Advanced Measurements

Transcription

1 Albaha University Faculty of Engineering Mechanical Engineering Department Lecture 11: Force, Strain, and Tactile Sensors Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University 2013 Aims This lecture aims: to identify strain gauges. to differentiate between tactile sensors such as: switch, piezoelectric, piezoresistive, MEMS, capacitive touch, acoustic touch and optical sensors to identify piezoelectric force sensors. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (2)

2 Outline Strain Gauges Tactile Switch Sensors Piezoelectric Sensors Piezoresistive Sensors MEMS Sensors Capacitive Touch Sensors Acoustic Touch Sensors Optical Sensors Piezoelectric Force Sensors Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (3) Introduction Classical mechanics deals with moving objects whose velocities are substantially smaller than the speed of light. Newton had found that acceleration is proportional to the acting force F and inversely proportional to the property of a body called the mass m, which is a scalar value: This equation is known as Newton s second law. In SI terms, mass (kg), length (m), and time (s) are the base units (see Table). Force and acceleration are derivative units. The force unit is the force that will accelerate 1 kg mass to acceleration 1 m/s 2. This unit is called a Newton. Mechanical units (bold face indicates base units) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (4)

3 Introduction Force sensors can be divided into two classes: quantitative and qualitative. A quantitative sensor actually measures the force and represents its value in terms of an electrical signal. Examples of these sensors are strain gauges and load cells. The qualitative sensors are the threshold devices that are not concerned with a good fidelity of representation of the force value. Their function is merely to indicate whether a sufficiently strong force is applied or not. That is, the output signal indicates when the force magnitude exceeds a predetermined threshold level. An example of these detectors is a computer keyboard where a key makes a contact only when it is pressed sufficiently hard. The qualitative force sensors are frequently used for detection of motion and position. A pressure sensitive floor mat and a piezoelectric cable are examples of the qualitative force sensors. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (5) Force Sensor The various methods of sensing force can be categorized as follows: 1. By balancing the unknown force against the gravitational force of a standard mass. 2. By measuring the acceleration of a known mass to which the force is applied. 3. By balancing the force against an electromagnetically developed force. 4. By converting the force to a fluid pressure and measuring that pressure. 5. By measuring the strain produced in an elastic member by the unknown force. In the modern sensors, the most commonly used method is 5, while 3 and 4 are used occasionally. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (6)

4 Force Sensor In most sensors, force is not directly converted into an electric signal. Some intermediate steps are usually required. Thus, many force sensors are the complex sensors. For instance, a force sensor can be fabricated by combining a force-to-displacement transducer and a position (displacement) sensor. The former may be a simple coil spring, whose compression displacement x can be defined through the spring coefficient k and compressing force F as: Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (7) Spring-loaded force sensor The sensor shown in Fig. a, is comprised of a spring and Linear variable differential transformer (LVDT) displacement sensor. Within the linear range of the spring, the LVDT sensor produces voltage, which is proportional to the applied force. A similar sensor can be constructed with other types of springs and pressure sensors, such as the one shown in Fig. b. The pressure sensor is combined with a fluid-filled bellows, which is subjected to force. The fluid-filled bellows functions as a forceto-pressure converter by distributing a localized force at its input over the sensing membrane of a pressure sensor. Spring-loaded force sensor with LVDT (a). Force sensor incorporating a pressure sensor (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (8)

5 Strain Gauges Strain is deformation of a physical body under the action of applied forces. A strain gauge is a resistive elastic sensor whose resistance is function of the applied strain (unit deformation). Since all materials resist to deformation, some force must be applied to cause deformation. Hence, resistance can be related to applied force. That relationship is generally called the piezoresistive effect and is expressed through the gauge factor S e of the conductor: For many materials S e 2 with the exception of platinum for which S e 6. For small variations in resistance not exceeding 2% (which is usually the case), the resistance of the metallic wire can be approximated by a linear equation: where R o is the resistance with no stress applied, and x = S e e. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (9) Strain Gauges A wire strain gauge is composed of a resistor bonded with an elastic carrier (backing). The backing, in turn, is applied to the object where stress or force should be measured. Obviously, that strain from the object must be reliably coupled to the gauge wire, while the wire must be electrically isolated from the object. The coefficient of thermal expansion of the backing should be matched to that of the wire. Many metals can be used to fabricate strain gauges. The most common materials are alloys constantan, nichrome, advance, and karma. Typical resistances vary from 100 to several thousand ohms. To possess good sensitivity, the sensor should have long longitudinal and short transverse segments (See Fig.), so that transverse sensitivity is no more than a couple of percent of the longitudinal. The gauges may be arranged in many ways to measure strains in different axes. Typically, they are connected into Wheatstone bridge circuits. It should be noted, that semiconductive strain gauges are quite sensitive to temperature variations. Therefore, interface circuits or the gauges must contain temperature compensating networks. Wire strain gauge bonded to elastic backing Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (10)

