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PMX as a Central Measurement and Control System for the Investigation of Haptic Actuators with Shape Memory Effect

zaf, germany



Shape memory alloys (SMA) can be operated as noiseless and lightweight small drives in minimalized installation spaces. Despite their small size, these drives have the highest power density of all known actuators.


What Is the Shape Memory Effect?

If a shape memory element is mechanically deformed in the martensitic state, a high elongation (up to approximately 8%) occurs in the form of a plateau when a critical stress is exceeded. If the temperature is subsequently increased to such an extent that a phase transformation of martensite into austenite occurs, the reshaping of the shape memory element takes place. This process is hysteretic and reversible [1]. In this way, the shape memory wire shown in Figure 1 (below) can be switched back and forth between states 1 and 2 under load by electrically heating it up. During the material conversion, a significant change in the electrical resistance can be detected.

An FG wire actuator usually consists of a NiTi alloy and can generate maximum tensile stresses of 400 MPa in continuous operation, 800 MPa for one-off operations [2]. For example, a 1g dead weight FG wire can move loads of 5000g. An electromagnet of the same power class would weigh more than 200g.

Because of the above properties, for example, the realization of very lightweight and compact haptic elements is possible.

Figure 2 also shows an arcuate FG drive. The hinged FG wire is mechanically clamped at both ends, while the middle part is connected to an actuator. This actuator can be used to transmit mechanical stimuli in the form of force stimuli to humans.

The special feature of force-tactile FG actuators for the haptics is the positioning behaviour of FG actuators. Here, quasi-static stimuli can be transmitted, which can be differentiated via haptic receptors much better in terms of intensity than is the case with vibration actuators.

Concept: Intelligent Automobile-Human Communication via Feel

Figure 3 shows a concept for using haptic information feedback in automobiles. For this purpose the vehicle detects the environmental conditions through sensors and can e.g. Distances to other vehicles while driving, obstacles when parking or detect hazards in the vicinity. The signal is detected, for example, by an ultrasonic sensor and forwarded to a control unit.

Now the information about the haptic stimulus channel can be transmitted to the driver via various skin receptors. As shown in [4], the distance information to the vehicle in front can be transmitted via analogue haptic elements on the steering wheel, and location-related stimuli can be generated in an ergonomically adapted seat. Thus, it is possible that in the lumbar region actuators are used, which make a reference to the rear distance sensors in the vehicle.

A particularly interesting point for stimulus perception is also the foot area. This can be stimulated with an actuator that is integrated into a shoe. In the area of the arch that remains unloaded when walking, an FG actuator can be used, which must produce a maximum travel of 2 mm.

 Here, the actuator overcomes 1.2 mm sole elements in the shoe and generates a maximum stimulus of 5 N with a travel of 0.8 mm. The connection to the control unit is made by a wireless information transmission. Energetically, the haptic control system is supplied via an inductive interface.

System Development and Experimental Setup

To evaluate the concept, a pattern was built in a men's shoe. The FG actuator was installed in the area of the rear foot arch in a groove machined in the shoe. The FG actuator is mounted so that the haptic pin has to penetrate the shoe sole in order to reach the resting foot arch in the shoe.

The test is controlled by a universal measuring amplifier PMX with integrated PLC, which was programmed with Codesys. The actuating path of the FG actuator and thus also the stimulus intensity of the haptic signal is detected by a potentiometric sensor and sent to the measuring amplifier. The triggering of the FG actuator is event-controlled, so that, depending on the servo-motor excited shoe load, the travel of the FG actuator can be measured.

At the start of a cycle, the controller issues a signal to close the relay, closing the circuit between the power source and the actuator being tested. As a result, the FG wire shortens, creating a stroke as shown in the sketch in Figure 2. The experimental setup is shown in Figure 4.

PMX a Component for Control and Data Recording

The PMX measuring amplifier system provides a tailor-made solution for operating the test bench. Its flexible design allows precise measurement with a wide variety of sensor types and can control the entire test bench via digital inputs and outputs and the integrated CODESYS Soft-PLC. All measured values are sampled at 20kHz and visualized and stored via the integrated Ethernet interface on the HBK DAQ software CATMAN.

In the following experiment, a pressure load in the shoe was simulated in addition, similar to when walking. Two servo motors were used to create the foot pressure on the shoe sole (from the top of the shoe). Figure 6 shows the associated dynamic measurement (over time). The load forces were measured below the shoe soles and occur independently of the travel of the FG actuator.

If the shoe is loaded, the load on the FG actuator also changes. If the travel reaches the zero position in the attempt, the current supply of the FG wire is switched off and the load is switched on. If the actuator is still in the austenitic state, it is elongated by the mechanical force, or experiences a pseudoelastic strain. After switching off the counterforce, the FG actuator now jumps back to an extended position, depending on its temperature to the transformation temperatures. Due to the opposing force results in a lower, noticeable for the user, pulse time of only 4 seconds.

About the ZAF

The ZAF, the Center for Applied Shape Memory Technology, is a member of the Forschungsgemeinschaft Werkzeuge und Werkstoffe e.V. Its main objective is industrial research and development for the tool and cutlery industry, solid forming, automotive, precision mechanics, machine tools and valve technology. It is an institute of the Bergische University in Wuppertal where Dr.-Ing. Alexander Czechowicz is employed.

Alexander Czechowicz

Technology used