BR(E)AKING NEWS!!!

Introduction:

As we venture onto our highways we hardly give much thought to the operation or complexity of our vehicle braking system. We only think about it when we have to suddenly slam on the brakes to avoid an impending catastrophe. At that instant we hope and pray our car will stop in time for our safety, and the safety of others. Most don’t realize the extensive design and testing that goes into braking systems to insure we stop in time.

Take the brake rotor for example (see figure 1). Under normal operating conditions it is subjected to extreme temperatures and forces, which causes rotor distortion and possible failures. To make certain it can withstand these conditions, extensive laboratory testing is performed to refine the design for longevity, short stopping distance and vibration free operation. Production lines are equipped with inspection equipment to provide 100% assurance that poor quality products are not used. Extensive on-vehicle testing is done under real life conditions to test prototype units and engineering designs. Test vehicles are driven for long periods of time while brake performance is monitored by a variety of sensors and data acquisition systems.

For efficiency reasons brake rotors are becoming lighter, thinner and designed with cooling vents to improve performance. These changes continue to reduce the braking surface, forcing designers to consider alternative materials and designs. This is where MTI Instruments (MTII) comes in.

The Driving Test:

In order to simulate what a driver encounters on a daily basis, test vehicles are equipped with both displacement and temperature sensors to actively monitor the brake rotor. Data on disk runout, disk thickness variation (DTV), disk “coning” (warpage), and temperature are continuously collected and monitored from inside the vehicle. A combination of city, country and highway driving courses are set up along with many designated braking locations to fully simulate in days what a driver typically experiences in weeks or even months. This information is used so that brake designers can determine how long a braking system will operate and its overall performance. Once a design is proven, mass production begins.

The Problem:

No brake rotor is perfect. Inconsistencies in manufacturing processes introduce thickness variation and runout. It is important to identify these conditions prior to installation in a vehicle. In order to accomplish this, dynamic measurements of the rotor are required on the production line. Conventional non-contact capacitance displacement sensors are used to monitor the distance between the probe and the rotor while spinning. Unfortunately, rotating targets frequently have an intermittent, uncertain or nonexistent ground path which causes inaccuracies in typical capacitance sensors.

The Solution:

MTII offers a unique “Push/Pull” Capacitance sensor (see Figure 2) which eliminates the need to electrically ground the rotor. This novel approach uses 2 sensing elements built into one probe body that work together to complete the measurement circuit. The push/pull probe design provides a clean and consistent electrical path resulting in decreased noise, higher accuracy and significantly more stable and repeatable measurements.

Tests were performed to compare the results of a standard capacitance probe to that of MTII’s Push/Pull technology. As the rotor was rotated, the output of each sensor was captured on an oscilloscope and is presented in Figure 3. Clearly it can be seen that the Push Pull technology provides a much cleaner and more accurate output signal.

MTII’s newest Multi-Channel brake rotor measurement system, the Accumeasure ™ 500 (see Figure 4) provides up to 6 independent measurement channels in a rugged, compact amplifier package. It provides the manufacturer the ability to measure thickness in 3 separate areas along the brake rotor radius “on the fly”. The AS-500 can operate on 12 Vdc power for in-vehicle testing and its compact design is ideal for installation into small, confined spaces. The amplifiers are specifically designed to be protected from shorts caused by the probe contacting the brake rotor or from contaminants such as water or oil.

Based on the push/pull advantages, several major Japanese and Korean vehicle manufacturers have standardized on MTII’s Accumeasure sensors for their testing requirements. In addition to brake testing MTII’s sensors have been used in demanding applications such as spindle and shaft runout, engine vibration, thermal expansion and contraction, suspension travel and fuel injector motion, to name a few. If you have a challenging noncontact measurement application, contact MTII’s experienced Application Engineers for more details on laser, capacitive, fiber-optic and eddy current sensors. With over 50 years of product line history MTII will provide a practical, cost effective solution to meet or exceed your requirements.

Using the Accumeasure 9000 Capacitance Transducer Amplifier to Measure Position and Vibration Displacement of Very Small Targets

Advances in nearly all fields of science, technology, and manufacturing has brought with it the need to measure the position and/or vibration displacement of very small mechanical parts and assemblies.

Since these devices often operate at high speed and with relatively small movements, it usually dictates the use of noncontact sensors to insure that the natural movements are not compromised by the sensor itself as may happen when using contacting sensors. In addition, the noncontact sensor must be very small and have an even smaller spatial sensing capability.

