IPhone only solution
In whatever UITextField you're getting these values from, you can specify the kind of keyboard you want to appear when somebody touches inside the text field.
E.G. a numeric-only keyboard.
Like this screenshot:
Ipad
The iPad does not support the numeric keyboard, so your options are to either not support the iPad, validate the field post submit, or follow one of the other suggestions here to create same behaviors while running on an iPad.