Requirements Document

Submitted by Team E

Scott Hancock, Vishal Golia, Greg Sitrick, and Donnie Yazzie

Project Scope/Overview/Description

The device we are designing is a universal strain gauge amplifier. The strain gauge is simply a transducer and, therefore, will be simulated by feeding an analog signal as the input to the system. The resistance of a strain gauge changes as the surface it is mounted to experiences mechanical strain. Typically they are made from Silicon and used as force sensors. Since the signals from strain gauges are highly varied, our amplifier must have the capability of downloading configuration data to the amplifier in order to ensure that zero and span controls are not needed. The range of signals produced by the strain gauge can be simulated for this prototype unit.

The circuitry of the system consists of some op-amps, a programmable gain amplifier (PGA), a 12-bit analog to digital converter (ADC), and a digital to analog converter (DAC). A block diagram of this system is included in Figure 1.1 below.

The resistors simulate the strain gauge and provide a range of signals to be fed into the system. The op amp is then used to remove the DC offset into the system. By simulating the characteristics of a strain gauge we are able to test the amplifier circuit without an actual strain gauge device. After receiving the signal from the strain gauge, the PGA sets the gain of the amplifier to x1, x2, x4, or x8. The signal is then sent to an ADC, which in turn feeds the converted data back to the microprocessor for further analysis. The microprocessor receives, through a serial communication port, the data for the gain stage, the ADC, and the DAC. It sets the gain of the PGA, calibrates the system via the DAC in the control loop, and tells the ADC whether a digital or analog signal is required.

The focus of this project is to prototype the amplifier system. The final product would be customized by Dataforth to fit specific applications, due to the variety of uses of strain gauges in industry. Some examples in industrial processes that could utilize this amplifier are steel processing, automobile manufacturing, airplane testing, etc.

Figure 1.1, Block Diagram of Strain Gauge Amplifier.

Requirement Categories

1. Mechanical

This section describes the physical characteristics of the entire system. The [DRY1] amplifier is, at this time, primarily a prototype; as a result the mechanical aspects are not limited in size and weight. However, the entire circuit will be constrained to the following dimensions, which will allow for greater flexibility in both the manufacturing and design of the product.

· Size will be kept within 2ft width, 2ft length, and 2ft height.

· Weight will be less than 20 pounds.

· The components will be centralized fully on a breadboard.

· The interconnectivity of the components will be with wires and solder.

2. Electrical

This section contains the specifications for the electronic components and electrical interfacing of the system. It provides detailed characteristics of the individual components, as well as the general requirements for their interfacing.

· Bandwidth is not critical. However, it will be limited to a maximum of 100Hz.

· The PGA will have preset gains of 1, 2, 4, and 8.

· The resistors should be rated at 1/4 watt and have a tolerance of 1 or 2%.

· The bridge is powered with a laboratory supply, adjustable from 1 volt to 10 volts.

· The Microchip PIC family PIC17C756 microprocessor will be used.

· The microprocessor communicates through a serial port with the computer and receives commands from the host computer and sends data back in response.

· The microprocessor sets the gain of the PGA and loads the data into the DAC from the computer.

· The DAC (D/A) provides the analog signal to zero the bridge.

· The data will be displayed on the screen of the computer (PC).

3. Environment

This section provides specifications on the environment that the system will operate in. Since environment conditions will vary greatly, depending on the use, these specifications must encompass a wide range of possible scenarios.

· Must be able to operate in environments of 0C to +55C.

· The system should work under normal atmospheric conditions, with no vibration or shock to the system.

4. Documentation

This section provides a description of the documentation needed to properly operate the system, as well as the functionality.

· A user’s guide for the amplifier will be made available after the testing has been completed.

5. Testing

This section discusses the processes for which testing will be implemented. It describes the conditions for which the amplifier must satisfy.

· Simulation of strain gauges through the use a four-resistor bridge of 3 100-ohm (±2%) resistors and a single resistance adjustable from approximately 95 ohms to 105 ohms.

· Verify that the computer sends commands to our system and receives back data through the serial ports.

· Verify that with a 1.0-volt supply, the range of the strain gauge simulator circuit is ±12.8mV. Then increase the supply to 10 volts and verify that the range is ±128mV.

· Verify that the microprocessor will calibrate the system.

· To check the accuracy of the system, measure the input voltage with a digital voltmeter and compare to the output.

6. General

This section provides general requirements that the system must comply with. The device will be used in various situations, each of which will require its own unique set of specifications. In order to accommodate this flexibility, the device will be designed to allow the user to easily change the configuration of the device with minimal problems.

· The system must not need zero and span controls.

· The accuracy of the system must be at least 0.1%.

· A simple control loop should calibrate the system.