6 Tactile Sensors The tactile sensors loosely can be subdivided into three subgroups: Touch Sensors These sensors detect and measure contact forces at defined points. A touch sensor typically is a threshold device or a binary sensor, namely touch or no touch. Spatial Sensors These sensors detect and measure the spatial distribution of forces perpendicular to a predetermined sensory area, and the subsequent interpretation of the spatial information. A spatial-sensing array can be considered to be a coordinated group of touch sensors. Slip Sensors These sensors detect and measure the movement of an object relative to the sensor. This can be achieved either by a specially designed slip sensor or by the interpretation of the data from a touch sensor or a spatial array. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (11) Tactile Sensors In general, the tactile sensors are a special class of force or pressure transducers that are characterized by small thickness. This makes the sensors useful in the applications where force or pressure can be developed between two surfaces being in close proximity to one another. Examples include robotics where tactile sensors can be positioned on the fingertips of a mechanical actuator to provide a feedback upon developing a contact with an object very much like tactile sensors work in human skin. They can be used to fabricate touch screen displays, keyboards, and other devices where a physical contact has to be sensed. A very broad area of applications is in the biomedical field where tactile sensors can be used in dentistry for the crown or bridge occlusion investigation, in studies of forces developed by a human foot during locomotion. They can be installed in artificial knees for the balancing of the prosthesis operation, etc. In mechanical and civil engineering, the sensors can be used to study forces developed by fastening devices. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (12)

7 Tactile Sensors Requirements to tactile sensors are based on investigation of human sensing and the analysis of grasping and manipulation. An example of the desirable characteristics of a touch or tactile sensor suitable for the majority of industrial applications is as follows: 1. A touch sensor should ideally be a single-point contact, though the sensory area can be any size. In practice, an area of 1 2 mm 2 is considered a satisfactory. 2. The sensitivity of the touch sensor is dependent on a number of variables determined by the sensor s basic physical characteristics. In addition, the sensitivity depends on the application, in particular, any physical barrier between the sensor and the object. A sensitivity within the range N, together with an allowance for accidental mechanical overload, is considered satisfactory for most industrial applications. 3. A minimum sensor bandwidth of 100 Hz. 4. The sensor characteristics must be stable and repeatable with low hysteresis. A linear response is not absolutely necessary as information processing techniques can be used to compensate for any moderate nonlinearities. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (13) Tactile Sensors: Switch Sensors Several methods can be used to fabricate tactile sensors. Some of them require a formation of a thin layer of a material, which is responsive to strain. A simple tactile sensor producing an on off output can be formed with two leaves of foil and a spacer (See Fig.). The spacer has round (or any other suitable shape) holes. One leaf is grounded and the other is connected to a pull-up resistor. A multiplexer can be used if more than one sensing area is required. When an external force is applied to the upper conductor over the hole in the spacer, the top leaf flexes and upon reaching the lower conductor, makes an electric contact, grounding the pull-up resistor. Membrane switch as a tactile sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (14)

8 Tactile Sensors: Switch Sensors The output signal becomes zero indicating the applied force. The upper and lower conducting leaves can be fabricated by a silk-screen printing of conductive ink on the backing material, like Mylar or polypropylene. Multiple sensing spots can be formed by printing rows and columns of a conductive ink. Touching of a particular area on a sensor will cause the corresponding row and column to join thus indicating force at a particular location. Membrane switch as a tactile sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (15) Tactile Sensors: Piezoelectric Sensors Good tactile sensors can be designed with piezoelectric films, such as polyvinylidene fluoride (PVDF) used in active or passive modes. An active ultrasonic coupling touch sensor with the piezoelectric films is illustrated in the shown Fig. where three films are laminated together. The upper and the bottom films are PVDF, while the center film is for the acoustic coupling between the other two. The softness of the center film determines sensitivity and the operating range of the sensor. The bottom piezoelectric film is driven by an AC voltage from an oscillator. This excitation signal results in mechanical contractions of the film that are coupled to the compression film and, in turn, to the upper piezoelectric film, which acts as a receiver. Active piezoelectric tactile sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (16)

