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Sensors are simple, why are they so complicated to connect?

Sensors are simple, why are they so complicated to connect?

Sensors all have a basic function of measuring something, and they have to output that data in some computer readable format. This is the point where there are so many options and limitations but I will call them "interfaces". The most popular interfaces are digital, analog voltage, analog current, 1-Wire, RS232, RS485, CAN bus, and Bluetooth/ANT. Some of these interfaces can support multiple sensors per connector by daisy chaining or unique addresses which increase complexity, and others are simply 1 sensor per input. To make things even more complex, there is no standard method for each type of sensor to communicate so the sensor manufacturer determines what data and/or ranges are defined per sensor. Also, sensors require power that is not delivered via the interface, except for 1-Wire, so you also need to design the power supply for the sensor.

Lets review the various interfaces and ease of deployment.

Digital

This is simply an input that is either on or off, such as a push switch or tamper switch. The on/off condition is simply a change in voltage past a threshold.

Deployment - Easy
Daisy chain/multiple sensors: No

Analog Voltage / Analog Current

An analog input is sampled by an ADC (analog to digital converter) inside a microcontroller to determine either the voltage or current of the input, and uses a dedicated input for each sensor. The ADC has a bit resolution determined by the microcontroller which is typically in the 12 to 16 bit range. A 12 bit ADC has a range of 0-4098 and a 16 bit ADC has a range of 0-65535 through the set input voltage range. This range is scaled across the input voltage range so if a input range is 0-100V, a 12 bit ADC can measure a difference of .025V and a 16 bit ADC can measure a difference of .0015V. You do have to factor in noise on the inputs, shielding, and tolerances of various input components so accuracy is normally in the 1-2% range.

Deployment - Easy
Daisy chain/multiple sensors: No

1-Wire

1-Wire is a very nice input if your device has an input as power and data are delivered over a single wire to the sensor/input device. The most common type of sensor is a temperature probe or a key fob/unique ID for identifying a person or item. The complexity comes with daisy chaining sensors for multiple probes as you move from a 2 wire probe to a 3 wire interface that connects probe 1 to probe 2 to probe 3, etc. Also, since the data is delivered to the microcontroller as various data values, there is often a bit more processing to do before displaying the data. Still a very simple interface to utilize but more complex than the average digital/analog input.

Deployment - Medium
Daisy chain/multiple sensors: Yes
Visit Wiki - 1-Wire

RS232

There are many complexities behind RS232 but data is delivered very efficiently once it is working. The largest issue is that diagnosis is difficult and requires a variety of tools to determine the cause or fault. It is also becoming more common to see "TTL level" RS232 which means the voltage translation from a microprocessor operating at a common 0V-3.3V has not been translated to RS232 levels of -12V to 12V. Although the signalling looks the same, the voltage offset does not match and it does not work. TTL RS232 has to interface to another TTL RS232 interface, or go through a level translator. After matching the interfaces, you must cross the TX/RX pairs between the host and client so one side transmits to the other's receive pin. The wiring is typically a DB9 port, or just 3 bare wires, and requires some reading on which way the wires go. Then you can set the baud rate and serial port settings to match the sensor you are receiving data from. There is no concept of releasing the TX pin to allow multiple transmitters on the same bus so RS232 is a single sensor per RS232 port. RS485 is the industrial version with longer cable lengths and multiple transmitters per bus. There is no standard for the type or structure of data to be transmitted over RS232 so each manufacturer develops their own.

Deployment - Difficult
Daisy chain/multiple sensors: No
Visit Wiki - RS232

RS485

RS485 is similar to RS232 and all the complexities are the same, but RS485 allows for multiple transmitters on the same bus by adding intelligence and transceiver interface hardware to release the bus when not actively transmitting. The electrical interface specs are also more robust to allow for much more noise tolerance and longer cable lengths by using differential pair technology. Deployment is quite complex.

Deployment - Difficult
Daisy chain/multiple sensors: Yes
Visit Wiki - RS485

CAN bus

By adding data delivery structures and standards to RS485, you get CAN bus. There is a massive database of standard interfaces, data structures, and manufacturers that all collaborate to work on interchangeable data formats via CAN bus. This is the largest standard in the automotive industry and has a large user base. The large complication with deployment is the amount of hardware and structure you need in place to get on the CAN bus. As there are lots of sensors and computers on most CAN bus's, there are also multiple layers of protection at the interface level which requires more specialized hardware. On the plus side, the data is relatively standard as well as the communication layers, on the negative side, the hardware required to connect to it is complex.

Deployment - Difficult
Daisy chain/multiple sensors: Yes
Visit Wiki - CAN bus

Bluetooth/ANT

As wireless sensors become more popular in retail applications, like outdoor temperature and humidity sensors that connect to mobile apps, they are often asked about. The largest issue is the battery in these sensors as a retail setting is easy to notify a user to change a battery whereas an industrial application is unrealistic to have a network of battery operated sensors. There are definitely applications where running a wire is not feasible yet there is power on both ends. In this case, use a robust sensor and industrial communications link such as Zigbee that is designed for industrial sensors.

Deployment - Difficult
Daisy chain/multiple sensors: Yes
Visit Wiki - BluetoothVisit Wiki - ANT

Zigbee

By far the best integration wireless communication protocol for industrial sensors would be Zigbee. There are plenty of sensors and options to choose from for your application. There are a couple of possibilities though, and internal Zigbee chipset, or an external radio that is interfaced to with RS232. The internal option is much easier to integrate and work with but both operate much like RS232 for raw data delivery. There is not a strict spec or standard on how to transmit data over Zigbee. There is also some additional setup on how to pair the devices together on the same channels and filtering of unwanted devices too. Overall, Zigbee has very good range and good industry adoptance and has become the standard for industrial wireless sensors.

Deployment - Difficult
Daisy chain/multiple sensors: Yes
Visit Wiki - Zigbee

END
About the Author

Jeffery Palmer is an American entrepreneur, inventor, computer programmer, and engineer. In 2004, Palmer co-founded Global Satellite Engineering (GSE); a design and engineering firm for the satcom industry. During his tenure as Director at GSE, he has promoted and inspired the growth of technology for satellite communications. Palmer has given many lectures and works closely with industry leaders to offer custom solutions to clients of the satcom industry.

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Jeffery Palmer
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For more technical information, please contact Jeff@gsat.us or call +1.954.459.4001

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