Category Archives: USB2250

A Closer Look at the USB2250 Data Acquisition

As a leading manufacturer of variable reluctance pressure transducers, we at Validyne Engineering make it our top priority to provide our customers with the latest technological advances. That is why we are pleased to introduce the USB2250 Data Acquisition.

What is a USB2250 Data Acquisition?

usb2250bThe USB2250 is a sensor interface that provides real world data acquisition for your PC through the USB port. Up to 16 different sensor inputs that are accepted by the USB2250 in any mix or combination with no external signal conditioning required.

All in One Configuration via the USB2250 Data Acquisition

When we say that up to 16 sensors are accepted no matter the mix, we mean no additional equipment or configuration is required. Thermocouples, RTDs, strain gages, LVDTs, potentiometers, VR sensors and low-level DC voltages are all wired directly to the terminal block..
In addition, the USB2250 Data Acquisition features 10 full scale input ranges from ±20 mV to ±10.24V full scale all with 16 bits of resolution. There is zero offset correction for low-level measurements. The USB2250 Data Acquisition also provides polynomial linearization for thermocouples and RTD’s. It produces a floating-point value directly in engineering units.

Included Software

Data Acquisition

Software includes a GUI configuration utility that gives the user the opportunity to set sensor type, gain range, channel, and other parameters. Easy Sense 2250 data acquisition software is included with Graphic Capabilities and Trigger Functionality included in the Premium Easy Sense version.

A LabVIEW VI is available for seamless integration to your existing software setup.

Exciting Uses and Possibilities

Data acquisition is the process of measuring electrical or physical phenomenon like voltage, current, pressure, temperature, or position with a computer. The USB2250 is a vital tool for those working in the oil, automotive, or medical industries and many more!

At Validyne, we work hard to provide our customers with the best equipment experience no matter where they operate. Providing equipment like the USB2250 not only makes the job easier, but is a more cost effective solution for sensor inputs to a PC.

Connecting the USB2250 Data Acquisition System to a String Pot

A string pot is a displacement measuring sensor that can conveniently measure displacements of a few inches to a few feet. The string pot is something like an ordinary tape measure but instead of a ruler there is a cable attached to a spring-wound potentiometer so that the distance the cable moves changes the position of the pot wiper. Simply mount the string pot securely and connect the cable to the moving part to be measured. When the potentiometer is powered, the output of the string pot is a voltage proportional to the displacement of the cable.

String pot construction is shown below along with a typical enclosed unit.

String Pot

 

 

 

 

String Pot

 

 

 

 

 

Electrically, the string pot is a simple potentiometer and it can be powered by the USB2250 and the signal received as a single-ended DC voltage. The connection diagram is shown below:


Data Acquisition

The +5 V power is supplied by the USB2250 terminal block. The position of the wiper changes the voltage at the A-In terminal from 0 to +5 Vdc, depending on the position of the wiper.

 

A simple scale factor is all that is needed to convert the voltage into a reading in inches. For example if a string pot has a 7 inch displacement, the signal will be 0 to +5 V from 0 to 7 inches. The scale factor for Easy Sense to convert the voltage signal into a reading in inches is 7 inches/5 Vdc = 1.4

Interfacing 4-20 mA Current Loops to the USB2250

Many pressure transducers and other field instruments use the two-wire 4-20 mA current loop for both power and signal. The 4-20 mA current loop is economical to install, using the same two wires for power and signal. It is also ideal for sending a signal over long distances – up to a mile or more – with high resistance to noise. Most data acquisition devices, however, are configured to accept voltage signals. This application note will describe how to interface a standard two-wire 4-20 mA current loop to the USB2250 sensor interface.

A typical 4-20 mA transmitter receives power from an external power supply. The power supply must be able to provide enough voltage and current to power the transmitter under all operating conditions. The transmitter will require some voltage just to produce a signal and the power supply must provide this plus any power needed to overcome any resistances placed in the current loop. The maximum amount of current required by the transmitter will be at least 20 mA, but it is best to select a power supply that will provide for 25 or 30 mA through the loop to allow for over-range indication by the transmitter.

