This text is an introduction to admittance controlled haptics.
The book starts off with a series of preliminary chapters, which the reader can skip on a first reading if desired.
First comes an introduction to haptics in general, and the place of admittance control in this field. Then there is an introduction to control theory which is needed to appreciate the difference between impedance and admittance control, especially as regards contact stability. Impedance control is discussed separately because it is probably the background for many readers, and it helps the understanding to frame it in words which will carry over to admittance control in a more natural way than is offered by the traditional view of impedance control.
Chapter 6 gives a brief introduction to the history and types of admittance control and its applications.
Chapter 7 starts the real discussion of haptic admittance control with the virtual mass model. Expert readers can skip directly to this chapter if desired.
Admittance control is loosely defined as that form of force control which uses a force sensor as an input, and displacement as an output.
This method of force control has been re-invented many times since at least he 1980's, in different guises, and under different names. This report defines admittance control more restrictively as the method using a pure virtual mass as the virtual model.
A large fraction of the literature however adds a damping term to the basic virtual mass model, and some authors even leave out the mass term altogether and just use a virtual damper as the model. This latter form of admittance control is sometimes called "accommodation control". The user force is then interpreted as a velocity command, instead of as an acceleration command. The user has to push, usually quite hard, to keep the device moving.
Accommodation control is more suited to industrial tasks, because it is more stable against hard surfaces. However, it is not suited to human interaction, since the robot feels like it has massive friction.
The virtual mass idea can also be seen as an extension to accommodation control, where the "intent of the user" is identified and turned into a more permanent command by an extra integrator on the velocity.
Dubbele integrator was duur ! Dus het bleef bij tamme D" : lead-lag en lag-lead networks in plaats van zuivere integratoren (en differentiatoren.). At one time, lead-lag instead of P and D. Analog integrators $1000 in 1980, one reason why the virtual mass model was not implemented before.