9 Tactile Sensors: Piezoelectric Sensors Since piezoelectricity is a reversible phenomenon, the upper film produces alternating voltage upon being subjected to mechanical vibrations from the compression film. These oscillations are amplified and fed into a synchronous demodulator. The demodulator is sensitive to both the amplitude and the phase of the received signal. When compressing force F is applied to the upper film, mechanical coupling between the three-layer assembly changes. This affects the amplitude and the phase of the received signal. These changes are recognized by the demodulator and appear at its output as a variable voltage. Active piezoelectric tactile sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (17) Tactile Sensors: Piezoresistive Sensors Another type of a tactile sensor is a piezoresistive sensor. It can be fabricated by using materials whose electrical resistance is function of strain. The sensor incorporates a force-sensitive resistor (FSR) whose resistance varies with applied pressure. A conductive elastomer is fabricated of silicone rubber, polyurethane, and other compounds that are impregnated with conductive particles or fibers. For instance, conductive rubber can be fabricated by using carbon powder as an impregnating material. FSR tactile sensor through-thickness application with an elastomer (a); transfer function (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (18)

10 Tactile Sensors: Piezoresistive Sensors Operating principles of elastomeric tactile sensors are based either on varying the contact area when the elastomer is squeezed between two conductive plates (Fig. a) or in changing the thickness. When the external force varies, the contact area at the interface between the pusher and the elastomer changes, resulting in a reduction of electrical resistance. At a certain pressure, the contact area reaches its maximum and the transfer function (Fig. b) goes to saturation. FSR tactile sensor through-thickness application with an elastomer (a); transfer function (b) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (19) Tactile Sensors: MEMS Microelectromechanical systems Sensors Miniature tactile sensors are especially in high demand in robotics, where good spatial resolution, high sensitivity, and a wide dynamic range are required. A plastic deformation in silicon can be used for the fabrication of a threshold tactile sensor with a mechanical hysteresis. In one design, the expansion of trapped gas in a sealed cavity formed by wafer bonding is used to plastically deform a thin silicon membrane bonded over the cavity, creating a spherically shaped cap. The structure shown in the Fig. is fabricated by a micromachining technology of a silicon wafer. Micromachined silicon threshold switch with trapped gas Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (20)

11 Tactile Sensors: MEMS Sensors In another design, a vacuum, instead of pressurized gas, is used in a microcavity. This sensor, shown in the Fig., has a silicon vacuum configuration, with a cold field emission cathode and a movable diaphragm anode. Schematic of a vacuum diode force sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (21) Tactile Sensors: Capacitive Touch Sensors A capacitive touch sensor is based on fundamental equations for the parallel-plate and coaxial capacitors. A capacitive touch sensor relies on the applied force that either changes the distance between the plates or the variable surface area of the capacitor. In such a sensor, two conductive plates are separated by a dielectric medium, which is also used as the elastomer to give the sensor its force-tocapacitance characteristics (Fig. a). To maximize the change in capacitance as force is applied, it is preferable to use a high permittivity dielectric in a coaxial capacitor design (Fig. b). Schematic of a vacuum diode force sensor Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (22)

12 Tactile Sensors: Capacitive Touch Sensors The capacitive sensors are popular in touch screen panels that typically are made of glass or a clear polymer coated with a transparent conductor such as indium tin oxide (ITO) that combines electrical conductivity and optical clarity. This type of sensor is basically a capacitor in which the plates are the overlapping areas between the horizontal and vertical axes in a grid pattern. Each plate may be a dual interdigitized or single electrode (See Fig.). Since the human body also conducts electricity, a touch on the surface of the sensor will affect the electric field and create a measurable change in the capacitance of the device. Micromachined silicon threshold switch with trapped gas Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (23) Tactile Sensors: Capacitive Touch Sensors Consider two electrodes deposited on a glass screen as shown in Fig. a. One of the electrodes (G) is grounded and the other is connected to the capacitance meter (C-meter). Some small baseline capacitance C o exists between the two electrodes and that capacitance is monitored by the C-meter. When a finger comes in proximity of the electrodes (Fig. b), it develops a capacitive coupling C 1 with each electrode. If the finger is pressed harder, because of the fingertip elasticity, the contact area with the touch screen increases and that causes a larger capacitive coupling C 2 > C 1 as shown in Fig. c. This will further increase the combined monitored capacitance and thus can be used as an indication of a harder pressing. A dual-electrode touch screen. No touch (a), light touch (b), strong touch (c), and a water droplet (d) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (24)

