User Defined function SQL 2008

I'm trying to create a calculated column that returns an INT value, I ve created a function and need to pass the ndx number to the function and having issues with returning multiple values within the sub query. how do I pass the ndx number to the function, I'm assuming that the calculated column looks at values from the same row!?

Msg 512, Level 16, State 1, Line 1

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

CREATE FUNCTION dbo.Nat_Weight(@me38_cycle_data_ndx INT)
RETURNS INT
AS 
BEGIN
DECLARE @nat_weight INT =0;
DECLARE @mattype1 INT;
DECLARE @mattype2 INT;
DECLARE @mattype3 INT;
--DECLARE @me38_cycle_data_ndx INT;
-- get material type, need only hoppers 1-3, hopper 4,5,6 material type will never = 2
SET @mattype1 = (SELECT typehopper_01 FROM mm_Cycle_Data);
SET @mattype2 = (SELECT typehopper_02 FROM mm_Cycle_Data );
SET @mattype3 = (SELECT typehopper_03 FROM mm_Cycle_Data );
-- if material type=2 then add to @nat_weight ,  
IF @mattype1 = 2
    set @nat_weight = (SELECT cyclehopper_01 FROM mm_Cycle_Data WHERE me38_cycle_data_ndx=@me38_cycle_data_ndx );
IF @mattype2 = 2
    set @nat_weight =@nat_weight+ (SELECT cyclehopper_02 FROM mm_Cycle_Data WHERE me38_cycle_data_ndx=@me38_cycle_data_ndx );
IF @mattype3 = 2
    set @nat_weight =@nat_weight+ (SELECT cyclehopper_03 FROM mm_Cycle_Data WHERE me38_cycle_data_ndx=@me38_cycle_data_ndx )
RETURN @nat_weight
END