Error (10482): VHDL error at QuadratureDecoder.vhd(34): object "conv_std_logic_vector" is used but not declared

I dont know why I am getting this error (Error (10482): Error (10482): VHDL error at QuadratureDecoder.vhd(34): object "conv_std_logic_vector" is used but not declared

Please point out what do I need to change.. Here is my code

library IEEE;

use ieee.std_logic_1164.all;

use ieee.numeric_std.all;

entity QuadratureDecoder is

Port ( QuadA : in STD_LOGIC;

QuadB : in STD_LOGIC;

Clk : in STD_LOGIC;

conv : out STD_LOGIC_VECTOR ;

position : out STD_LOGIC_VECTOR (7 downto 0));

end QuadratureDecoder;

architecture Behavioral of QuadratureDecoder is

signal QuadA_Delayed: unsigned(2 downto 0) := "000";

signal QuadB_Delayed: unsigned(2 downto 0) := "000";

signal position1 : std_logic_vector (7 downto 0):="00000000";

signal Count_Enable: STD_LOGIC;

signal Count_Direction: STD_LOGIC;

signal Count: unsigned(7 downto 0) := "00000000";

begin

process (Clk)

begin

if Clk='1' and Clk'event then

QuadA_Delayed <= (QuadA_Delayed(1), QuadA_Delayed(0), QuadA);

QuadB_Delayed <= (QuadB_Delayed(1), QuadB_Delayed(0), QuadB);

if Count_Enable='1' then

if Count_Direction='1' then

Count <= Count + 1;

position <= conv_std_logic_vector(Count, 8);

else

Count <= Count - 1;

Position <= conv_std_logic_vector(Count, 8);

end if;

end if;

end if;

end process;

Count_Enable <= QuadA_Delayed(1) xor QuadA_Delayed(2) xor QuadB_Delayed(1)

xor QuadB_Delayed(2);

Count_Direction <= QuadA_Delayed(1) xor QuadB_Delayed(2);

end Behavioral;