13 Tactile Sensors: Capacitive Touch Sensors Now, let us assume that a droplet of water is deposited on the touch screen above the electrodes as shown in Fig. d. Being electrically conductive with a dielectric constant between 76 and 80, water forms a strong coupling C 3 with the electrodes, which is comparable with that of a finger and, as a result, the touch screen will indicate a false touch. Sensitivity to water droplets is a disadvantage of a dual-electrode touch screen where one electrode is grounded. A dual-electrode touch screen. No touch (a), light touch (b), strong touch (c), and a water droplet (d) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (25) Tactile Sensors: Capacitive Touch Sensors To resolve a sensitivity to water droplets, an improvement of a capacitive touch screen that contains a single-electrode pattern was proposed. No electrode in that screen is grounded. Under the no-touch condition, only a small capacitance Cg exists between the electrode and ground (earth) as indicated in (Fig. a) and it is monitored by the C-meter. A human body naturally forms a strong capacitive coupling C B to the surrounding objects. This capacitance is several orders of magnitude larger than C o. Hence, a human body may be considered a ground. When a finger comes in the vicinity of the electrode (Fig. b), a capacitance C1 is formed between the finger tip and the electrode. A single-electrode touch screen. No touch (a), light touch (b), strong touch (c), a water droplet (d), and touching through a water droplet (e) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (26)

14 Tactile Sensors: Capacitive Touch Sensors This capacitance is electrically connected in parallel to the baseline capacitance C o, causing the C-meter to respond. Like in a two-electrode screen, a stronger pressing will create a larger capacitance as shown in Fig. c. However, when a water droplet is deposited on the screen, it will cause no detection as the water droplet is not couplet to ground as shown in Fig. d. It is interesting to note that as shown in Fig. e touching the water droplet will form a capacitive coupling to ground and the touch will be correctly detected. A single-electrode touch screen. No touch (a), light touch (b), strong touch (c), a water droplet (d), and touching through a water droplet (e) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (27) Tactile Sensors: Acoustic Touch Sensors Acoustic touch screen is based on the recognition of sound waves propagating in an object when the user touches it. A touch of an object produces a pattern of sound waves propagating through the material. This pattern creates an acoustic signature that is unique to the location of the impact. This property is called Time Reversal Acoustics, which can be used to precisely identify location of the source of radiated waves. An acoustic sensor picks up the vibration in the material and passes them to a microcontroller that captures the audio vibrations within an object, generates, and stores the acoustic signatures. This is done during the training of the sensor. In use, when touching the object at the same spot that was previously touched in training, the detected acoustic pattern is compared with the database of signatures and matches a response to the stored pattern. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (28)

15 Tactile Sensors: Acoustic Touch Sensors Another acoustic touch sensor are based on the surface acoustic waves (SAW) technology that uses ultrasonic waves passing over the touch screen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touch screen panels can be damaged by outside elements. Contaminants on the surface such as water or oil droplets can also interfere with the functionality of the touch screen. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (29) Tactile Sensors: Optical Sensors Conventional optical-touch systems use an array of infrared (IR) light-emitting diodes (LEDs) on two adjacent bezel edges of a display, with photo detectors placed on the two opposite bezel edges to analyze the system and determine a touch event (See Fig.). The LED and photo detectors pairs create a grid of light beams across the display. An object (such as a finger or pen) that touches the screen changes the reflection due to a difference between refractive properties of air and a finger. This results in a measured decrease in light intensity at the corresponding photo detector. The measured photo detector outputs can be used to locate a touch-point coordinate. Concept of optical touch screen. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (30)

16 Piezoelectric Force Sensors While the tactile sensors that use piezoelectric effect as it was described above are not intended for the precision measurement of force, the same effect can be used quite efficiently for the precision measurements in different sensor designs. Piezoelectric effects can be used in both passive and active force sensors. In the sensor applications, the goal is just the opposite a sensitivity to force along certain axes should be maximized. For example, the diametric force has been used for a high-performance pressure transducer (See Fig.). Concept of optical touch screen. Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (31) Piezoelectric Force Sensors Another design of a sensor that operates over a relatively narrow range from 0 to 1.5 kg, however, with a good linearity and over 11-bit resolution is shown in the Fig. quartz To fabricate the sensor, a rectangular plate is cut of the crystal where only one edge is parallel to the x axis, and the face of the plate is cut at the angle of approximately = 35 with respect to the z axis. This cut is commonly known as AT-cut plate (Fig. a). The plate is given surface electrode s for utilizing a piezoelectric effect, which are connected in a positive feedback of an oscillator (Fig. b). A quartz crystal oscillates at a fundamental frequency f o (unloaded), which shifts at loading. Each resonator is connected into its own oscillating circuit and the resulting frequencies are subtracted, thus negating a temperature effect. A commercial force sensor is shown in Fig. c. Quartz force sensor AT-cut of a quartz crystal a); structure of the sensor (b); outside appearance (c) Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (32)

17 Thank You Ossama Abouelatta Mechanical Engineering Department Faculty of Engineering Albaha University Albaha, KSA Assoc. Prof. Ossama Abouelatta, Mechanical Engineering Department, Faculty of Engineering, Albaha University (33)