SpriteKit: Uniform Circular Motion - ios

Question

I'm trying to move a sprite in a circular motion using a reusable function (that I need to use on different objects).
The first result is given by this function:
func circularMotion(angle: CGFloat, radius: CGFloat, center: CGPoint) -> CGPoint{
let x = radius * cos(angle) + center.x
let y = radius * sin(angle) + center.y
return CGPoint(x: x, y: y)
}
And then I update the position of my sprite like this:
override func update(currentTime: NSTimeInterval) {
player.position = circularMotion(ang, radius: 200, center: CGPoint(x: size.width/2, y: size.height/2))
ang += speed
}
Where ang (the angle) and speed (0.05) are two initialized properties.
And this works pretty fine, but the actual problem is that I need to have the velocity of the sprite that rotates, since it has a PhysicBody.
My approach, though, changes only the position of the sprite. So I decided to do something like this:
override func update(currentTime: NSTimeInterval) {
let prevCirc = circularMotion(ang, radius: 200, center: CGPoint(x: size.width/2, y: size.height/2))
let Circ = circularMotion(ang + 0.05, radius: 200, center: CGPoint(x: size.width/2, y: size.height/2))
let v = CGVector(dx: Circ.x - prevCirc.x, dy: Circ.y - prevCirc.y )
player.physicsBody!.velocity = v
}
I just calculated the difference of position, finding the perpendicular Vector respect to the position vector. This still "works", the body follows a circular motion, but it has not the radius and center given (it's a smaller circle).
I know it's because of the length of the vector that it gives this result.
So the real question is, having the radius and the center of the circle I want (and the speed of how the angle changes) how can I calculate the perpendicular velocity so that it gives me a motion with those properties?