One such recent application required measurement of the position and vibration displacement amplitude of taut wires .002" to .005" (50 to 125 microns) in diameter.

After several unsuccessful attempts at making this measurement using a variety of conventional noncontact sensors employing magnetics, optical, and capacitance technology, the customer turned to MTI Instruments for a solution to the problem.

Fortunately, MTI Instruments had recently completed the development and market introduction of the AccumeasureT 9000 Capacitance Transducer Amplifier, which can operate with up to 100X smaller capacitance sensing area than most previous capacitance sensors, while still maintaining acceptable levels of standoff distance and sensing range.

This permitted MTI Instruments engineers to design a capacitance sensor with a sensing electrode only .005" wide x .050" long, which when operated with the Accumeasure 9000 amplifier at full sensitivity could give a sensing range of up to .008", as required by the application. The .005" wide dimension of the sensing element is aligned with the .003" diameter of the wire target and the .050" length dimension of the sensing element is aligned with the axial length of the wire target. Most previous capacitance transducer amplifiers could only give about .0005" (500 micro inches) range with this size sensor, which would be far short of the customer's requirement.

Figure 1 shows the dimensions of the face of the sensor; Figure 2 shows the gap response, and Figure 3 shows the lateral sensitivity when operating at a standoff distance of .005". Figure 4 shows a sketch of the electric field lines from the sensing electrode to the wire.

This is only one example of the unique capabilities of the Accumeasure 9000 amplifier, which has permitted MTI Instruments engineers to solve different and demanding applications for customers in a wide variety of research, engineering and manufacturing fields.

Although the MTI Instruments Accumeasure ™ capacitance based instrumentation is designed primarily to make noncontact measurements of position, displacement, vibration, or runout, it can also be used to make noncontact thickness measurements of dielectric¹ materials. If the thickness is known or can be independently measured, the Accumeasure System can also be used to measure the dielectric constant of insulating materials.

The Accumeasure System uses a constant current signal source at either 16kHz (Accumeasure 500 & 1500) or 100kHz (Accumeasure 5000) carrier frequency. The transducer (probe) is a passive element, in that it contains no active electronic circuitry. All of the active circuitry is contained within the Accumeasure electronic amplifier and supplied to the probe through low noise coaxial cable. The Accumeasure constant current amplifier circuitry supplies the control voltage required to keep the sensing current at a constant level over the rated displacement sensing range of the probe and amplifier combination. A high precision buffer amplifier is used to electrically drive the coaxial cable shield and the coaxial capacitance probe structure at the same amplitude and phase as the sensing signal. This effectively cancels all stray capacitances and permits the amplifier to respond only to the capacitance between the face of the sensor probe and the target surface. This results in the Accumeasure amplifier having a linear response to either gap changes, or dielectric material thickness changes.

Figure 1

The Accumeasure equipment operates as a classic parallel plate capacitor system with the face of the probe being one of the plates, and the target or surface being measured as the other plate. As shown in Figure 1, a ground return path must be provided from the target back to the low side of the Accumeasure amplifier in order to complete the current path. In many practical situations or applications, the target is already at ground potential and the ground return path is automatically provided through the power line grounding and the grounded power cords, but it is usually best to attach a grounding wire directly from the target to the ground return connector on the Accumeasure amplifier. When measuring thickness of dielectric materials, a fixed gap is established between the probe face and the grounded return plate. The dielectric material to be measured is placed in the fixed gap, or placed in contact with the surface of the ground return plate. If the material to be measured is moving, it may be passed through the gap without contact. The MTI Instruments KD-CH-IIID, a precision calibration micrometer fixture is recommended for conducting these tests.

The Accumeasure System operates on the basic principle that C_p= K(A/D) where C_p= the capacitance formed between the face of the capacitance probe sensing element and the target surface; K =the dielectric constant of the air, plus other physical constants; A = the area of the sensing electrode at the face of the probe, and, D = the distance between the sensing electrode and the target surface. A more complete description of the Accumeasure operation and circuitry can be obtained by referring to the Users Manual for the Accumeasure amplifier in use.

Since the Accumeasure operates as a constant current supply, then:

V_o α (l/Cp) where V_o = amplifier output voltage
C_p α (1/D)
then V_o D

A block diagram of the Accumeasure circuitry and sensing technique is shown in Figure 2. An active guarding current is used to cancel the coaxial cable capacitance and to prevent divergence of the electric field at the probe face.

