As I understand it Roger, you have a means of detecting this 'velocity' but what I found peculiar last time I engaged in this topic was that you had not quantified this velocity as a specification. I'm pretty sure you're not talking about risetime/slew rate.The question is, have you quantified this circuit operation with a spec? Or put another way, have you quantified the 'velocity' spec?
At least you confirmed one thing- that you are not talking about risetime/slew rate.