Capacitance Sensor System: How a Small Electric Field Can Save You Big Money
1) Introduction
In an ideal world, machine parts would run in a frictionless state, never coming in contact with each other, never needing to interact. Bearings would be unnecessary, gears would not need to mesh perfectly, and imperfections on springs would go unnoticed. However, this is not an ideal world and physical interaction of machine parts does exist. Under such conditions, it is essential that these interactive parts be well-manufactured and as defect free as possible. Even the most seemingly innocuous imperfections in a bearing or on a gear shaft can create tremors that can build into noticeable vibrations capable of reducing or derailing a machine’s performance.
A perfect example of this is the basic spindle. It plays a critical role in a number of common machine systems including automobile suspensions, computer hard disc drives, and rotating axis assemblies in machining tools. Performance in all three of these systems benefits substantially from spindles with few surface imperfections. Surface defects, such as “chatter”, can lead to nuisance vibrations, unwanted noise, and wear of connected parts, like bearings.
As an example, consider a Tier 1 supplier of automotive parts producing spindle components for some of the world’s top automakers. Defective spindles can cause a number of problems for these automakers, especially in terms of wear and tear of connected parts like bearings, wheel hubs, steering arms, and tie rods. Depending on the level of defect, the steering system itself may even feel residual effects from the resultant vibrations. While these vibrations may not be immediately noticeable to car passengers, over time they can significantly reduce life span, increase maintenance costs, frustrate car owners, and ultimately hurt the automaker’s reputation.
The Origins of Spindle Defects
Spindle defects generated during manufacturing come from a few different sources including vibration of the grinding mechanism, an improperly dressed grinding wheel, and grinder wheel runout. Most manufacturers are well aware of these issues, and take precautions to reduce their impact as much as possible, but prevention is not guaranteed. Depending on the application, even the smallest imperfections in a wheel spindle can lead to future problems.
A spindle is generally finished using a cylindrical grinding process. The non-repetitive runout of the grinding wheel typically manifests itself as defects in the part being ground. For example, if a finished spindle is laid out flat and closely observed the surface would appear as tiny waves from the grinding process. As the spindle interacts with other moving parts, such as bearings, a regular but undesired vibration occurs that can have disparaging effects, such as premature bearing wear. Depending on the application, even patterned defects invisible to the human eye can catalyze these problems.
Tackling the Defect Problem
In a busy manufacturing plant, the battle to discover and prevent such defects is compounded by volume and speed, as assembly lines whisk huge numbers of spindles through the fabrication process. In order to weed out defected spindles from the bunch, the manufacturer really needs an inline system that can quickly and reliably analyze a spindle surface and quantify the level of defect. This system will need to be robust and able to handle the rigors of a manufacturing environment while still providing precise measurements that often require accuracy to the micro-inch. A laser triangulation system, commonly used to generate precise measurements for general manufacturing applications, would not work well here as the mirror-like surface of the spindle would create too much light scatter, preventing the sensor from producing accurate readings.
A more viable solution is a custom designed capacitance sensor system. A capacitance sensor uses a key principle of electromagnetic theory, capacitance, to generate an electric field that can then be measured and converted to the distance between the sensor and a target surface. Integrating this relatively simple concept into a tightly crafted and user friendly tool offers a number of significant benefits to spindle manufacturers or any manufacturer requiring a defect free surface on their product.
Returning to our Tier 1 supplier: in their busy production environment they need a way to provide quality control checks to measure and classify spindle imperfections. Our supplier is intrigued by the potential for using capacitance sensors in this process but wants to make sure these devices can meet their key needs: detect very small defects accurately, provide enough room to load and unload parts, and handle the rigors of a production environment. However, before discussing how well capacitance sensors can handle these demands it is worthwhile to understand how these sensors work in the first place.
