### From Analog to Digital Actuation  
Planar motion could be based on continuous (analog) actuators, able to attain and keep any position inside the actuator stroke. These actuators present high performances and are generally controlled in closed-loop with high-precision sensors, making their integration in compact systems complex. Another architecture based on a collaboration of digital actuators could also be used for planar motions. The stroke of digital actuators is divided into well-defined, repeatable and stable positions that enables sensorless solutions. Once the mobile part reaches one of the discrete position, the stability of the position maintains the mobile part in place. Energy is only needed to switch the mobile part between the discrete positions, enabling the use of a simple pulsed control signal. Due to the digital behavior, an open loop control can be used, simplifying the integration of digital actuators in compact or highly integrated systems. In the literature, digital actuators are used as switches or arranged in a collaborative actuation array for more complex tasks. Different physical principles have been used to create digital actuators. The authors in [13] used an electromagnetic bistable actuator as a micro switch, with 160 um stroke and a closing time of 0.96 s. In [14], the authors propose a chain of two DoF digital electromagnetic tilting actuators to perform complex movements and manipulations as a modular robot chain. The authors in [15] built a 50 \(\times\) 50 optical switch matrix using digital electrostatic actuators to guide the light. As their strokes were 1 um, they obtained switching times of 0.85 us. The authors in [16] proposed a tactile display with an elastomer actuator working with an electrostatic actuator producing digital actuation. The resulting actuator had a stroke of 680 um and 185 mN blocking force. A combination of electromagnetic and piezoelectric principles for multistable digital actuation was presented in [17]. The actuator consists of permanent magnetic rings, defining multiple stable positions: a lower one in contact with a piezoelectric stack actuator, and upper ones located at predefined heights, where the magnetic object is stably levitated. The piezo stack actuator provides a fast initial acceleration of the magnet, improving performance, and then solenoids control which final position is attained. In [18], the authors fabricated a DAC system based on a multistable electrostatic actuator array. Their system showed a total stroke of 150 um with eight defined positions at a control voltage of 60 V. The minimum switching time was 0.1 ms. The authors in [19] presented a shape memory alloy actuated anti-slip mechanism for the wheels of step climbing robots. The proposed mechanism used a bi-stable four-bar mechanism to clamp on the edges of the stairs to avoid slipping, blocking the wheel of the robot while activated and passively returning to the closed position when not needed. In addition to the presented applications, digital actuators have been used for planar conveyance devices. In [20], the authors presented a 20 \(\times\) 20 array of ciliary electrothermal digital actuators. The conveyor had a minimum step of 2 to 18 um and a maximum conveyance speed of 0.65 mm s\({}^{-1}\). In [21], the author developed a \(4\times 4\) array of digital electrothermal actuators as a micro-conveyor based on lift and slide principle. Each actuator had a force of 80 uN and a stroke of 3 um. The minimum step size was 0.28 um, with a maximum speed of 56.8 um s\({}^{-1}\). Both systems in [20,21] are electrothermal, known to generate large strokes and deflections with important forces, but have slow response times (thus slow conveyance speeds) and consume more power than other actuation principles. Planar motion conveyors based on digital actuators produce a step by step output which can be limiting if a smooth continuous motion is required.