Figure 2

If an insulating material having a dielectric constant greater than air (1.0) is inserted into the sensing gap, then the capacitance C_p will be changed even if the gap between the probe face and the ground plane remain constant. This effect can be used to measure the thickness of a test sample.

If the dielectric constant K of the test material is known, then the thickness can be measured using the following procedure:

Since C_p = K(A/D), if A and D remain constant, then C_p α K. Also, since V_o α (1/Cp), then V_o α 1/K. This effect can be seen in Figure 3, which shows the output voltage versus gap (or D) when the gap is completely filled with air, having a dielectric constant of 1.0, or completely filled with some other nonconducting material such as oil or plastic having dielectric constant greater than 1.0.

Figure 3

When dielectric constant of material to be measured is known:

If an Accumeasure amplifier and capacitance probe are set up at any air gap within the operating range of the system, the Thickness Sensitivity Factor for a material having a particular dielectric constant can be calculated as follows:

Thickness Sensitivity Factor =    Operating gap   

		                VoAir- (VoAir ÷ K)

Example A1:

If the fixed operating gap is 0.020" and the Vo(AIR) is 10.00 volts, and the dielectric constant K of the material to be measured is known to be 3.00, then:

Calculated Thickness Sensitivity Factor =          0.020"           =  0.003" / Volt

		                          10.00 - (10.00 ÷ 3.00)

Therefore, if a test sample of the same dielectric constant of 3.00 but having an unknown thickness were to be introduced into the same gap, the thickness could be calculated as follows:

Test sample thickness = Thickness Sensitivity Factor x (Vo Air - Vo Sample)

If the output voltage with the test sample in place reads, for example, 6.50 volts, then:

Test sample thickness = 0.003" / Volts x (10.00 - 6.50) = 0.01050"

Example A2:

If the operating gap was 0.008", the V_o AIR was 9.00 volts, and the dielectric constant K was known to be 2.30, then the Sensitivity Factor could be calculated as follows:

Sensitivity Factor =          0.008"           = 0.00157" / Volt

		       9.00 - (9.00 ÷ 2.30)

If a test sample of the same dielectric constant of 2.30, but having an unknown thickness were introduced into the same 0.008" operating gap, and the voltage then dropped to 7.30 volts, then:

Sample thickness = 0.00157" / Volt x (9.00-7.30) = 0.00267"

If the dielectric constant of the material to be measured is unknown, or cannot be determined by contacting the manufacturer or referring to handbook references, then there are two methods to measure it using the Accumeasure equipment.

When the dielectric constant is unknown, but a sample of known thickness is available or can be measured by independent means:

Insert the sample into the operating gap and measure the change in the output voltage. The sensitivity factor can then be calculated as follows:

Sensitivity Factor =      Sample Thickness, in inches       = 0.0021" / Volt

		                 (10.00 - 7.63)

The thickness of additional pieces of continuous strips of the same material can now be measured without contact by multiplying the sensitivity factor by the change in the output voltage due to the presence of the material being measured. If the change in voltage was 3.1, then the thickness would be: (0.0021"/ VOLTS) X 3.1 VOLTS = 0.0065" thickness.

When the dielectric constant is unknown and a sample of known thickness is unavailable

If the dielectric constant is unknown or uncertain, then it can be measured with the Accumeasure equipment by placing a sample of the material in the operating gap and then adjusting the gap until the test material completely fills the gap. A reading of the output voltage is taken and the material carefully removed without changing the gap setting. The reading of the output voltage is now taken without the material in the same gap. The dielectric constant can be calculated as follows:

K=   Vo AIR 
   Vo Sample

After the dielectric constant is established, then Method A can be used to measure additional samples of the same material.

Some thin film dielectric materials may be slightly conductive or may be coated on one or both sides with very thin films of conductive material. If this is the case, then care should be taken before using the Accumeasure System to measure thickness or dielectric constant to insure that the results are valid. This can be done by placing the test material in the operating gap and then moving it so that it alternately touches the probe face and the ground return reference plate while noting the change in output signal. Another check that can be made is to reverse the test material sides within the operating gap and noting the change in output voltage. In either case, if the output voltage changes more than about ±10 millivolts, the material may have some bulk conductivity or thin film plating on one or both sides.

The output voltage variations, due to positional changes or reversals of the test material within the operating gap, can then be multiplied by the Sensitivity Factor to determine the uncertainty or possible error in the thickness reading. If the dielectric is coated on one side only, then the coated side should be placed in contact with the grounded reference plate to obtain proper thickness measurements.

