This is my code-------

class account_voucher(osv.Model):

_inherit = "account.voucher"

_columns={

'sequence_number':fields.char('Sequence Number'),

}

def create(self, cr, uid, vals, context=None):

vals['sequence_number'] = self.pool.get('ir.sequence').get(cr, uid,'account.voucher')

return super(account_voucher, self).create(cr, uid, vals, context=context)

_defaults={

'sequence_number': lambda obj, cr, uid, context: ' /',

}

account_voucher()

=============================

<?xml version="1.0" encoding="utf-8"?>

<openerp>

<data noupdate="1">

<!-- Sequences for loan_id -->

<record id="seq_cloan_code_inh1" model="ir.sequence.type">

<field name="name">SEQ ID</field>

<field name="code">account.voucher</field>

</record>

<record id="seq_cloan_code_inh2" model="ir.sequence">

<field name="name">SEQ ID</field>

<field name="code">account.voucher</field>

<field name="prefix">SEQ</field>

<field name="padding">2</field>

</record>

</data>

</openerp>