The Blues had already had one try disallowed and scored one that counted through Casey Laalala to take a 10-3 lead after half an hour.
After a good start, Gloucester were rocking and seemingly in need of half time to regroup.
However, Freddie Burns' interception try in injury time proved to be crucial as Gloucester went in level and then took the game by the scruff of the neck in the second half.
Simpson-Daniel ran in three tries after the break as Gloucester made the game safe despite Sweeney's late try and a place in next week's final was secured.
International fixtures over the weekend had a huge impact upon this game. Both sides were missing several players and the shiny and new Cardiff City Stadium looked barely half ful despite the semi final status and competitive ticket pricing.
The Gloucester supporters to their credit had turned out in force and the far side of the crowd was a sea of cherry and white which greeted their team with gusto during the warm ups.
Gloucester looked fired up in the game's opening minutes and determined to spread the ball wide. The home defence were getting in lots of early tackling practice.
Akapusi Qera was the first man to so nearly break through but his offload ws picked off by a retreating Blue. However, Cardiff killed the ball only for Robinson, who had just received lengthy treatment, to drag his penalty attempt wide.
Gloucester kept up the pressure and kept hammering away. Eventually Cardiff were caught offside and Robinson had another chance, this time from wide out on the left and this time he was on the money for a 3-0 lead after 11 minutes.
The Blues needed a break and got one when James Simpson-Daniel fumbled a high kick near his own 22. The Cardiff pack forced a penalty from the scrum and Ben Blair slotted the penalty to level the scores.
It prompted a better spell from Cardiff and they almost got a try out of nowhere on 21 minutes. Wing Tom James wriggled his way clear from a ruck on halfway and chipped over Freddie Burns.
Simpson-Daniel looked to win the chase but couldn't hang onto the ball and James seemed to ground for an opportunist score. However, after consulting the video referee, referee Clancy awarded Gloucester a penalty to the obvious disdain of the home crowd.
However, the score wasn't long in coming. Casey Laulala bumped off a couple of weak tackles on midfield near the ruck and powered clear. He stepped Burns and dived over under the posts. It was a poor score from a Gloucester perspective and made even worse when Blair converted for 10-3.
It could have got worse as Sweeney jinked his way clear from fly half straight afterwards, evading Azam on the way, but Tindall got back to secure a priceless turnover in his own 22.
At this stage, there were simply too many errors from Gloucester. Free kicks were conceded at scrums, tackles were being missed and passes either going forward or being dropped in attack.
Half time looked as though it would come without further event until, with Gloucester seemingly out of ideas, Ceri Sweeney floated a pass out side and Freddie Burns picked it off for an interception try. Robinson converted to send the teams in level at half time.
It was a huge break for Gloucester who had spluttered after a good start. Cardiff had not really offered anything aside from Sweeney's kicking from hand and their two main breaks came from missed tackles.
However, Gloucester would have to resect possession and stiffen up in defence if they were to emerge victorious.
Adam Eustace replaced Jake Boer at half time but it was the South Sea Island trio of Vainikolo, Fuimaono-Sapolu and Qera who very nearly opened up the Blues in the opening minutes as they traded passes into the home 22 only for the defence to scramble back and kill the danger.
However, the cultured left boot of Robinson reapplied the pressure as his measured kick caught Tom James near his own line.
Gloucester's catch and drive was repelled but Cardiff did so illegally. Robinson opted to kick for goal from left of the posts but dragged his kick across the face to keep the score at 10 all.
The Gloucester fly half didn't let his head drop and his half break and offload led to Qera powering into the 22. Gloucester were within a couple of metres and Cardiff openside Ben White killed the ball and was yellow carded.
Gloucester could have kicked the ball but kept the ball in hand and Cardiff eventually ran out of tacklers. Tindall put Simpson-Daniel clear and the winger did the rest. Robinson slotted an excellent conversion for 17-10 after 56 minutes.
It prompted a furious response from Cardiff and Filise, Powell and Rush almost bashed their way to the line and only desperate defence kept them out.
Wave after wave of Blues forwards hammered their way towards the line but Gloucester dug deep and Cardiff had to settle for a Blair penalty to make the score 13-17 on 61 minutes.
They were tense times for Gloucester as the Blues dominated possession for the next few minutes and the defence had to work overtime as the game went into a tense final 10 minutes.
However, the tension was relieved as 72 minutes as Gloucester won a key turnover on midfield. Qera made the initial yards, linked with Molenaar and his inside pass found Simpson-Daniel who sprinted home for his second try. Robinson converted for 13-24.
The pack took over and almost muscled their way over from a fantastic rolling maul but Freddie Burns measured chip was the icing on the cake as it sat up nicely for Simpson-Daniel to complete his hat trick.
There was just time for Sweeney to take advantage of a Gloucester handling mistake to score a late consolation but nothing was going to deny Bryan Redpath's men by that stage.
|Cardiff Blues Score Card|
|Gloucester Rugby Score Card|