Quality Magazine

Develop an SPC System

April 1, 2003
Choosing a new SPC system is more than just picking out software. For many companies, sending statistical process control (SPC) data to a customer is mandatory, but where does one begin when choosing an SPC system? Creating a budget for a new SPC system requires taking many things into consideration. The type of gaging in use is one factor because it influences the SPC software choices, as well as the amount of operator training that will be required to maintain the system.

Many types of measuring devices are available and many are designed for specific manufacturing processes. How these devices output measurements is key to software selection. The majority of SPC software programs can collect the output of a serial gage or a gage interface product. A gage interface will take the output from a variety of devices and standardize that output to a serial string the software can accept.

Which one?

The type of gage used will determine the type of gage interface required to collect data. Three main types of outputs are commonly used by measuring instruments: digital, analog and linear variable differential transformer (LVDT).

Digital output gages have the ability to output an actual reading. The form of the output may be a serial RS232 output or another proprietary format. These devices measure a part and display or send a reading that matches the actual dimension in question.

The output of a digital device is captured by a gage interface or directly tied to a computer’s communication port. Because the output of a digital device is a numeric value, it is recorded and charted by most SPC software. Devices such as weigh scales, calipers, indicators and micrometers are examples of digital gages.

Analog output gages are different. These devices output a voltage that is in direct proportion to the change in measurement. To take a reading, an analog interface is needed to interpret the reading. This interface can be a PC card or an external device, but either way the interface has to take a reading of the voltage. This reading is then converted to a numeric value so it can be recorded by the SPC software.

For example: An analog output from a temperature device will output a voltage range—usually 0 to 5 VDC or ±10 VDC—that is in direct correlation to temperature.

The output of these devices is a voltage range that varies with the temperature being measured. The device will output the representative voltage, and the interface between hardware and software interprets the voltage as a measurement of temperature. Prior to using an analog output, the relationship of output volts to temperature measurement must be established. This requires measuring known values, such as minimum (MIN) and maximum (MAX) readings, to create a slope of the output. This process is called calibration.

Measurements are taken at known values and recorded by the interface hardware and software. The slope of the analog curve is created to establish the relationship of an analog signal to a known measurement. From this slope, any value can be calculated from the analog output signal.

Because every probe is different, each one needs to be calibrated before use, so the hardware and software understand how to interpret the data.

LVDT output devices—mainly probes—have a different type of analog output that requires a specific interface to signal condition the output of the gage to a reading. In simple terms, the LVDT output is a ratio of the input voltage to the output voltage. This ratio measurement allows for precise measurement because both the input and the output of the device are taken into consideration.

Like analog gages, LVDTs convert voltage readings into measurements, and thus need to be calibrated periodically. The actual ratio is then used against known values—a MIN and MAX master that allows calibration of each gage.

Variants of the LVDT probes are available, such as newer digital products. These products output their measurement in numeric form. The output is a numeric counter for the displacement of the probe. This makes it easier to connect LVDT products by reducing the signal conditioning hardware required for standard LVDT probes. These devices still require calibration so that the relationship of the output to the actual measured value is known. The output values are used to create a slope of the output and to compute a measurement value anywhere on that slope.

System requirements

How does this affect the SPC system available to a manufacturer? Most SPC software will connect directly to a digital device or interface. Both analog and LVDT devices require an interface and a protocol for calibration. Gage interface hardware can be selected that will do the calibration internally, allowing any SPC software to be used for the collection process.

Check with the chosen SPC vendor to see how, or if, the software can connect to the chosen device. Using a gage interface will help connect most devices to SPC software, but not all. Some of the options for LVDT interfacing are supplied as a complete system: gaging, interface, collecting and reporting.

The use of gage interfaces allows greater flexibility in selecting the type of gage to be used. The options available with a gage interface allow operators to control how data is sent to the SPC software, standardizing setups across the company. Some gage interfaces will perform calibration setup without the need for software. These interfaces allow any software to collect from virtually any device.

Many other factors need to be considered. The periodic need to calibrate analog and LVDT devices requires operators who understand the process of calibration and its importance in the accuracy of the devices. The use of analog and LVDT gaging can initially be more expensive and time consuming but can be offset by their durability in the field. The replacement savings over time may overshadow the initial cost of these devices. Durability, accuracy and size may override the added calibration requirements for the LVDT and analog systems.

The ease of use and understanding by all operators may lead to the use of digital gaging. Digital gages are getting smaller and more accurate every year. Long battery life and ease of implementation have made digital tools a mainstay in most applications. Matching the manufacturing process to the type of gaging used and the involvement of trained operators will come to play an important role in choosing the overall SPC data collection and reporting system.