How Capacitance Sensors Work
Because it is based on a key principle of electromagnetism, the capacitance sensor is uniquely qualified to work with electrically conducting materials. This does, of course, remove its usefulness for nonconductive materials like brick, wood, and ceramics; although technologies like the previously mentioned laser triangulation system can fill the void there. However, for any type of conductive material – namely metals – the capacitance sensor offers a remarkably accurate, reliable, and stable system for identifying even the smallest surface imperfection. Capacitance sensors work using the idea that if two conductive surfaces are separated by a distance, and a voltage is applied to one of the surfaces, an electric field is created. The property of capacitance essentially measures the ability of a body to hold an electrical charge. So the generated field is actually the result of the difference between the two charges stored on each of the surfaces. Capacitance is proportional to the surface area of the sensor and the dielectric constant of the material between the surfaces and inversely proportional to the distance between the two surfaces.
In a capacitance sensor system, the sensor itself acts as one of the surfaces and the target, such as a spindle, acts as the other. As a result of this parallel arrangement, the total surface area remains constant. The material in the gap between the two surfaces is generally air which has a known and constant dielectric property. This leaves only one variable – distance. To determine this distance an amplifier converts capacitance into a linear voltage proportional to the distance between sensor and surface. Depending on system configuration, the voltage change relative to distance change can go up or down. The precise amount of voltage change indicating a change in distance is known, in capacitance sensor terms, as sensitivity.
Because this system works on the idea of mapping voltage changes to distance changes, the accuracy of the sensor is tied directly to the linearity of the voltage. To maintain this linearity, the electric field between the sensor and the target system needs to remain uniform. To tackle this problem the sensor is fitted with a guard ring. The guard ring is an additional field surrounding the sensor adjusted to the same phase and voltage, acting primarily as a shield for the sensing area against warping and stray capacitance. Typically this guard ring is designed to extend to twice the width of the sensor’s measurement range; however, some systems allow this range to be reduced to the full scale measurement range of the system. This allows the use of smaller sensors for applications where real estate is limited.
One particular challenge posed by using capacitance sensors on wheel spindles is target curvature. Naturally, the sensor needs to be calibrated before use, and typically this calibration is performed under the assumption that the target surface is flat. However, if the target area is curved, like on a cylindrical surface, the calibration zero point shifts, offsetting the measurement values. There are two solutions to handle this. One involves an in place calibration that corrects for the offset, although this can reduce the sensor measurement range. The other option is to use a sensor significantly smaller than the diameter of the surface being measured (typically 10X smaller). In essence, the probe is now “seeing” a flat surface, much like the world appears flat from the average person’s vantage point.
Key Advantages of a Capacitance Sensor
In addition to accuracy and speed, capacitance sensors offer a number of highly useful advantages in a manufacturing environment. As it turns out, several of these advantages fit right in with our example Tier 1 supplier’s requirements for a quality control system. First, the system is non-contact by design, meaning it can obtain measurements without having to actually touch the surface being measured. This prevents inadvertent damage to the target surface and makes it easier and safer to integrate into a high-speed assembly line. In addition, capacitance sensors can also be placed relatively far from the target. This also reduces the possibility of damage and allows more room to maneuver parts in and out of the measurement fixture. And finally, the high frequency response of these sensors allows for dynamic measurements while the spindle is rotated.
Some custom designed capacitance systems offer the ability to “push” their range, giving larger measurement ranges to smaller sized probes. In other words, a probe with a 1/2mm measurement range can be configured to offer ranges of 1mm or even 2mm. For an assembly operation with limited space, or a target surface with odd angles, this pushing grants the manufacturer the flexibility to use a small probe and still have a sizable measurement range and standoff distance.
Small size can be important. Capacitance sensors come in different sizes, and these sizes affect the sensor’s spatial resolution. Spatial resolution refers to the smallest area under which a sensor element can accurately detect surface features. Due to the design of a typical capacitance sensor, the field generated is slightly larger than the diameter of the probe tip. A general rule is to use a probe tip that is 25% smaller than the smallest feature targeted for identification. In other words, smaller sensor probe tips can detect smaller features. Fortunately, capacitance sensors can come in extremely small sizes, down to the tenths of a millimeter. For our Tier 1 supplier, this not only offers them a sensor with an ideally small spatial resolution, it solves the issue discussed earlier of having a sensor small enough to appear parallel to the cylindrical surface of the spindle. In addition, sensor engineers can incorporate custom designs with radius or chamfered faces in order to accommodate any number of different spindle configurations or features.
