function query() on a non-object error

I am trying to create a plugin for Wordpress which allows the user to enter a title into a text box. The user then clicks 'submit' and that title is taken and added to the database. However currently when you press submit, it returns a PHP error:

Fatal error: Call to a member functio开发者_运维知识库n query() on a non-object in /home/matthew/public_html/demo/wp-content/plugins/premium-slider/admin.php on line 49

Line 49 is the following code:

$wpdb->query($sql);

However this is perfectly valid code according to Wordpress's tutorials.

Here is the overall code segment:

function print_admin_form() {
 if (isset($_POST['addnew'])) {

 $new = $_REQUEST['name'];

 $name = escape($new);

  $sql = "INSERT INTO $table_name_cat (name) VALUES('$new')";
  $wpdb->query($sql);
  echo $name; } 
 else {
  echo '<div id="message" class="error" style="width:750px;"><p><strong>Failed to add new slider.</strong></p></div>'; }?>

<div class="wrap">
<h2>Easing Slider</h2>
 <form method="post" action="<?php echo str_replace( '%7E', '~', $_SERVER['REQUEST_URI']); ?>&updated=true">
   <div class="metabox-holder" style="width:402px;float:left;">
    <div class="postbox">
      <h3><span>Slider title:</span></h3>
      <h4 style="margin:10px;">Name:</h4><input type="text" name="name" style="width: 380px;margin:10px;margin-top:0px;"/>
    </div>
 </div>
 <input type="submit" class="button-primary" name="addnew" value="Save Changes" style="" />
 </form>
 </div> <?php }

try :

global $wpdb;

as first line of your print_admin_form function.