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

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.