PLC/DCS I/O Module: Analog Input

I/O modules are building blocks of industrial control systems used in end equipment such as PLC (Programmable Logic Controllers), PAC (Programmable Automation Controller) and DCS (Distributed Control Systems). Analog input modules capture and measure signals such as temperature, pressure, flow, level, vibration and motion. These parameters are transmitted from equipment such as sensors and other field devices on the factory floor in the form of current or voltage. Current-loops typically range from 4-20mA while voltage inputs can be single ended or differential ranging from 0-5V, 0-10V, +/-5V and +/-10V.
Signal Chain
The measurement accuracy is typically determined by the ADC resolution with thermocouple applications needing up to 24 bits. Delta sigma ADCs are a good choice with some products having a built-in PGA and Mux. Some products also offer ratio-metric measurement with built-in current sources and voltage biases for ungrounded thermocouples. Typical sampling rates top out at 1 KSPS to measure slow changing variables like temperature, pressure, level and flow. Specialized applications like plastic injection and systems which aggregate a large number of channels need larger sampling speeds of up to 500 KSPS. SAR ADC’s are a good choice for these applications. Simultaneous sampling architectures are used for modules used in condition monitoring, motion control and power automation.
Signal conditioning considerations include high input impedance when interfacing to sensors with low source impedance which is achieved with input buffers and INA's (Instrumentation Amplifiers). Other considerations are large common mode voltage range and common mode rejection of interference from motors, ac power lines and other sources which inject noise on the analog inputs. Since the inputs could be signals from thermocouples and other low level sensors, low offset voltages as well as minimal drift of the offset voltage over temperature is needed to maintain accuracy. Precision bipolar op amps are used extensively. Differential amps can be used when the common mode range is greater than the power supply rails. Finally, PGA's (Programmable Gain Amplifiers) can be used when the input voltage range can vary over multiple sensor types.
Processor
Development platforms and software tools in addition to support which quickens time to market are highly desired. Connectivity options ranging from Ethernet ports to UARTs to simple SPI ports are desired. Ethernet enables integration of field devices to the corporate enterprise network. Most industrial control applications need simple low cost PLC's with lots of digital I/O and integrated functions. These include fault diagnostics, watchdog timers, low power, multiple SPI ports, UART's, integrated analog peripherals like ADCs and PWM outputs which perform similar functions to DACs. MCU's (Micro Controller Units) are a good choice for these applications. For applications which push capabilities and need higher loop rates along with advanced control algorithms to handle multiple functional domains like logic, PID control and motion, higher performance MPU's (Micro processing Units) are needed.
Industrial applications need to support a multitude of communication options. Many of TI's processors include a Real time Processing Unit (PRU) to support a truly programmable approach for these communication standards. In addition, the ARM architecture offers the performance and low power needed for mission critical functions.
Interface
There are a number of field bus options – both serial (RS-485, DeviceNet, Profibus, CAN etc.) and Ethernet based. Extended cable reach and diagnostics to debug wire breaks provides system designers significant value. Integrated digital isolation on the interface transceivers is highly desirable to reduce board space. In addition the interface between the ADC and the controller can be digitally isolated either by using opto-couplers or TI isolators built with capacitive technology.
Microcontroller
The Hercules™ Safety MCUs offer an ARM Cortex-R4F based solution and are certified suitable for use in systems that need to achieve IEC61508 SIL-3 safety levels. These MCUs also offer integrated floating point, 12 bit ADCs, motor-control-specific PWMs and encoder inputs via its flexible HET Timer co-processor. Hercules Safety MCUs can also be used to implement scalar and vector-control techniques and support a range of performance requirements.
Power
To protect against transients and ground loops, the field side which interfaces to sensors is electrically isolated from the control side. This is done on a per channel basis or by isolating groups of channels from each other and from the control side. The design can be customized for performance and cost by using a DC-DC converter and discrete components or pre-built isolated DC-DC converter modules can be used. Other considerations include high efficiency and integration along with smaller packages. Picking a DC-DC converter with a large input range will protect again supply transients. Finally, using LDO's with good PSRR (Power Supply rejection Ratio) to supply the precision analog circuitry will reduce the power supply ripple and preserve system accuracy and resolution.

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