^1. Dielectric materials are those materials which behave more like insulators than conductors.

They are classified as having very few electrons available for conduction in contrast to metals which have an abundance of free electrons which can travel over large distances inside the metal. Inserting a dielectric material between two capacitor plates has the effect of increasing the capacitance between the two plates.

The following is a list of the dielectric constant of some common gases:

The following is a list of dielectric constant for some common plastics, rubber, glasses and liquids:

* The dielectric constant and the conductivity of water is very dependent on the mineral content and impurities. It is not recommended as a gap medium for use with capacitance probes.

1. Most gases have a dielectric constant very close to 1.000 and therefore have little effect on the output signal of a capacitance probe when operated as a displacement sensor relative to a conductive target.

2. Most plastics, glasses, and oils have dielectric constants in the order of 2 to 5 times higher than air and are therefore good candidates for capacitance based thickness measurements, or for use in the operating gap of capacitance displacement sensors, but the exact dielectric factor must be known or measured if accurate thickness or displacement information is required.

-- Curt Kissinger
MTI Instruments Applications Engineer

In order to maintain proper pressure profiles along the glass furnace, the quench gap plate spacing must be maintained at predetermined gaps. Unfortunately, due to process changes and temperature swings, this gap can vary throughout the day. This variation causes the glass to change in thickness, producing uneven temperature gradients and creating potentially weak glass panels. Finding a method of maintaining this constant gap proved to be difficult because of the harsh environment where sensors would be required.

The final probe configuration was a custom 8” long, ASP-100-CTA sensor with 0.5” (12.5mm) measurement range. Even with such a wide range, the resolution obtained was well within the customer’s requirements. Continuously monitoring the gap provided real-time feedback to the quench furnace and resulted in substantial decrease in downtime, and greatly improved product quality.

MTI Instruments Inc. offers several styles and types of non-contact sensors. Our high precision laser, fiber-optic and capacitance systems provide resolutions to 0.04 micro-inch (1 nm) and frequency responses to 500 kHz. Contact MTI’s experienced Application Engineers for solutions to your difficult measurement needs.

MTII’s Accumeasure 1500 capacitance sensor provides solution to difficult thermal expansion measurement application

As today’s automobile engine become more sophisticated the need to perform additional testing becomes essential. Many components now serve several purposes in order to produce higher horsepower in a smaller overall package. This requires each component to be manufactured to tighter tolerances. An example would be the oil pan.

For years the sole purpose of the oil pan was to act as a reservoir for the engine lubricant. Some modern engines now utilize the pan as a structural component. It is machined from a rigid steel or aluminum casting and designed to support the lower portion of the crankshaft bearings and rear main seal. This development has introduced a potential problem area that must be analyzed.

As engines go through heating and cooling cycles it is essential that the engine block and oil pan expand and contract at nearly the same rate. If not, stresses can be introduced to the pan gasket, main seal and bearing races. This can cause oil leaks, premature bearing failures and unwanted noise and vibration.

GM engineers approached MTII looking for a way to monitor the relative motion between the block and the pan to determine if excessive stresses were occurring. The Accumeasure 1500 capacitance system was selected, because of its high accuracy and multi-channel capabilities. A test was performed using 16 capacitance probes strategically mounted to an engine. Each probe had an operating range of 0.04” (1mm) with a resolution of better than 10 micro-inches (0.25 microns). Before starting the test, all outputs were set to zero volts. As the engine was started, and brought up to operating temperature, the output of each sensor was recorded. This data was used to determine the relative displacement between the two components.

It was found that the relative motion was well within the seal and bearing manufacturers specification. Had movements of 0.01” (250 microns) or greater been encountered the manufacturer would have had to consider redesigning the lower motor components.

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 1400F (760C). 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. Contact MTI’s experienced Application Engineers for solutions to your difficult measurement needs.

Measuring Machining Imperfections and Runout in Automobile Wheel Spindles

Introduction

Problem

No grinder produces a perfect surface. Grinder wheel runout, improperly dressed wheels and fixture vibrations can cause chatter on the spindle surface finish. Chatter is a series of microscopic, repetitive imperfections, or waves, in the surface of the material being machined. A large, Tier 1 Supplier to the automotive industry approached MTII looking for a method to measure and classify these imperfections as a 100% quality control check. They required a production environment sensor that had small spatial resolution, high frequency response and large standoff distance for easy loading and unloading of parts. Additionally, the measurement accuracy had to be less than 4 micro-inches (0.1 microns) with a resolution of 1 micro-inch (0.025 microns), or better, in order to properly quantify the defects. System noise had to be kept to a minimum in order to prevent unnecessary parts rejection and scrap.

