transmissive window. An adapter is available for development and design purposes that can be lashed up to the USB interface of a normal PC or notebook. Figure 3 depicts the iC-TW3 graphic user interface for the conditioning of signal paths A and B. During development, this allows the designer to determine all parameters for the gain and offset of the preamplifier, filter, and output amplifier. Operating mode settings (differential or unipolar) and sensor error monitoring can also be programmed using this setup. All new settings are written by the software straight to the EEPROM connected up to iC-TW3, if this option is selected. The current temperature measured by iC-TW3 is also displayed visually, as are the alarm messages for EEPROM checksum, excessive temperature, and sensor errors. Each individual signal gain path can be set to power-down mode to save on power dissipation.
Figure 3. Signal conditioning via a USB interface for development and production
Similar functions can be set in the third channel for the Z signal path. This can be used to scan incremental encoder reference tracks for angle and motion measurement, for example, or as an adjustable comparator with gain and offset settings for alarms. Automatic offset compensation for cyclic signals, such as those from sine/cosine scans with the maximum adaption frequency and the target amplitude (1/2 Vpp internally or a predetermined external value), is selected for all sensor signal channels using the Miscellaneous (Misc.) menu. This can also be used to switch temperature compensation on and off and to set the upper temperature limit. The interpolating points and parameters for the temperature compensation of the characteristic curves (up to 16 look-up tables) are entered via an integrated editor (accessed under Extras).
Sensor Bridge Applications
Figure 4 is a circuit diagram for a motion sensor that scans two incremental tracks via magnetic or optical sensor bridges and then conditions the periodic sine/cosine signals, amplifying them to 1 Vpp and transmitting them differentially via the connecting cable to a 120 Ω line termination. Optionally, an index sensor signal can be conditioned and transmitted using the iC-TW3's third channel. The advantage of this approach is that the differential sine/cosine transmission is virtually immune to interference, to the point that its logical verifiability ensures functional safety in critical applications. On the receiver side the conditioned sensor signal can then also be digitized with a very high resolution, enabling a line break or short circuit on the receiver side to be easily identified.
Figure 4. Motion sensor with sine/cosine signal conditioning and differential analog transmission
After a power up, the iC-TW3 extracts its mode of operation and calibration data from the EEPROM and fills its internal RAM. Access is still possible via its 1-wire interface, enabling renewed compensation or a change in operating mode. These alterations can then be written by the iC-TW3 to the EEPROM. If the iC-TW3 detects an error (such as excessive temperature, an EEPROM checksum error, or a line break in the sensor element, for instance), the NERR output is activated. This alarm can then be transmitted via a digital output driver across longer lines or cables.
While the system shown in Figure 4 allows safe differential line driving on 120 Ω, the system in Figure 5 provides 100 Ω line driving. In the magnetic incremental encoder example, shown in Figure 5, the magnetic sensor bridge or optical signals are amplified and conditioned by the iC-MSB in a similar way to the iC-TW3. On a line with a 100 Ω termination, the iC-MSB provides a signal swing of 1 Vpp and is short-circuit-proof and error tolerant. The circuitry of the iC-MSB is such that it passes a Failure Mode and Effects Analysis (FMEA) and is therefore suitable for use in safety applications, such as the SINUMERIK control system from Siemens.
Figure 5. Magnetic encoder with analog signal transmission for safety-critical applications (Click image for larger version)
As described, the sensor signal conditioning should always include, besides flexible signal conditioning options, the entire signal transmission path, containing both signal conditioning and analog line drivers. This helps to reduce system cost and meet functional safety requirements. On-chip temperature and automatic offset compensation offer new ways to increase system performance and reduce the effort in the control system.
ABOUT THE AUTHORS
Marko Hepp, Sales and Applications Manager for BiSS Products ([email protected]) and Joachim Quasdorf, Sales and Applications Manager for Interpolation and Encoder Products, ([email protected]) can be reached at iC-Haus GmbH, Bodenheim, Germany; +49 6135-9292-300.