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Lachlan

Isolated DC Remote for Wide Area and Local Area Control & Monitoring

Status: Proposal
1585
1
votes
May 18, 2014
DC Remote PCB.jpg

This device provides isolated sensor measurement and control to allow equipment to be monitored and controlled without the need to consider polarity or high common mode voltages. The device provides the following measurements and controls.

4x Digital Inputs / Counters / Timers:

- Each of pair of two digital inputs share a common return, and each pair is isolated from other pairs and from all other circuitry.

- One 6 pin detachable plug for the 4 digital inputs and two returns.

- Propagation delay through the optocoupler and software for edge detection is 5μs for high and 65μS for low, therefore each pulse must have a minimum pulse width of 65μs and total period of greater than 70μs (14kHz max frequency).

- Input voltage 5 - 15V.

- Channel configuration can be:

     * Normal or inverted, positive or negative edge triggered.

     * Digital, 32 bit Counter or high speed timer.

8 x Digital / PWM Output:

- Each of pair of two digital outputs share a common return, and each pair is isolated from other pairs and from the remaining circuitry.

- Two 6 pin detachable plugs for the 8 digital outputs and four returns.

- Each of the digital outputs can operate in PWM mode but each set of four PWM outputs share a common timer and must operate with the same frequency.

- PWM channels are 16 bit and can operate up to 14 kHz, and a duty cycle from 0-100% in up to 1000 steps.

- All outputs can be independently configured as either PWM or standard Digital output.

- Maximum switching Voltage is 40V and switching current is 50mA.

2 x DC Voltage:

- Two bipolar DC voltage measurements of +/- 50V with common neutral.

- The DC measurements are not isolated from all other inputs, outputs, power and communications channels, except the Temperature measurement.

- The voltage range can be set at time of construction.

- Resolution is 0.05% and accuracy around 0.2%. Accuracy can be improved by calibration.

- Anti-aliasing filter at 20Hz limits DC variation to around 5Hz. 1 x DC Current: - This current measurement is isolated from all other circuitry.

- One high power bipolar current channel of +/- 20A. Range can be set at time of construction to +/- 5A, or +/- 30A.

- 2 pin screw terminal used for input.

- Resolution is 0.2% and accuracy is around 1.5%. Accuracy can be improved by calibration.

- Anti-aliasing filter at 20Hz limits DC variation to around 5Hz.

1 x Thermocouple:

This interfaces to a K-type thermocouple.

-The thermocouple is compensated using an onboard RTD to cancel the effects of changes in terminal temperature.

- Temperature range set to -50 - 150⁰C, but is capable of measuring beyond these values.

- Accuracy around 1%.

Other hardware:

- Processor is a 32 MHz ATxmega32A4 processor set with a sensor scan rate of 200mS. Although this scan rate can be increased, other limitations limit the benefit of doing so.

- Communications is via fully isolated RS485 half-duplex capable of up to 115,200 baud over short distances, or fully isolated SPI interface up to 1MHz over very short distances.

- Power supply can be between 7 – 30V. This is also fully isolated from any of the sensor measurements.

Comments:

To record these measurements over time, this device seamlessly integrates with the Wide Area Web Server project currently in elector-Labs. These measurements can then be displayed and graphed on a web page. In addition, control actions can be established to turn on/off devices when thresholds are exceeded. This allows functionality like monitoring a motor and shutting it down when it is not operating correctly, and also monitoring the change in voltage/current/temperature prior to the shutdown.

Relay Board:

The digital output is usually not used directly, but controls other elements. A typical enhancement is using relays. Attached is a description of a 4 channel Relay board that directly interfaces with one of the 4 channel digital outputs. Each of the relays provides normally open or normally closed contacts and can switch 250V 16A. The relay operate/release time is 8ms and contact bounce time of 4ms so the minimum high and low times are 12ms each.

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