Solution

Results

Brake Rotor Analysis Using Non-Contact Measurement Sensor

MTI’s Accumeasure 9000 capacitance sensors provide the ideal measurement solution for brake rotor thickness, wear, runout and coning

Introduction

Today’s automobile braking systems are becoming more complex each year. Manufacturers are striving to provide improved braking force and performance in smaller, lighter packages. Additionally, consumers are demanding smooth, vibration free braking. This requires the brake system to be manufactured to tighter tolerances and perform under extremely harsh conditions. To accomplish this, manufacturers are required to perform 100% inspection and improve rotor performance techniques.

Problem

No brake rotor is perfect. Inconsistencies in manufacturing processes introduce thickness variation and runout. It is important to identify and correct these inconsistencies in order to produce a reliable system that provides vibration free operation. In order to do this, dynamic measurements of the rotor are required. Conventional capacitance methods use a non-contact measurement probe which monitors the distance between the probe and the electrically grounded target (rotor). Unfortunately, rotating targets frequently have an intermittent, uncertain or nonexistent ground path. This introduces unwanted noise, instability, drift and reduces accuracy of the measurement.

Solution

MTI Instruments offers a unique “Push-Pull” capacitance measurement system that does not require a grounded target. It utilizes two probes, built into one body, that work together to complete the ground path. One probe pushes current into the brake rotor while the adjacent probe pulls the current out. The result is a “clean”, consistent electrical sensing path.

Results

Tests were performed to compare the results of a standard capacitance probe to that of the Push-Pull technology. Figure 1 shows the amplifiers and fixture used during the test. As the rotor was rotated, the output of each probe was captured on an oscilloscope and is presented in Figure 2. Clearly the Push-Pull technology provides a much cleaner output signal. The resolution obtained was better than 0.5 microns at a measurement range of 1.5 mm.

Figure 1	Figure 2

MTII also manufactures high precision laser and fiber-optic systems with resolution to 0.04 micro-inch (1 nm) and frequency response to 500 kHz. Contact MTI’s experienced Application Engineers for solutions to your difficult measurement needs.

Popular Marine Engine Manufacturer uses MTI Capacitive Sensors to Precisely Measure Engine Oil Film Thickness

Introduction:

Problem:

MTI was challenged by a top marine engine manufacturer to design a non-contact sensor to measure the microscopic oil film layer between the engine cylinder wall and the piston ring. Oil film thickness is an important parameter in engine development, and directly relates to oil consumption and engine life as well as efficient engine performance and emissions reduction. Engine testing is typically performed at high RPM’s with piston linear speeds approaching 500 inches/second. A measuring system with high frequency response and small sensor size was required to keep engine machining to a minimum, and not change the engine operating characteristics along with the ability to obtain dynamic measurement data as the piston ring passes the sensor face.

Solution:

MTI engineers arrived at a unique miniature sensor design with a diameter of 5.5-mm, measurement range of 50 microns, resolution better than 0.25 um and a response time of 20 kHz. The sensor was threaded to offer ease of mounting into the cylinder bore and precisely machined to allow the sensor face to be flush or slightly recessed into the cylinder bore. Accumeasure™ capacitive sensors measure the minute capacitance change due to the oil thickness variation and convert this change into a thickness measurement. The sensor offered a rectangular 0.12-mm x 1.25-mm sensing area and the resulting footprint was designed to be considerably smaller than the piston ring width to allow it to measure only the piston ring oil film thickness. High temperature epoxy and materials were used in the probe fabrication to enable it to operate at temperatures as high as 285 C (545 F) and to withstand high pressure.

The Equipment:

Other Applications:

MTI Accumeasure sensors can also be used in other applications such as determining the ratio of oil/air present in an engine passage. The Accumeasure equipment provides a distinct voltage output, depending upon whether oil, air or a mixture of both (foam) is present. The system works equally well in transmission research to determine the amount of cavitation in the fluid. Any foaming of the fluid indicates air is present in the mixture and will cause a noticeable change in the output signal. This type of data proves to be invaluable in determining the effect of mixing various additives into transmission fluids and the resultant knowledge helps researchers in designing a superior, longer lasting product.

Benefits:

If you have a demanding noncontact sensing requirement contact MTI’s team of experienced application specialists who will thoroughly and accurately analyze your requirements and provide you a practical, yet cost effective solution.