To interface to a data acquisition device such as the USB2250 a resistor is placed in the loop and the voltage drop across the resistor will be connected to the USB2250 as a single-ended voltage input. To see how this works, assume the following conditions:

Minimum voltage required by the transmitter = 12 Vdc
Maximum loop current = 25 mA
Interface Resistor = 250 Ohms
Wire or other miscellaneous resistances in the loop = 20 Ohms

To calculate the voltage required for the power supply, we add up the voltage drops in the loop:

12 Vdc for the transmitter
Voltage drop through the wire = 0.025 * 20 = 0.5 Vdc
Voltage drop through the interface resistor = 0.025 * 250 = 6.25 Vdc

Adding these voltage drops together, the minimum voltage provided by the power supply must be 12 + 0.5 + 6.25 = 18.75 Vdc to push 25 mA through all the resistances in the loop. For a single loop the power supply will need to be rated for at least 18.75 * 0.25 = 0.47 W, but typically a single power supply will power many loops before wattage ratings become an issue.  We can use a 24 Vdc power supply – just as long as it is greater than 18.75 Vdc.

The diagram below shows how the current loop is interfaced to the USB2250 terminal block. Note that the power return is common to the USB2250 signal ground. The voltage drop across the resistor is 1 Vdc when the signal is 4 mA and 5 Vdc when the signal is 20 mA, being proportional in between.

4-20 mA USB2250

USB2250 scale and offset factors in Easy Sense software are used to convert the 1 to 5 Vdc input into engineering units. If, for example, the 4-20 mA signal is for a 100 psig pressure transducer, then at an input of 1 Vdc the pressure is 0 psig and at 5 Vdc the pressure is 100 psig.

The algebra is simplified by first determining the scale factor = change in pressure/change in voltage = 100/4 = 25.

Multiply the scale factor by -1 to obtain the offset factor: 25 * -1 = -25.

Check by determining the readings at each end point:

At 1 Vdc the reading will be R = (25 * 1) -25 = 0 psig

At 5 Vdc the reading R = (25 * 5) -25 = 100 psig

The USB2250 will read the input from the current loop and provide readings in psig or any other engineering units.

Connecting Strain Gages to the USB2250, Part II

The quarter-bridge strain gage configuration can be improved upon to increase the output signal into the USB2250 by adding another strain gage to the system. For the previous example, the second strain gage could be affixed to another part of the steel bar, as shown below.

steelbar2

The applied load and stress are the same as before, so the two strain gages will change in resistance by the same amount.

dR = Rsg * GF * E

Where:
dR = Change in resistance of the strain gage, Ohms
Rsg = nominal resistance of the strain gage, Ohms
GF = Gage Factor of the strain gage (stated by the manufacturer but usually about 2)
E = Strain, inches/inch

Assuming a gage factor of 2, the change in resistance in Ohms = 350 * 2 * 3.333 e-4 = 0.233 Ohms for each gage.

Because there are two strain gages we can construct a half-bridge circuit that will double the signal going into the USB2250:

sg3

The output of the bridges is now doubled.

In the example the strain gage resistance has changed from 350 Ohms at no load to 350.233 Ohms when 10,000 pounds is applied to the bar. The output signal of the Whetstone half-bridge is calculated as follows:

Vb = ((Ve * Rsg)/(R2 + Rsg)) – (Ve * R3)/(R3 + Rsg) = 0.0016637

Where:

Vb = voltage output of the bridge, Vdc
Ve = USB2250 excitation voltage, Vdc = 5
Rsg = Rsistance of the strain gages, Ohms = 350.233
R2 = Completion resistor, Ohms = 350
R3 = Completion resistor, Ohms = 350

So the circuit now produces 1.6637 millivolts at full load.

We can also express that as millivolts output per volt of excitation, or mV/V:

mV/V = 1000 * (Vb/Ve) = 0.33274

So the USB2250 will receive 0 to 0.33274 mV/V of signal from this circuit as the bar is loaded from 0 to 10,000 lbs.

To determine the scale factor for the USB2250 that will indicate applied load, we proceed as before:

Lb = mV/V * SF

Where:

Lb = Applied force in pounds
mV/V = signal from Whetstone bridge
SF = Scale Factor

Re-arranging this we get SF = Lb/mV.V = 10000/0.33274 = 30053

By entering 30053 into the Scale Factor box in the USB2250 software, we will now receive the readings as pounds of force applied to the bar.

Twice the signal coming from the strain gage circuit means that any noise present will be less of the total available signal. Because strain gages have such a small output, any chance to increase the signal should be implemented.

