  In unstructured environments, robots run the risk of unexpected collisions.
How well they react to these events is determined by how transparent they are
to collisions. Transparency is affected by structural properties as well as
sensing and control architectures. In this paper, we propose the collision
reflex metric as a way to formally quantify transparency. It is defined as the
total impulse transferred in collision, which determines the collision
mitigation capabilities of a closed-loop robotic system taking into account
structure, sensing, and control. We analyze the effect of motor scaling,
stiffness, and configuration on the collision reflex of a system using an
analytical model. Physical experiments using the move-until-touch behavior are
conducted to compare the collision reflex of direct-drive and
quasi-direct-drive actuators and robotic hands (Schunk WSG-50 and Dexterous
DDHand.) For transparent systems, we see a counter-intuitive trend: the impulse
may be lower at higher pre-impact velocities.
