CodeIgniter使用回车键登录

在我的登录表单中有这个：

<div class="middle-box loginscreen  animated fadeInDown">
<div>
    <div>
        <h1 class="logo-name"><?= $this->lang->line('GGG')?></h1>
    </div>
    <h3><?= $this->lang->line('GGG')?></h3>
    <form class="m-t" role="form" action="welcome/check_user" id="check_user" method="POST">
        <div id="save_result"></div>
        <div class="form-group has-feedback">
            <label class="control-label" for="username"><?= $this->lang->line('Username')?><sup class="mandatory">*</sup></label>
            <input type="email" class="form-control required email" placeholder="<?= $this->lang->line('Username')?>" name="username" id="username" maxlength="50">
        </div>
        <div class="form-group has-feedback">
            <label class="control-label" for="password"><?= $this->lang->line('Password')?><sup class="mandatory">*</sup></label>
            <input type="password" id="btn" class="form-control required" placeholder="<?= $this->lang->line('Password')?>" name="password" id="password">
        </div>
        <button type="button" onclick="submit_form('#check_user')" class="btn btn-primary block full-width m-b"><?= $this->lang->line('Login')?></button>

需要得到输入按钮提交，它不会。