Capacitance sensors also have extremely high resolution. In terms of a sensor system, resolution refers to the smallest amount of distance change that can be reliably measured. The high resolution characteristics of capacitance sensors makes them ideal for finding tiny imperfections and allows them to be used in a host of other applications. Combining this high resolution with a sensor design that promotes linearity produces a system that can not only detect sub-micron sized defects; it can do so extremely accurately – an attribute critical to the successful quality control operation at our Tier 1 supplier.
A manufacturing environment, such as that found at our Tier 1 supplier, can be highly rigorous, and any sensor system needs to survive in such an environment. Because capacitance probes are passive by design they are incredibly robust and can handle high shock and vibration conditions. Sensors made from stainless steel offer additional advantages. Stainless steel has a low thermal coefficient of expansion. Thermal expansion refers to the rate at which a metal expands or contracts due to temperature changes. A metal with a high thermal expansion rate will significantly expand and contract with temperature, changing the sensor area and therefore the measurement range. In addition, this expansion and contraction can actually change the probe to target operating distance, providing inaccurate results.
Conclusions – Quality Control and the Bottom Line
Any manufacturer that chooses to incorporate capacitance sensors into their assembly process will be abundantly rewarded. They can greatly simplify their entire quality control procedure using a method that is quick and reliable, and that offers output data that is highly usable for either manual validation or automatic qualification. Capacitance sensors come in many different sizes and shapes and can be fitted into a variety of different fixture designs. With a wide variety of configuration options available, they can be integrated into virtually any product assembly line, and handle nearly any kind of shape, from the flat surface of a computer disc, to the circular shape of a wheel spindle. Their high resolution and sensitivity mean manufacturers can ask these sensors to find even the smallest defects, and do so quickly and efficiently.
After considering the substantial benefits of incorporating capacitance sensors into their quality control process, our Tier 1 supplier opted to have a custom capacitance sensor system installed. They soon realized a number of additional rewards. First, they were able to develop a system that, in conjunction with an ultra high-resolution capacitance amplifier, quickly measured and sorted spindles, providing real time data on the quality of each part. Secondly, by placing the sensor system early in the machining process defective parts were discarded or reworked before additional manufacturing or assembly occurred. And finally, engineers were able to examine data over time and use it to identify root causes in the assembly line that may be generating these defects, a great way to cut operating costs and improve efficiency. Our Tier 1 supplier found this process so effective that they expanded the use of capacitance sensor systems to additional applications such as wheel flange runout and bolt position.
By using capacitance sensors, a manufacturer can know that each piece leaving the warehouse has been scrutinized to high standards. A consistent, quality product that is reliably defect free means happy clients. Whether it’s a spindle manufacturer making parts for major automobile makers or a plant handling other devices such as computer disc drives or rotating axis assemblies, sending out defective product can not only be costly in terms of returns and liability claims it can also damage reputations. Developing a quality control system based around the robust and powerful capabilities of capacitance sensors protects against this highly undesirable scenario and may, in fact, contribute substantially to the all-important goal of repeat business. In addition, these sensors can ultimately lead to reduced waste and operating costs, a tangible value that company accountants can happily measure in real dollars and cents.
MTI Instruments Inc offers several styles and types of non-contact capacitance sensors. The passive probes offer excellent thermal stability and are capable of temperatures in excess of 600°C. MTII also manufactures high precision laser and fiber-optic systems with resolutions to 0.04 micro-inch (1 nm) and frequency responses to 500 kHz.
MTI Instruments is a worldwide supplier of precision non-contact physical measurement solutions, portable balancing equipment and semiconductor wafer inspection tools. These products use a comprehensive array of technologies to solve complex real world applications in numerous industries. Our products consist of electronic gaging instruments for position, displacement and vibration applications within the design, manufacturing/production, test and research markets; semiconductor products for wafer characterization of semi-insulating and semi-conducting wafers; and engine balancing and vibration analysis systems for both military and commercial aircraft.
We are very proud of the crucial role we play with many of the world’s largest companies. Whether it’s basic research, quality, or process control, we offer solutions to both end users and OEM’s alike. Products you have contact with every day most likely have passed by an MTII sensor.
