0LAUK0 2018Q1 Group 2 - Design Plans Research

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Introduction

This page contains research performed by group 2 for the course Project Robots Everywhere (0LAUK0) regarding the state-of-the-art on actuators and face tracking. This literature study is divided into the following relevant topics:

  • face tracking
  • monitor-arm actuation


Face Tracking

content


Monitor-Arm Actuation

To design the monitor arm we first needed to consider what actuation type to use. This actuation type is dependent on a few variables such as:

  • Precision
  • Torque
  • Speed
  • Noise
  • Cost
  • Design and looks
  • Compatibility
  • Input and output

Basically there are four convenient options for actuation. These are: Stepper Motors, Servo Motors, Direct DC Motors and Hydraulic Actuation. In the following paragraph we will explain what the pros and cons are of these actuation types based on the variables described above.

Stepper Motors

The principle of magnetic attraction and repulsion are used by stepper motors to move a screw. The torque is created by alternately applying current to the individual windings in the motor. This torque turns a permanent magnet rotor. Once the windings are energised, a holding torque is generated. Unlike DC (servo) motors, stepper motors have an inherent holding, detent or torque that can be used to maintain the position of devices in the power off state for a period of time. Steppers provide an inexpensive open-loop method to achieve high accuracy depending on the resolution. Furthermore, no feedback is required for position or speed control. Stepper motors are also compatible with modern digital equipment. They are also less expensive than servo motors, because of the removal of position feedback. On the other hand, steppers are bulky, noisy, generate a much amount of unwanted heat and do not provide smooth continuous motion. The large size of these motors can make it difficult to incorporate them into small mechanisms. Furthermore, friction can be a problem in the high accuracy control.

Continuous Rotation Servo Motors

When used with an encoder, servo motors provide smooth, continuous motion as well as high speeds and submicron accuracy. It consists of coils of wire around a metal core (called an armature) inside a magnetic field. When electric current is applied to the windings, the armature interacts with the magnetic field causing the armature to turn. These motors require constant power to maintain position, so it is not an ideal solution for set-and-hold applications. It also generates unwanted heat (not as much heat however than stepper motors) and requires a feedback mechanism for controlling position and velocity. Even when holding a specific position, these motors often oscillate around the position. In general, servo motors are considered to be more accurate than stepper motors, but just like with the stepper motors, friction can also be a problem in the high accuracy control.

Direct DC Motors

Direct DC motors are only usable when implemented in a way comparable to servo’s. The rotational speed of a direct DC motor is really high, and thus always needs some sort of gearing which makes a lot of nice and can be hard to implement. Direct DC motors also need an encoder to control properly. By doing this one actually makes a servo of the direct DC motor, so the rest is comparable.

Hydraulic Actuation

Hydraulic actuation can be a real silent type of actuation. This is only true when the pressurizer is not close to the monitor arm system. When controlled correctly it can be fairly precise, but it will always need some form of closed-loop feedback system. It may be difficult to find a sensor that is precise enough to use for this closed-loop feedback system. One could think of a linear solenoid actuator with some sort of encoder to drive the hydraulics, but this may make the system difficult to employ. Another problem with hydraulics is that it is based on a lot of assumptions on the flowing medium. This means that the output is always a bit different than the expected results. Hydraulics are a real powerhouse and always make sure the monitor arm stays in the right position as long as there are no leaks. Hydraulics are good at linear and rotary actuations. The speed of hydraulic actuation is not high, but probably fast enough for our system. Hydraulic systems are expensive compared to other actuation types. A hydraulic system can be implemented real neat when done right, but due to a lot of components it may look real messy and can give a lot of problems.

Chosen Actuation Type

We have chosen to use stepper motors for our robotic monitor arm. The stepper motor can precisely rotate to each wanted position for the robot arm. It is a good solution for the radial actuations needed. When implemented with the ‘Gispen duo zitsta desks’, which will be used in Atlas, the linear movement in the z direction of the monitor can be actuated by the same actuation used in the desk. This means that for the monitor arm only rotational actuation is needed. The biggest downside of the stepper motor is the noise it makes, but this can be solved by implementing stealthChop™ technology by Trinamic. When using a TMC controller device which uses stealthChop™ and spreadCycle™ the stepper motors noise can be reduced with more than 10 dB which makes the whole system silent enough to use in a workplace. When using a standard stepper motor such as the Nema 14, precision up to 1.8 degrees can be reached which makes the stepper motor accurate enough for the monitor arm. When the torque is not sufficient enough we could implement a spring system in the arms which reduces the force need to move the monitor arm but also makes sure the monitor arm stays in the right position without needing to much extra force from the stepper motor. The speed of the stepper motors is more than sufficient. SpreadCycle™ technology makes sure the stepper motor always operates smoothly. The costs for the actuation are not high compared to other solutions. Stepper motors are quite bulky, but should be easy compatible in a system. When integrated neatly, the motors should look quite nice.

Sources

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