Connecting Strain Gages to the USB2250

Strain gages are sensors that change in resistance proportional to the strain of the item to which they are affixed. Strain gages can also measure the stress at the points they are attached and have been used extensively in aircraft frame testing and other structural studies. The relationship between stress, strain and resistance change as well as the proper way to use a strain gage in a circuit to interface to data acquisition can be a complicated subject. Here is a simplified look at the use of strain gages and how to connect them to the USB2250.

A typical strain gage, as shown below, is a very thin piece of wire bonded to flexible backing material with pads for connecting the strain gage into a circuit.

Strain Gage

 

The strain gage is affixed with epoxy to the structural piece that is to be measured. Strain gages are normally available in 120 Ohm, 350 and 1000 Ohm resistances. The change in resistance of a strain gage in use is often very slight, typically less than 1 Ohm. The bonding of the strain gage to the structural piece must be very tight so that small amounts of strain are efficiently transmitted to the gage – and this is something of an art.

So how much change in resistance will occur in a typical application? Here is a simple example: a 1 inch square bar of steel is loaded with a 10,000 lb weight. If a 350 Ohm strain gage is affixed to one of flat sides, what will be the resistance change?

Strain Gage

The first thing to do is calculate the stress on the structural part to which the gage is affixed, and that is simply the force applied divided by the area.

S = F/A

Where:

S = Stress, psi
F = Applied Force, Lbs
A = Area, Sq In

In this case 10,000 lbs/1 sq in = 10,000 psi.

How much strain occurs? Strain is expressed in inches per inch, and is a dimensionless number. It is simply the number of inches of strain that occurs over every inch of length in the direction of the load. A 10,000 lb load on a 1 foot long bar will have less total strain than the same load on a 100 ft long bar. But the amount of strain per unit length is constant.

E = S/M

Where:

E = Strain, inches per inch (dimensionless)
S = Stress, psi
M = Modulus of elasticity, dimensionless

In this case the strain is 10,000/30,000,000 = 3.333 e -4 inches per inch.

Where 10,000 is the applied force in psi and 30,000,000 is the modulus of elasticity for steel.

Once the strain is known for the applied load, the change in resistance can be calculated.

dR = Rsg * GF * E

Where:
dR = Change in resistance of the strain gage, Ohms
Rsg = nominal resistance of the strain gage, Ohms
GF = Gage Factor of the strain gage (stated by the manufacturer but usually about 2)
E = Strain, inches/inch

Assuming a gage factor of 2, the change in resistance in Ohms = 350 * 2 * 3.333 e-4 = 0.233 Ohms.  So in this case the change in resistance of the strain gage is just one quarter of one ohm out of to total strain gage resistance of 350 Ohms.

The USB2250 can measure such small changes in resistance when the strain gage is configured in a Whetstone Bridge circuit and connected to the USB2250 terminal block.

Note that there are a total of four resistances in the Whetstone bridge and only one is the active strain gage. This is called a quarter bridge configuration. Each of the other resistances must be 350 Ohms exactly, and these are called completion resistances. Completion resistor networks are provided by strain gage manufacturers. The quarter bridge interface to the USB2250 is shown below:

Strain Gage

In this example, the strain gage resistance has changed from 350 Ohms at no load to 350.233 Ohms when 10,000 pounds is applied to the bar. The output signal of the Whetstone bridge is calculated as follows:

Vb = ((Ve * Rsg)/(R2 + Rsg)) – (Ve/2) 

Where:

Vb = voltage output of the bridge, Vdc
Ve = USB2250 excitation voltage, Vdc = 5
Rsg = Rsistance of the strain gage, Ohms = 350.233
R2 = Completion resistor, Ohms = 350

So the circuit in this case produces just 0.83186 millivolts at full load.

We can also express that as millivolts output per volt of excitation, or mV/V:

mV/V = 1000 * (Vb/Ve)

So the USB2250 will receive 0 to 0.1664 mV/V of signal from this circuit as the bar is loaded from 0 to 10,000 lbs.

If we wanted the USB2250 to indicate the weight applied to the bar – making it into a kind of a scale – than we can determine the scale factor needed to convert the mV/V reading into pounds of force:

Lb = mV/V * SF

Where:

Lb = Applied force in pounds
mV/V = signal from Whetstone bridge
SF = Scale Factor

Re-arranging this we get SF = Lb/mV.V = 10000/0.1664 = 60106

By entering 60106 into the Scale Factor box in the USB2250 software, we will now receive the readings as pounds of force applied to the bar.